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1st Generation 1964 1/2 - 1973 - Questions & general discussions that apply to a specific year => 1967 Mustang => Topic started by: 3stangs2fordtrucks on February 09, 2018, 07:47:04 PM

Title: 1967 convenience low fuel light remains on
Post by: 3stangs2fordtrucks on February 09, 2018, 07:47:04 PM
Hi, this is my first post to your forum. The forum is quite impressive.  I spent last night reading on your forum the information related to the 1967-68 convenience package.  I am presently trying to trouble shoot my convenience system on a 1967 GT390 fastback.  The car is disassembled so I can dig through the wiring fairly easily.  Three of the lights function properly but the low fuel light comes on when I attach the battery to the car and remains on.  I understand from previously posts that the main cause of failure is that the low fuel sensor fails; however, I have a new low fuel sending unit that I purchased from WCCC.  I have also replaced the low fuel relay with a nos one.  To test the low fuel system, I have suspended the sending unit in a bucket of water so that the sensor is submerged but not the top of the unit.  I grounded the sending unit to a frame bolt.  Nothing I have done will result in the light going off.  If anyone has advice I would appreciate it.  I must be missing something.
Title: Re: 1967 convenience low fuel light remains on
Post by: jwc66k on February 09, 2018, 08:25:20 PM
First, welcome. That's a real "downer" entrance.
I went thru this low fuel light circuit on a friend's 67 Fastback a few years back. It was the Thermistor.
For your situation, my first recommendation is to disconnect the low fuel relay with power applied. The low fuel light should go out.
Another sanity check (relay back in) would be to look at the solenoid. On the stud that goes to the starter there is a ring lug connection wire that provides a circuit check function for the low fuel light. That lug can be attached to the battery side of the solenoid by mistake which will keep the light on. A visual check.
Good luck.
Jim
Title: Re: 1967 convenience low fuel light remains on
Post by: 67gtasanjose on February 09, 2018, 08:31:43 PM
First, welcome. That's a real "downer" entrance.
I went thru this low fuel light circuit on a friend's 67 Fastback a few years back. It was the Thermistor.
For your situation, my first recommendation is to disconnect the low fuel relay with power applied. The low fuel light should go out.
Another sanity check (relay back in) would be to look at the solenoid. On the stud that goes to the starter there is a ring lug connection wire that provides a circuit check function for the low fuel light. That lug can be attached to the battery side of the solenoid by mistake which will keep the light on. A visual check.
Good luck.
Jim

Good guess Jim, you beat me to that answer! I hope this is it.

If you GOOGLE "Mustang low fuel schematic", you will find some help. It helps to also understand where the wires are SUPPOSED to go.
Title: Re: 1967 convenience low fuel light remains on
Post by: midlife on February 09, 2018, 08:36:01 PM
First, welcome. That's a real "downer" entrance.
I went thru this low fuel light circuit on a friend's 67 Fastback a few years back. It was the Thermistor.
For your situation, my first recommendation is to disconnect the low fuel relay with power applied. The low fuel light should go out.
Another sanity check (relay back in) would be to look at the solenoid. On the stud that goes to the starter there is a ring lug connection wire that provides a circuit check function for the low fuel light. That lug can be attached to the battery side of the solenoid by mistake which will keep the light on. A visual check.
Good luck.
Jim
And beat me to it as well.  +2.
Title: Re: 1967 convenience low fuel light remains on
Post by: ruppstang on February 09, 2018, 08:45:36 PM
That was my first thought too but he stated that the other three lights were working.
Title: Re: 1967 convenience low fuel light remains on
Post by: midlife on February 09, 2018, 10:01:00 PM
That was my first thought too but he stated that the other three lights were working.
It is only that line to the starter solenoid that powers up the low fuel light as a proof-out circuit while cranking the engine.  The other 3 systems have nothing to do with that circuit.
Title: Re: 1967 convenience low fuel light remains on
Post by: ruppstang on February 09, 2018, 10:33:52 PM
It is only that line to the starter solenoid that powers up the low fuel light as a proof-out circuit while cranking the engine.  The other 3 systems have nothing to do with that circuit.
Thanks. I was not aware of that LOL and I have it in my car.
Title: Re: 1967 convenience low fuel light remains on
Post by: 3stangs2fordtrucks on February 09, 2018, 10:35:49 PM
Thanks, I will check it out first thing tomorrow. 
Title: Re: 1967 convenience low fuel light remains on
Post by: 196667Bob on February 10, 2018, 04:42:41 AM
Thanks. I was not aware of that LOL and I have it in my car.

Marty : Maybe the attached might help clarify it. Note that the red wire from the starter solenoid only goes to the plug for the low fuel relay (and thus the low fuel bulb) ; the blue w/red stripe wire (shown in violet) from the ignition switch  provides the "hot" side to the other 3 bulbs.

Bob
Title: Re: 1967 convenience low fuel light remains on
Post by: ruppstang on February 10, 2018, 08:39:05 AM
Thanks Bob,
I probably knew that 12 years ago when I restored my car, just remembered that the red wire was the test power. I do not drive that car very much.
Marty
Title: Re: 1967 convenience low fuel light remains on
Post by: jwc66k on February 10, 2018, 02:48:48 PM
The wiring diagram posted by Bob is slightly different than the one in my 67 Mustang Electrical Assembly Manual (AM0018 pg 107, E7-8855-1) dated 1-5-67, which indicates a different view of the low fuel relay, namely only one resistor, Bob's picture shows two. I think it's the same resistor, and is most likely incorporated into the harness.
Jim
Looking at the assembly drawing (Am0018 pg 55, E7-8618-1, view B), you can see what might be a resistance wire at the relay connector.
Title: Re: 1967 convenience low fuel light remains on
Post by: 196667Bob on February 10, 2018, 02:59:54 PM
Jim : The two resistor symbols on my drawing are on the same wire - it forms a loop. Since it was so close to the main harness, I guess the second resistor symbol was added t0 show that it was a continuation, not a separate wire from the main harness. Your drawing clearly shows the separation, so no need for the second symbol on the same resistor wire.

Bob
Title: Re: 1967 convenience low fuel light remains on
Post by: 196667Bob on February 10, 2018, 03:14:25 PM
Thanks, I will check it out first thing tomorrow.

Should you find that you have the system wired correctly, the only other things that I can think of are 1) that the Relay contacts are stuck shut, or 2) the Relay itself. I know you said earlier that you had an NOS Relay ; is it stamped C6AF-10A968-A ? The reason I ask is because there were 4 different Relays used for different 1967 and 68 applications (see attached), and none were interchangeable. Each had their own use, and none was ever "Replaced By" another. Each part number was unique and used until it was deemed "NR"(Obsolete).

Bob
Title: Re: 1967 convenience low fuel light remains on
Post by: 3stangs2fordtrucks on February 10, 2018, 04:35:43 PM
Hi, you guys were correct.  I had incorrectly attached the wire to the starting solenoid.  I attached it to the correct side and the light went out.  I am not sure it works correctly now because when I took the sending unit out of the bucket of water the light did not come on.  However, I may have a grounding issue since the system is just laying in the car and the indicator lights, relay and sending unit are not attached.

 I am on the way out of town for the weekend to play with grand kids (higher calling) but will check it out Monday.

The nos relay is indeed C6AF-10A968-A.

I really appreciate the interest and excellent advice you guys have provided.  I would never have figured this out.  I am strictly a newbie.  Thanks again.
Title: Re: 1967 convenience low fuel light remains on
Post by: 3stangs2fordtrucks on February 12, 2018, 06:49:29 PM
Hi,  Well, I am still chasing dead ends.  I can't get the low fuel light to go on regardless of what I do.  I removed the starter cable from the starter solenoid so I could turn the key on easier.  Using the 1967 Osborn Electrical Assembly Manual as a guide, I found that wire 365A has power at the Low Fuel Relay and that wire 366 from the starter solenoid does not apparently even when I turn the key to start. I rechecked the ground  (wire 57) to make sure it was good (re-sanded the attachment spot).  It appears to be good.
I am not sure how to test the system since I am not sure what happens when the sensor is out of the gas (water??).  Should the light be on if it is dry and then go out when submerged.  Not sure what the plan of attach should be.  Thanks for your help.
Title: Re: 1967 convenience low fuel light remains on
Post by: midlife on February 12, 2018, 07:15:12 PM
The proof-out circuit should work regardless of the state of the sensor itself; the purpose was to show that the bulb still functioned.  The circuitry is quite complicated to understand.  Mach1 Driver figures out true schematics about how things work, and he did so for the low fuel gauge on this page: http://1969stang.com/forum/index.php?/topic/56764-a-real-schematic/&page=2&tab=comments#comment-198900.  Look for a Sept. 18, 2017 second post by him showing a PDF with all three states of the circuitry.  Your lamp should light up with the ring connector attached to the starter solenoid when the key is in CRANK. 
Title: Re: 1967 convenience low fuel light remains on
Post by: 67gta289 on February 12, 2018, 10:00:18 PM
***Edited based on posts below - sketch removed, refer to the modified one that Richard posted below**

In an attempt to simplify the matter, I put the attached diagram together.

In order to troubleshoot this effectively, you need a volt meter with a basic working knowledge of it.

Here is a sequence of events of operation:

1. Fuel level normal, ignition switch moved to run, but not crank.  Power through the 7.5A fuse through the relay coil (with parallel resistor wire), through the thermistor to ground.   Because the thermistor is covered with fuel, it dissipates the heat developed by the electrical current passing through it to ground 29, and the thermistor resistance is low (relative term).  It is low enough that the voltage drop across the resistor coil is enough to pick the relay up.  This opens the relay contact, and the light is out.

2. While cranking, the power to the 7.5A fuse is off (so that all available power is available for the starting load).  The relay drops out, the contact returns to the normally closed shelf state, and power from the starter relay makes its way to the low fuel light through ground 57.  This is the "proof out" circuit that confirms the bulb is good.

3. When cranking is done, and engine started, the key returns to run.  We are back to step #1 and the light is out.

4. After driving, using enough fuel to expose the thermistor, there is no liquid heat transfer to remove the heat developed by the current passing through it.  The thermistor heats up, and by its nature the resistance increases.  There will be some time for this to occur, but I'm guessing perhaps 10's of seconds, not minutes.  Once the thermistor resistance increases enough, the voltage drop across the relay gets to the point where is not enough to sustain it, and it drops out.  The relay contact swings back to the normally closed shelf state, passing current through power from the starter relay through wire 366 to the low fuel light, to ground through 57.

I do have plans within a few weeks to do a functional test on my car.  I will document the voltage levels (over time) of wire 367, which is key.

What I would recommend for you is this:

1. Measure from wire 366 to 57, and wire 365 to 29.  Pull the connectors and probe the pins.  With the key on, 365 to 29 should be 12V.  With the key in "crank" position, 366 to 57 should be 12V  If neither of these hold true, fix this problem before proceeding.

2. With previous disconnected plugs connected, turn key to crank.  Low fuel light should come on.  If it doesn't, fix this before proceeding.

3. Put key to run.  Disconnect plug in trunk to thermistor.  Temporarily jumper wire 367 to a clean ground.  Low fuel light should come on.  If it doesn't, fix this before proceeding.

4. Up to this point we have checked everything EXCEPT the thermistor.  With the thermistor disconnected, measure resistance of the thermistor, from wire 367 to 29, on the harness that goes to the sensor.  Report in a reply to this post what you measure.

5. Connect everything back up, and figure out a way to measure wire 367.  One option is to stick a small needle through the insulation and attach the voltmeter to needle, with reference to a clean ground.  Measure the voltage when the thermistor is covered with water, and then measure again with the thermistor exposed to air, waiting about 1 minute.  Report these voltages back a reply to this post.

Title: Re: 1967 convenience low fuel light remains on
Post by: 3stangs2fordtrucks on February 12, 2018, 10:49:00 PM
Thanks so much. I will work on it tomorrow and get back to you guys 
Title: Re: 1967 convenience low fuel light remains on
Post by: 67gtasanjose on February 13, 2018, 07:06:38 AM
Awesome write up John. Thank you for the description. Jeff suggested we could (perhaps one day) assemble an article for this system that can cover the highlight points such as this. The Low Fuel light is the most complicated to understand since it functions on resistor values instead of basic switch functions.

I do have a question on your image of the circuit. Something looks off to me on the switch function of the relay. Do you see it?
Title: Re: 1967 convenience low fuel light remains on
Post by: midlife on February 13, 2018, 07:46:35 AM
The contact pivot piece (output)  of the relay is incorrect.
Title: Re: 1967 convenience low fuel light remains on
Post by: 67gtasanjose on February 13, 2018, 08:36:44 AM
The contact pivot piece (output)  of the relay is incorrect.

Correct! The way the image is drawn, the relay wouldn't ever energize the lamp and the only function would be "bulb test".

In actuality, the way the image depicts the circuit, that drawing would just blow the 7.5a fuse whenever the fuel was low, trying to start the engine (run the starter) off of the 7.5a fuse ;)
Title: Re: 1967 convenience low fuel light remains on
Post by: 196667Bob on February 13, 2018, 12:23:42 PM
Correct! The way the image is drawn, the relay wouldn't ever energize the lamp and the only function would be "bulb test".

In actuality, the way the image depicts the circuit, that drawing would just blow the 7.5a fuse whenever the fuel was low, trying to start the engine (run the starter) off of the 7.5a fuse ;)
The contact pivot piece (output)  of the relay is incorrect.

So, is the attached (in my crude red mark-up) the way you two are saying the drawing should be ?

Thanks,

Bob
Title: Re: 1967 convenience low fuel light remains on
Post by: 67gtasanjose on February 13, 2018, 01:16:24 PM
So, is the attached (in my crude red mark-up) the way you two are saying the drawing should be ?

Thanks,

Bob

Not how an electrical engineer might have drawn it, but it lends to the correct switching operation at least. No, I am no engineer but I have read a great deal of electrical drawings (schematics).
Title: Re: 1967 convenience low fuel light remains on
Post by: 196667Bob on February 13, 2018, 02:05:48 PM
Well, John is an Electrical Engineer, and me being (a) Civil, I just followed his lead.

Bob
Title: Re: 1967 convenience low fuel light remains on
Post by: midlife on February 13, 2018, 02:31:59 PM
So, is the attached (in my crude red mark-up) the way you two are saying the drawing should be ?

Thanks,

Bob
No.  Pivot will be at bottom right of the triangle.  When energized, the relay will contact the low fuel line; when not energized, the arm will not touch anything.
Title: Re: 1967 convenience low fuel light remains on
Post by: 67gtasanjose on February 13, 2018, 03:29:10 PM
No.  Pivot will be at bottom right of the triangle.  When energized, the relay will contact the low fuel line; when not energized, the arm will not touch anything.
I think the confusion may be in Fords illustration.
Below, I have attached a typical relay. this particular schematic, is for a fuel pump. The right hand side of the image shows the windings of the electromagnet. The left hand side of the image, shows an on and off switch. For this particular relay, there is no dual function. Notice how the pointer is pointing at a dot. The dotted line arch, represents the movement that would occur when the electromagnet is energized. The straight dotted line represents the connection (for movement) to the electromagnet. This is a typical drawing I commonly seen used in automotive schematics for relays. If the relay were to have a dual function, there would be a pin "87a" drawn in to the left of the pin 87 of the relay. (and would show a connecting line).

Now, to complicate things back in the day, Ford liked to use "special relays" (like what we use on the Low Fuel Relay). Whatever the situation, the original schematics shared DO NOT look to show the correct movement. I have attached what I am seeing in "How this works",  maybe I am wrong, I am OK with that. It would be nice to have a DEFINITE, clear idea of how this relay works internally.

Sorry John, if it looks like I "attacked" the integrity of your schematic. I know you are one smart cookie ;) I now want to get get a relay and bench-test it!


I believe if I were to take a functional test of the low fuel relay that is used on the 67 Mustangs, you would find that the relay holds continuity for the bulb-test except when the electromagnet is energized. When the electromagnet is energized, you would find continuity from the 7.5amp fuse (circuit 365) through the relay to the bulb.

NOTES ON LATEST EDIT: I see how John came up with his illustration but I still feel it may be incorrect, (based upon Ford's drawing, which Ford's drawing does not really make sense to me when I examine it closely.)
Title: Re: 1967 convenience low fuel light remains on
Post by: 3stangs2fordtrucks on February 13, 2018, 04:39:15 PM
I did not know my questions would create such a mess. 

However, I did not get far in completing the check list.  I turned the key on. I tested wires 29 and 365 with the positive probe at the disconnected connector for wire 365 and for wire 29 the ground probe at the backside of the sending unit I get about 11.46 v (close enough?).  However I either do not know how to probe wires 366 to 57 or there is no voltage there.  I put the ground probe on the connector for wire 57  but was not sure where to connect for wire 366.  I put the probe into the the back of the low fuel relay (also between the relay and wiring connector) where wire 366 is attached.  There was no voltage. Also there is no voltage on wire 366 leaving the battery.  I tried testing with the ignition on start and still no voltage.  Not sure if I am doing this incorrectly or am missing something.  Thanks for any help.
Title: Re: 1967 convenience low fuel light remains on
Post by: 67gtasanjose on February 13, 2018, 04:57:16 PM
I did not know my questions would create such a mess. 

However, I did not get far in completing the check list.  I turned the key on. I tested wires 29 and 365 with the positive probe at the disconnected connector for wire 365 and for wire 29 the ground probe at the backside of the sending unit I get about 11.46 v (close enough?).  However I either do not know how to probe wires 366 to 57 or there is no voltage there.  I put the ground probe on the connector for wire 57  but was not sure where to connect for wire 366.  I put the probe into the the back of the low fuel relay (also between the relay and wiring connector) where wire 366 is attached.  There was no voltage. Also there is no voltage on wire 366 leaving the battery.  I tried testing with the ignition on start and still no voltage.  Not sure if I am doing this incorrectly or am missing something.  Thanks for any help.

I just uploaded my understanding of the inputs (see images in last reply). I numbered the sources to assist. #366 will have 12V power ONLY when the starter solenoid is engaged because the underhood wire extension connects to the STARTER SIDE on the starter solenoid (not the battery side). #57 goes through the bulb to ground so it would only show a resitor value from the bulb to ground. Connectors can be broken internally to cause failures too. The red wire from the solenoid should be really easy to ohm test since it runs all by itself.
Title: Re: 1967 convenience low fuel light remains on
Post by: 3stangs2fordtrucks on February 13, 2018, 05:45:39 PM
I tried testing wire 366 at the relay by turning the ignition to crank and holding it.  I did not get any voltage.  Looking at your latest diagram there is an internal bridge which should give power to 366 with just the key turned on but would this not give voltage to 366 back to starter solenoid?  I will test wire 215 with wire 57 and see if there is power.  I checked the fuse and bulb and they are not blown.
Title: Re: 1967 convenience low fuel light remains on
Post by: 196667Bob on February 13, 2018, 06:02:41 PM
Richard : In your second drawing, the way I am taking it, it is shown with the key in the "crank" position only. In the "ignition switch on" position, I would think that the contact would disconnect from 366, complete the circuit to the bulb. That way, when turning the switch to the "on" position, and being low on fuel, the bulb would light. I think I'll "mock" mine up a little sooner than I was going to, just to see what I can find (and hopefully not blow my Relay).
Title: Re: 1967 convenience low fuel light remains on
Post by: 67gtasanjose on February 13, 2018, 06:34:49 PM
I tried testing wire 366 at the relay by turning the ignition to crank and holding it.  I did not get any voltage. .

There is a connection in 366 where it comes through the cowl. It is a single bullet-style male connector. Try it at that location. CHeck continuity with an ohm meter perhaps.

... Looking at your latest diagram there is an internal bridge which should give power to 366 with just the key turned on but would this not give voltage to 366 back to starter solenoid?

NO. The relay isolates the two circuits. It can only make contact to one or the other points inside the relay.

  I will test wire 215 with wire 57 and see if there is power.  I checked the fuse and bulb and they are not blown.

215 is the OUTPUT voltage TO the low fuel lamp. It will only be hot (12V) in either crank position (relay installed) and/or if you ground 367 (relay installed) it will energize the relay and source 12V from the ignition source (7.5a fuse circuit 365A, across the bridge inside the relay).
Title: Re: 1967 convenience low fuel light remains on
Post by: 67gtasanjose on February 13, 2018, 06:42:44 PM
Richard : In your second drawing, the way I am taking it, it is shown with the key in the "crank" position only. In the "ignition switch on" position, I would think that the contact would disconnect from 366, complete the circuit to the bulb. That way, when turning the switch to the "on" position, and being low on fuel, the bulb would light. I think I'll "mock" mine up a little sooner than I was going to, just to see what I can find (and hopefully not blow my Relay).

1.) When relay NOT energized, circuits are ready for bulb test only. (bulb lights when starter solenoid energized)
2.) When relay is energized (by grounding 367 -OR- if the Thermistor with resistor wire reaches the right combined resistance) relay coil pulls the switch closed, connecting 215 to 365A through the fuse and ignition switch (key on).

The dotted line across the middle of my relay I drew, is a "linkage" to the "arrow" or contact arm. It looks simple to me. I bench-tested mine a while ago and all worked just fine as I have pictured so I feel confident. I HAVE NOT TRIED TO DO A BENCH TEST though the thermistor & resistor wire yet though. I figured to wait till I begin assembling the car for that test.
Title: Re: 1967 convenience low fuel light remains on
Post by: krelboyne on February 13, 2018, 07:25:09 PM
I picked up a testing device for checking some of the Ford fuel stuff. I guess it is Rotunda? One of my Estate Sale finds.
Title: Re: 1967 convenience low fuel light remains on
Post by: 3stangs2fordtrucks on February 13, 2018, 08:14:44 PM
Thanks guys,  I think I have a problem not related to the low fuel problem.  I tested for voltage going to the starter solenoid and there is no power to either the switched or ignition side.  Strange because earlier when I turned the ignition to crank the starter tried to start.  I disconnected the starter wire so this did not happen (the motor has been rebuilt but not started yet).  I think the problem may be related to the tilt/tiltaway system.  It tilts/away correctly when vacuum is applied and the door opened; however, there is a switch to prevent the car from being started while wheel is tilted.  I removed the fuse and the wire to the solenoid switch and that had no effect on getting power to starter solenoid.  So not sure this is a factor but I have checked fuses in fuse block, tilt wiring and convenience wiring and all are good.  It has a 4 speed so I moved shifter around in case problem with safety switch.  Strange thing was that the power was there when I started this process but now no power to solenoid. 

again I really appreciate you guys putting all this effort in this problem.  Hopefully, when I find out this problem low fuel problem will, with advice you guys have provided, be solved.  Another day, another fight. 
Title: Re: 1967 convenience low fuel light remains on
Post by: midlife on February 13, 2018, 09:05:52 PM
I think you are on the right track, suspecting the tilt-away system interfering with the NSS wiring.  Solve that problem, and you'll solve your no-power to the low fuel lamp.
Title: Re: 1967 convenience low fuel light remains on
Post by: 67gta289 on February 13, 2018, 09:17:02 PM
Wow a lot can happen after being at work 11 hours, then coming home to install a garbage disposal and stove now that the new kitchen countertops are installed.

Now that I have pulled the black feathers out of my mouth (eating crow) I did mess up the relay contact.  The note I had regarding connecting 366 to 365 not making sense is something I was going to go back and check, but you all did that for me.

Richard in particular gets the big gold star on his forehead.

As for the current status, the OP is correct in that the tilt wheel, when not locked and loaded, will prevent the starter relay from picking up (startus interruptus) and thus power being applied to 366.  If you look at that schematic (I'm not about to touch that now after what happened the last time :) ) the power used to energize the tilt solenoid to engage vacuum to trip the mechanism does not need to be in place.  All that you need is to have the tilt wheel locked in place.  Either that or gag the position plunger switch (looks suspiciously like a door switch used to turn the dome light on).  If the switch is not installed yet, find the two wires and jumper them together.  As Midlife inferred the tilt position switch interrupts the neutral safety switch circuit. 

Bob, as far as blowing out the relay, the only ways to do that are (1) hit the coil with a high potential, probably more than 24V for more than 15 minutes (the old design standard), or (2) use the contact to interrupt a large load, like a blower motor or starter motor.  Both of those are not likely.

Back to the tilt, when I was much younger, and (gasp) would drive my car, park it, and walk away, I would often reach down and pull one of the two red (if I recall the color correctly) wires of the tilt switch.  No cranking after that.  A poor man's theft deterrent system.
Title: Re: 1967 convenience low fuel light remains on
Post by: preaction on February 13, 2018, 10:30:26 PM
I picked up a testing device for checking some of the Ford fuel stuff. I guess it is Rotunda? One of my Estate Sale finds.
Scott, that was a great find and a real time saver.
Title: Re: 1967 convenience low fuel light remains on
Post by: midlife on February 13, 2018, 10:33:10 PM
As an aside, I've refurbished about 10 harnesses with the low fuel relay wiring.  Of them, only 2 had good resistor wires; the rest were broken.  This wire is covered in an asbestos-type material and is extremely fine.  How it is crimped to the standard spade connector is beyond my ken.  I suspect it cannot be soldered (except silver-solder? which I do not have the skills or tools to do so).  I suspect when that happens that the low fuel relay will not work correctly (either lamp is always on or lamp never comes on)...
Title: Re: 1967 convenience low fuel light remains on
Post by: 3stangs2fordtrucks on February 15, 2018, 05:40:00 PM
Sorry to take so long to get back but I am slow.  I jumpered the two wires from the starter safety switch located on the steering column but this did not solve the problem of no power to the starter.  I think it was not in the tilt system but the adjustment of the neutral safety switch.  I messed (my way of proceeding) with the shifter and switch and was able to get power to the starter solenoid so the starter bumped with I turned key to crank. I then disconnected starter wire to keep from bumping starter.

I was able to complete the list recommended by 67gta289.  I am extremely grateful for your help.  From your list:  (1) With connectors disconnected, I measured from wire 365 to 29 with key on and between wire 366 to 57 with key in crank position.  Both measured around 12v (11.20-12-20).  (2) I plugged in connectors and turned key to crank and light came on.  (3) Disconnected plug in trunk to thermistor (this sending unit is a new one recently purchased from WCCC) and jumped wire 367 to a ground and low fuel light came on.  (4) I disconnected the sending unit from the harness and measured the resistance from the thermistor.  I measured wire 367 (at the end of the harness and also on the plug of the sending unit) and wire 29 (back side ground on sending unit) and the resistence was around 1200 ohm.  I have about 10 additional low fuel option sending units and with the exception of one they all measured 0 ohm.  The one other one measured 90 ohm.  (5) with everything connected I wrapped a small wire around the thermistor plug on the back of the sending unit and tested with a ground on the chassis with the sending unit in a bucket of water located just so thermistor was submerged but electrical plug (including underneath of sending unit plate) was dry.  I also tested with thermistor out of the water.  I did this several times.  All readings were around 11.20 volts either in or out of water.  The low fuel sending unit light did not come on.  Maybe I am not measuring correctly but I took care to make sure the wire wrapped around the plug from the thermistor did not touch the sending unit or any other wire.  Please let me know if this measurement method seems ok.  If not I will try the needle method you suggested.  Thanks again for your help. 
Title: Re: 1967 convenience low fuel light remains on
Post by: 67gtasanjose on February 15, 2018, 06:06:40 PM
Good that all works as you described. There has to be an issue with the thermistor. Perhaps the specific gravity of water may be too heavy to operate the thermistor. It may be a huge fire risk to try gasoline on a bench-test so I won't suggest that. Perhaps if the sender was mounted inside a fuel tank and you could just plug everything in and add a jumper wire, to ground the fuel tank itself (to ensure a good ground). Put about 3-4 gallons of gas in the tank and see what you get. It is nice there is a drain plug to empty the gas back out again too ;)
Title: Re: 1967 convenience low fuel light remains on
Post by: 67gta289 on February 15, 2018, 07:09:29 PM
I don't think specific gravity matters.

Consider the drawing.  When the ignition switch is in the run position, if nothing is broken, there is always a current flow through the fuse, then the relay coil in parallel with the resistor wire, then through the thermistor, and then to ground.

When the thermistor is submerged in fluid, whether water or gasoline, enough heat is transferred away from the thermistor to keep the thermistor resistance low enough, resulting in the relay being energized.  There is enough voltage drop across the relay coil to pull it in and turn out the indicator light.

This is why the resistor wire is important, it is sized so that the "voltage divider" circuit works properly.

When the thermistor is not submerged in fluid, there is less heat transfer, so the thermistor heats up.  It is this repeatable property that is fundamental to success in this design.  When the thermistor heats up, the resistance increases.  As more voltage drop shifts from the relay coil to the thermistor, eventually the relay does not have enough potential and it drops out, and the indicator light comes on.

There is a time component to this, and as stated previously I would guess it is in the 10's of seconds, not minutes.  This allows for variability in relay coil, resistor wire, and thermistor manufacturing.  It also prevents the light from flipping on and off when the level is close to the low level and you hit the brakes causing sloshing.

For the OP, I would do these things next:

1. Figure out how to ground the wire 29 with the thermistor plugged in.  I understand that you would want to do this in a non-destructive manner.   Based on what you wrote, if you ground 29 the light will go on, and if you lift the ground (open circuit) the light will go out.  Is that the case?  If so everything is in place, but now we need to determine if the problem is in the thermistor or with a bad quality ground.

2. With the same setup, a temporary known good ground on 29, get the circuit hot again, with the thermistor in the air.  No need for a bucket until we can get the light on.  If we do, submerging in water should turn it off after an unknown (yet) time delay.  I would really be tempted to use thermography to measure the surface temperature of the thermistor.  If you have a quality meter, you could try to get in series and measure amperage.    If this works (I would probably wait at least 5 minutes just because I don't know how long it takes to heat up), then submerge and wait again to see it clear.   It this all works, then your wire 29 is bad and you need to chase that.  If it does not work, the thermistor is bad.

I'll try to get out and check mine this weekend.


Title: Re: 1967 convenience low fuel light remains on
Post by: 3stangs2fordtrucks on February 15, 2018, 08:13:55 PM
I think wire 29 is where I am confused.  It is a virtual wire.  It does not exist as a wire but is the ground on the sending unit face.  There are three prongs coming off the sending unit but only two wires in the harness.  One is for the low fuel guage in the instrument cluster and the other is the low fuel light in the convenience package.  The third prong on the sending unit is a dead end.  There is no wire in the pigtail coming from the sending unit for that prong.  Instead, I think the thermistor grounds to the sending unit itself which is grounded to the gas tank. 

In my tests I never got a light to go on from testing wire 367 to wire 29.  Per item 3 in my check list I jumpered wire 367 to a clean ground (on car frame) and the low fuel light came on.  If I connect wire 367 (one coming from low fuel thermistor) and the ground on the back of the sending unit (virtual wire 29) I do not get a light (maybe because sending unit is not grounded since it is not in the fuel tank but laying in the trunk or bad thermistor).  I will try to ground sending unit and repeat measurement to see if get light to come on. 

I tried to test wire 367 by tying a wire to the plug where it meets the sending unit while ensuring it did not touch face of sending unit.  I then ground  this to frame, ground on back of sending unit, etc; all without success.  I tried sticking needle inside pigtail harness and where plugs from harness meets pigtail.  All readings were around 12 v whether in the water or dry. 

I have two multimeters.  I have a Fluke 23 and Fluke 25.  Both exceed my capabilities but am willing to try to measure amperage.  I have seen probes that fit on multimeters that measures heat but dont
't believe I have one.  Always up for new toys.  U-tube maybe will be helpful to get started.  I think I have a grounding problem but not sure why light does not come on when thermistor is dry. 

 
Title: Re: 1967 convenience low fuel light remains on
Post by: 3stangs2fordtrucks on February 16, 2018, 03:58:37 PM
Hi,  I finally got the light to come on by attaching a small wire around the ground post on the sending unit and grounding it to the frame.  When I submerged the thermistor in water the light remained on.  Therefore, I guess the end of this road is that the thermistor is faulty.  I am not sure what the resistance reading indicated? 

I contacted WCCC to see about getting a replacement and they said no problem but they are out of stock.  I am on their notify list and hopefully when new ones come in I will be in business. 

Again, I want to thank the many people who participated in this thread.  I can't believe there are a group of you guys who are willing to take your time to help people like me.  Very impressive. Hopefully, this thread will be helpful for many others who have similar problems for years to come. 

When I get the replacement from WCCC and install it, I will send a final post. 
Title: Re: 1967 convenience low fuel light remains on
Post by: 67gta289 on February 16, 2018, 05:14:22 PM
Good news.  If I'm not at work tomorrow I will measure the resistance of the thermistors that I have - hopefully at least one is good.

Last week I installed an electric heating mat under a ceramic tile floor, and the sensor is...a thermistor.
Title: Re: 1967 convenience low fuel light remains on
Post by: 67gta289 on February 20, 2018, 10:05:21 PM
Took longer than expected.  I have two complete systems, one mostly installed in my car, the other in a box.  Turns out that I had two out of two bad relays, but I was able to repair both :)  I also have two out of two bad thermistors :(.

Curiosity got the best of me, so I did quite a bit of testing.  It turns out that both relays pick up at around 7.35V, and drop out at around 6.95V.  The relay coils are 42.6 ohms, and the resistor wires are around 47 ohms, which has a Thevenin equivalent of 22.3 ohms. 

At the pick up voltage drop of 7.35V, and 22.3 ohms, the current requirement is 0.33 amps.

At a nominal 12V, and 0.33 amps, the thermistor voltage drop is 4.65, which would make it around 14 ohms when hot.  Of course it could be less than 14 ohms, and by design, given tolerances, I would probably go for the 10 ohm range.

The one from WCCC was backordered, so I'll look around at other options, then test and report back.

Anyone care to double check my calcs?
 
Title: Re: 1967 convenience low fuel light remains on
Post by: jwc66k on February 21, 2018, 12:11:51 AM
The one from WCCC was backordered, so I'll look around at other options, then test and report back.
I got a thermistor from NPD, installed it on a 67 Fastback and it worked fine. That was about 12 years ago so I can't say what the quality of the current offering is.
Anyone care to double check my calcs?
Sober?
Jim
Title: Re: 1967 convenience low fuel light remains on
Post by: 67gta289 on February 25, 2018, 10:16:26 AM
I ended up doing the same, buying one from NPD.  I measured the resistance, or should say attempted to, while on the bench.  In one direction I saw 1 kohm, and in the other it was open.  The thermistor is not a simple resistor, so that should not be shocking.  Sorry for the electrical pun, I couldn't resist.

Then I installed it in the trunk.  I had the thermistor loose, not attached to the sending unit, so I used two jumper wires to complete the circuit.

At 13.2V (Ign), with the thermistor in the air (shop temp ~70F) it took just under 3 minutes for the thermistor to heat up enough to turn the low fuel light on.   

When I submerged the thermistor in a small cup of water, the light went out almost immediately.  After extracting the thermistor, it took another 3 minutes or so for the light to come on.

I'm including this information so that (1) if you don't measure resistance on your thermistor, you don't necessarily have one that is broken, and (2) you know the timing involved so that if you do your own test, you don't give up too soon and start spending more money!

John
Title: Re: 1967 convenience low fuel light remains on
Post by: 67gta289 on April 01, 2018, 12:02:45 PM
Here is a document that I put together to help understand how the system works, which should help in troubleshooting as well.
Title: Re: 1967 convenience low fuel light remains on
Post by: ruppstang on April 01, 2018, 12:12:14 PM
Nice work John!
Jeff I think this should go in the library.
Title: Re: 1967 convenience low fuel light remains on
Post by: 67gtasanjose on April 01, 2018, 02:01:55 PM
Nice work John!
Jeff I think this should go in the library.

+1 on both counts. I know it was suggested a few years ago to put together an article on the whole system. (This was by Jeff within the other lengthy thread on the whole system). Certainly, this Low Fuel circuit is crutial to the content needed for an article, this circuit being the most difficult to understand of the 4-lamp warning system.

Thank you for a beautiful job on this John.
Title: Re: 1967 convenience low fuel light remains on
Post by: jwc66k on April 01, 2018, 02:02:16 PM
Well done.
Nice work John!
Jeff I think this should go in the library.
Agree.
Jim