Author Topic: 1967 convenience low fuel light remains on  (Read 562 times)

Offline midlife

  • Wiring Guru---let me check your shorts!
  • Global Moderator
  • Hero Member
  • ****
  • Posts: 1417
    • Midlife Harness Restorations
Re: 1967 convenience low fuel light remains on
« Reply #15 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. 

Offline 67gta289

  • Hero Member
  • *****
  • Posts: 1597
Re: 1967 convenience low fuel light remains on
« Reply #16 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.

« Last Edit: February 13, 2018, 09:01:43 PM by 67gta289 »
John
67 289 GTA Dec 20 1966 San Jose
MCA 74660

Offline 3stangs2fordtrucks

  • Newbie
  • *
  • Posts: 11
Re: 1967 convenience low fuel light remains on
« Reply #17 on: February 12, 2018, 10:49:00 PM »
Thanks so much. I will work on it tomorrow and get back to you guys 

Offline 67gtasanjose

  • Hero Member
  • *****
  • Posts: 3149
  • "Take the MUSTANG PLEDGE"
Re: 1967 convenience low fuel light remains on
« Reply #18 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?
« Last Edit: February 13, 2018, 07:20:32 AM by 67gtasanjose »
Richard Urch

1967 (11/2/66, S.J.) GTA Luxury Coupe, 289-4V w/Thermactor Emissions, C-4, Int./Ext. Decor +many options

2005 (04/05) GT Premium Convertible, Windveil Blue, Parchment Top w/Med. Parchment interior,  Roush Body Appointments

Offline midlife

  • Wiring Guru---let me check your shorts!
  • Global Moderator
  • Hero Member
  • ****
  • Posts: 1417
    • Midlife Harness Restorations
Re: 1967 convenience low fuel light remains on
« Reply #19 on: February 13, 2018, 07:46:35 AM »
The contact pivot piece (output)  of the relay is incorrect.

Offline 67gtasanjose

  • Hero Member
  • *****
  • Posts: 3149
  • "Take the MUSTANG PLEDGE"
Re: 1967 convenience low fuel light remains on
« Reply #20 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 ;)
« Last Edit: February 13, 2018, 08:45:23 AM by 67gtasanjose »
Richard Urch

1967 (11/2/66, S.J.) GTA Luxury Coupe, 289-4V w/Thermactor Emissions, C-4, Int./Ext. Decor +many options

2005 (04/05) GT Premium Convertible, Windveil Blue, Parchment Top w/Med. Parchment interior,  Roush Body Appointments

Offline 196667Bob

  • Sr. Member
  • ****
  • Posts: 398
Re: 1967 convenience low fuel light remains on
« Reply #21 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
1966 Coupe, C Code, 3 Sp MT, 6T07C154XXX, Build Date 11/22/65
1967 Conv, C Code, C4, 7F03C154XXX, Actual Build Date 01/31/67

Offline 67gtasanjose

  • Hero Member
  • *****
  • Posts: 3149
  • "Take the MUSTANG PLEDGE"
Re: 1967 convenience low fuel light remains on
« Reply #22 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).
Richard Urch

1967 (11/2/66, S.J.) GTA Luxury Coupe, 289-4V w/Thermactor Emissions, C-4, Int./Ext. Decor +many options

2005 (04/05) GT Premium Convertible, Windveil Blue, Parchment Top w/Med. Parchment interior,  Roush Body Appointments

Offline 196667Bob

  • Sr. Member
  • ****
  • Posts: 398
Re: 1967 convenience low fuel light remains on
« Reply #23 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
1966 Coupe, C Code, 3 Sp MT, 6T07C154XXX, Build Date 11/22/65
1967 Conv, C Code, C4, 7F03C154XXX, Actual Build Date 01/31/67

Offline midlife

  • Wiring Guru---let me check your shorts!
  • Global Moderator
  • Hero Member
  • ****
  • Posts: 1417
    • Midlife Harness Restorations
Re: 1967 convenience low fuel light remains on
« Reply #24 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.

Offline 67gtasanjose

  • Hero Member
  • *****
  • Posts: 3149
  • "Take the MUSTANG PLEDGE"
Re: 1967 convenience low fuel light remains on
« Reply #25 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.)
« Last Edit: February 13, 2018, 05:05:07 PM by 67gtasanjose »
Richard Urch

1967 (11/2/66, S.J.) GTA Luxury Coupe, 289-4V w/Thermactor Emissions, C-4, Int./Ext. Decor +many options

2005 (04/05) GT Premium Convertible, Windveil Blue, Parchment Top w/Med. Parchment interior,  Roush Body Appointments

Offline 3stangs2fordtrucks

  • Newbie
  • *
  • Posts: 11
Re: 1967 convenience low fuel light remains on
« Reply #26 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.

Offline 67gtasanjose

  • Hero Member
  • *****
  • Posts: 3149
  • "Take the MUSTANG PLEDGE"
Re: 1967 convenience low fuel light remains on
« Reply #27 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.
Richard Urch

1967 (11/2/66, S.J.) GTA Luxury Coupe, 289-4V w/Thermactor Emissions, C-4, Int./Ext. Decor +many options

2005 (04/05) GT Premium Convertible, Windveil Blue, Parchment Top w/Med. Parchment interior,  Roush Body Appointments

Offline 3stangs2fordtrucks

  • Newbie
  • *
  • Posts: 11
Re: 1967 convenience low fuel light remains on
« Reply #28 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.

Offline 196667Bob

  • Sr. Member
  • ****
  • Posts: 398
Re: 1967 convenience low fuel light remains on
« Reply #29 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).
1966 Coupe, C Code, 3 Sp MT, 6T07C154XXX, Build Date 11/22/65
1967 Conv, C Code, C4, 7F03C154XXX, Actual Build Date 01/31/67