Topic: 1967-68 Safety Convenience System - Application, Components, Wiring, Lenses and Bulbs

[mikeljgt500kr]

Thanks very much for this detail, it helps a lot.

[jwc66k]

I used the 67 wiring diagram vs the 68 wiring diagram which are (slightly) different. The 68 appears to be what you've sketched.
Jim

[67gta289]

Jim, I would not mind if you could post the two diagrams you are referring to.  The diagram I made was actually from the harness and accessories from a Dec 66 car, not a 68 MY diagram.  John

[jwc66k]

They're in the 67 and 68 Mustang Electrical Assembly Manuals, pg 107 for 67, pg 98 for 68. The harnesses appear to be the same, the internal connections of the relay are different.
Jim

[midlife]

I thought about a power resistor as well, but decided it would be a heat issue with surrounding wire insulation if any got close enough to contact it.  I also thought about using a looooong run of pink resistor wire or another long run of alternator lamp resistor wire but the length would be on the order of 27 feet! 

This is one resistor wire that I simply have not found a good substitute for the original application.

Interesting that you show another resistor inside the relay (am I interpreting that correctly?)  Along these lines, does anyone know what the purpose of this resistor wire is?  I suspect it is for the initial power application to light up the bulb to ensure the owner that the system is operational.  Without knowing the details of the actual relay (I've never had one in my possession), I'm a bit dumbfounded.

[ruppstang]

Yes it should light briefly to proof the light.
Marty

[jwc66k]

Interesting that you show another resistor inside the relay (am I interpreting that correctly?)  Along these lines, does anyone know what the purpose of this resistor wire is? 

It's actually the coil of the relay. The accepted schematic symbol looks like an extended spring. It does have resistance plus another measurable commodity called reluctance. A coil "fights" the voltage activating it, and "fights" when that voltage is removed. That's the simple answer.
Jim

[midlife]

Great!  I thought that was the case, so I'm now more comfortable than before telling customers that without that resistor wire, the system will still work but won't light the bulb when power is first applied, but the low fuel light will work.

[67gta289]

As Jim said it's just representing the coil.  I took the lazy way out since I was using Excel for the diagram and not my Cad program due to my location at the time.

The light DOES go on via the connection to the starter relay.  It should be brief - or exactly as long as you are turning over the engine (starter relay energized).

The relay is a simple one with an SPDT contact (single pole double contact).  The normally closed contact routes power from the starter relay, and the normally opened contact is for an actual low fuel condition.

With that background, and since the relay coil has a resistance of about 44 ohms, we should ask why the parallel resistance is needed.  The resistor is NOT part of the lamp test circuit.  The resistor DOES drop the overall resistance in half (as shown on the diagram), and doubles the current flow to the thermistor.

My thought is that the thermistor size is such that it needs that amount of current to heat up enough to "switch" (drop in resistance) to pull in the relay.

I will be bench testing the function with and without the resistor in a couple days.  Have a "real" job that will get in the way this week.

It could be that it works without the added resistor, but would take much more time to do the job.

[67gtasanjose]

I figured I would bench test my low fuel light system too, using the reproduction thermistor I bought.

I know my light only worked for about a year or two after I first got the car and hadn't since about 1981. Without a wiring diagram, I found that if I grounded the wire at the tank plug, the light came on so I always felt my problem was the tank sender unit. It will be nice to see it working again

Education is good! I had, before getting a wiring diagram a few years ago, always thought there was some sort of "grounding pellet" floating in what I now know as  the thermistor, and thought it probably only needed cleaned of varnish. I also wondered how it didn't "spark" and possibly ignite!

Always good to figure things out with edjucation and remove the guess work.

Color codes on the wiring diagram would be nice in your images, John. Other than that...Spot-On!

Richard

[67gta289]

I'll add wire numbers and colors the next go around. Also will add thermistor curves (resistance vs temp at different voltage levels) plus Thevinin equivalent data for the curious.

Tonight I'll add some pics of the thermistor. My recollection is that they are soldered to ensure a good connection.

Grounding the wire at the sender resulting in the light coming on is a great Solomon's Sword troubleshooting technique.  Divide and conquer.

In your case the problem is most likely in the tank (thermistor or circuit/ground) but could also be an open resistor wire.  The wire was added for a reason, nobody including Ford adds stuff for no reason.  The resistor wire is easy to check. A nominal 23 ohms plugged into the relay, 46 separated.

[67gtasanjose]

The resistor wire is easy to check. A nominal 23 ohms plugged into the relay, 46 separated.

My recollection is that they are soldered to ensure a good connection.

Grounding the wire at the sender resulting in the light coming on is a great Solomon's Sword troubleshooting technique.  Divide and conquer.

Yes, soldered onto the fuel sender so the tip of the thernistor points towards the bottom of the tank. (wire comes out the top) I have a 67 type and a 68 type. The only difference is the size of the fuel line tubing (and the engineering part number)
 
[/quote]

I'll add wire numbers and colors the next go around. Also will add thermistor curves (resistance vs temp at different voltage levels) plus Thevinin equivalent data for the curious.

Tonight I'll add some pics of the thermistor. .

Always good to have more information. Thanks John!

Richard

[67gtasanjose]

An interesting thread, here is a picture of mine, it is on a late April '67 built Jersey GT 'vert. 

OFF TOPIC:
Looking at the picture of Mikes Radio and console face...Are the rear radio knobs correct for Mikes April 67 NJ build? Most 67's I had seen had the "tab" type knob on the tone and mock fader (with AM radio) I thought the type in Mike's picture is correct for 68's. (maybe I need educated here)


ON TOPIC:
Are Mike's lenses "original" or the reproduction? (his example has a "SEAT" not a "BELT" lamp)  Are they all installed in the correct locations for his build date & plant

Reference http://www.concoursmustang.com/forum/index.php?action=post;quote=49672;topic=8554.0
Marty's San Jose built (early type) system is the same as mine. The lamps are in different locations than Mike's are. Maybe setup wrong or different per assembly date or plant? Everything I understand about the system says Mike's is incorrect. Always room for more clarification. A later 67 Console I had that had all of the later style lenses also had the lamps in the same location as my car and Marty's but I never had any build info on that console.

Richard

[mikeljgt500kr]

Good questions, I would also be interested in the answers.

[mikeljgt500kr]

Bueller?  Bueller?