Operations Check - Low Fuel light

[Alan H. Tast]

Apparently there have been discussions on various show fields regarding the Low Fuel warning light that's part of the Safety Convenience Control Panel. In order to clarify how the system works, Jim Mills and I spent last night pouring through a stack of Owners Manuals, Wiring Diagrams, Electrical Assembly Manuals and Shop Manuals.

Owners Manuals: We looked through owners manuals for '64-'66. The '64 version 1st Printing 1963, page 26 states: "The optional low fuel level warning light is located on the instrument panel under the clock. The light will glow when there is less than approxmiately 3 gallons of gasoline in the fuel tank." It does not provide operational check information like the '65 manual - I suspect that the language was added either in a subsequent printing of the '64 manual (I don't have any 2nd or additional printings of the '64 Owners Manual, but I'm cc'ing "Fuz" Johnson for his input).

I don't have a copy of the first printing of the '65 Owners Manual, but we did find reference to operation of the low-fuel light in the '65 - 2nd Printing January 1965, page 31. It reads as follows:

"The low fuel level warning light is located on the instrument panel under the clock. The light will glow when there is less than approxmiately 3 gallons of gasoline in the fuel tank. The bulb can be checked by turning the ignition switch to START position."
(We would appreciate it if someone could check a copy of the first printing of the '65 Owners Manual to see if this same language appears.)


The first and second printings of the '66 Owners Manual give no information on usage/operation of the low fuel warning light, they just show a drawing of it. There is a third printing of the '66 manual but I apparently have it buried somewhere as a result of last year's move. I doubt it changed in respect to the Low Fuel Light's description of operation.

Shop Manuals: '64 pg 15-12 gives a description of the Low Fuel light system but no information on the check operation. '65 pg 15-12 reads the same as '64, and in turn the same as '66. '66 pg 15-13 gives a description of the operation of the Low Fuel light. It says toward the end of the first paragraph:

"...The warning lamp will light up just before the fuel gauge pointer indicates empty and/or when there are approximately 3-1/2 gallons of fuel in the tank."

The Shop Manuals don't give information on the check operation of the light when the car is started. It could be inferred that when the fuel gauge pointer is on E, i.e. where it would be before the On/Start positions are engaged, the light would momentarily come on when the switch is engaged and the pointer moves off of E. But, from our understanding of the wiring diagrams as noted below, operation of the light is dependent upon activating the Starter and in turn feeding power to the relay for the light.

Electrical Assembly Manuals: Jim and I went through copies of the Electrical Assembly Manuals for all 3 years and in short (no pun intended) found that the wiring circuit is the same for all three years.

1. When the ignition switch is turned to "Start" a wire from the ignition switch goes through the neutral safety switch and terminates at the "S" terminal of the starter relay. When power is applied to this wire, it energizes the starter solenoid and pulls the contact closed to operate the starter.

2. From the "S" terminal of the solenoid a wire runs to the Low Fuel Warning sensing relay (which is in the passenger side kick panel - above the fuse panel for '64 and '65, or in the area of the the end of the dash/fuse panel for '66). When power is applied to this wire, it goes through the normally-closed contact of the Low Fuel warning relay and to a wire going to the Low Fuel warning light.

3. Energizing the starter relay in turn sends power to the Low Fuel warning light through the relay and lights it up. When the solenoid disengages, power to the bulb is lost and the light extinguishes.

Therefore, when the starter is turned on, the Low Fuel light should also come on until the starter stops getting power.

For the Low Fuel warning light to work when the car is running,
1. When gas level drops below the thermistor on the fuel sender in the tank, the thermistor is exposed to atmosphere.

2. When the thermistor is exposed to air, it warms up, allowing resistance in wiring to drop, and in turn allowing enough voltage to reach the coil of the relay.

3. When the voltage increases to a sufficient level, it energizes the relay coil, causing it to go to ground at the fuel sender. The 12v source for the relay coil comes through the fuse panel to the normally-open contact and the positive relay coil connection. This allows the normally-open contact to close and energize the Low Fuel warning lamp.

According to the Electrical Assembly Manuals, all three years are the same in schematic except for wire number ID from year to year.

Now to complicate matters: In studying the '64 Wiring Diagrams and Electrical Assembly Manual, Jim thinks there is either conflict or a change/revision between the large-size schematic page 5.2 of the '64 revised 4-64 and what's in the reproduction of the '64 Electrical Assembly Manual on page E4-4821-2 for the Low Fuel Warning relay. We think the Electrical Manual from 4-64 shows a revision to the operation circuit to energize the relay from the thermistor with a spliced wire at the firewall bulkhead connector. It appears from the schematic that the light would be on all the time. Connections on the starter solenoid and/or the relay plug appear to be incorrect. You may want to compare the diagrams/schematics you have.

There is also a conflict between the Electrical Assembly Manual and Wiring Diagrams for '65. The '65 Low Fuel schematic is on page 6.2 (no revision date) of the '65 Wiring Diagrams, 1965 Thunderbird Accessories. It differs from the '64 Wiring Diagram from the relay to the Low Fuel light. The diagram shows the light bulb grounding through the coil and not the thermistor. We can see how the thermistor is engergized in the schematics for the Electrical Assembly Manual but not the Wiring Diagram. The wiring diagram shows a wire from the thermistor but shows the wire ending. We think the Wiring Diagram shows the plug at the relay wired backwards, similar to '64's diagram.

'66 Low Fuel schematic is on page 5-22 Thunderbird Safety Convenience Package of the '66 Wiring Diagrams (no revision date). It shows the third wire from the fuel sender for the thermistor and a resistance wire at low fuel waring relay (wire #488) paralleling the wire from the relay to the thermistor. It shows the schematic correctly for operation. We think this is some kind of "bleed" wire, but need to chat with an electrical engineer to figure out what's going on here. (We want to try to corner Greg Docktor at Minneapolis on this.)

Our Conclusion: for Operations checks, the Low Fuel light should come on as long as the starter switch is engaged, or when gas level drops below the level of the thermistor, i.e. 3.5 gallons.

Does anyone care to offer a different conclusion?

[redstangbob]

If your eyes are like mine, they should be sore by now after looking at those diagrams LOL. I wish i could help with the 65 owners manual but both my original and re-pop are January 65. This subject is interesting to me after we had a discussion a while back about a "mystery relay" that was integrated into the low fuel warning. My car and garage are in a shape where I can't easily trace that circuit. I agree on how the warning should appear to the driver while starting and driving. I would like to touch on a point just so there's no confusion (if thats possible LOL).

2. When the thermistor is exposed to air, it warms up, allowing resistance in wiring to drop, and in turn allowing enough voltage to reach the coil of the relay.

3. When the voltage increases to a sufficient level, it energizes the relay coil, causing it to go to ground at the fuel sender. The 12v source for the relay coil comes through the fuse panel to the normally-open contact and the positive relay coil connection. This allows the normally-open contact to close and energize the Low Fuel warning lamp.


I believe the 2 circuits involved in the fuel sender are both variable grounds. One of course is for the fuel level, the lower the resistance to electric flow the more the gauge deflects to full. The other is the thermistor for the low fuel warning. I see it as power is supplied to the relay through wire 365 which goes through the winding of the relay and then exits through wire 367. This wire is connected to ground through the thermistor. When the thermistor is no longer covered with fuel it warms up, and as a thermistor, it's resistance changes. This will allow more currant to flow to ground and that's what will close the relay, turning on the light. Of course the winding is such that heavy current is not needed for safety's sake. To check the operation of the circuit, have the key on or in acc and ground the green/white wire. the relay should click and the light comes on.
So to clear up my point, no power goes FROM the sender, it is a way to control the grounding of the circuits. Power for the relay is on with the key and is looking for a ground. I hope I haven't made this more confusing, or mis-understood what you were meaning. Thanks for your efforts, and I will get to the bottom of the "mystery relay" some day. Bob C

[tbirdjim]

I know sometimes people do things and wonder if anybody can use the info.

Well, I want to thank you guys for putting in the effort to do this here.

One of the items on my 'bucket list' is to sort out the wiring back at the sender and up to the gauge and low level light.

Right now it is cobbled together with butt splices due to chafed and worn wires back by the tank. I have fuel quantity but it is not 100% accurate and no low level light. (I removed relay as the light was on all the time)

Thank you for your efforts. The ops check will come in handy after I effect repairs.

[john6t6]

I agree with tbirdjim, the info is valuable & requires time & effort. Thanks for posting this information. By the way my low fuel light works just like you gents described it, however with a non accurate fuel gauge (annoying) I have not let it get too low to see if the light comes on but I will try it eventually.

[Rplacido]

Does anyone have a part number for the low fuel relay?

Also which wire at the sending unit is low fuel (green or yellow)?