My low fuel light stopped working a few years ago. Before you say “ELFI”, I already have a replacement thermistor that I purchased back when the light first stopped working. I’m currently in the fuel system replacing the old lines and I have the sending unit out for fuel tank inspection (looks clean in there still).
I would like to determine if the thermistor is bad, or if it is the relay… or both.
Symptoms:
Low Fuel light comes on when the key is turned to Start (prove out works and lamp is good).
Low Fuel light does NOT come on when the fuel is low.
I read up a bit and found that the thermistor is supposed to read between 10 and 40 ohms resistance between the sending unit prong and the sending unit body (ground) in air. Mine reads open / infinite.
I read that you can momentarily ground the low fuel lead at the gas tank with the key in Run and the low fuel light should come on. Mine did not.
I was trying to ground at the sending unit plug, and not at the connector in the trunk (which reminds me, I should verify that connection too).
I haven’t tried checking the low fuel relay yet, or verifying that it has a good connection.
Questions:
What should the thermistor resistance be?
Am I doing the ground test for the light properly?
When / if I replace the thermistor, do I desolder the whole little “can” from the bracket? Or do I bend the little crimps on the can and slide the guts out / slide the new thermistor into the little can?
Modern gas with methanol combines with water to produce sulfuric acid. The acid eats the thermistor. The good replacement thermistor is no longer available. I have found a replacement that is made in Taiwan and I have 500 on order.
Looking at the picture it appears that the thermistor is just held in by a spring clip. That whole connection looks bad. You should have roughly 900 ohms from the wire to a clean thermistor case at 70F. It goes down as it warms and up when colder. I’d check the resistance from the thermistor case to the sending unit flange (ground) before condemning the thermistor.
I got mine working but it took a new resistor wire, relay and a repro sending unit (plus a Meter Match unit to fix the level calibration on the repro sending unit). The ELFI would have been simpler.
NPD sells a replacement thermistor that they list for 67/68 Mustang/Cougar. I have successfully used these on two 67 Mustangs that have been working for several years, about 2 gallons and light comes on. However on one 68 Mustang and 68 Cougar the light came on one or two times then quit. Taking them apart the thermistors measured open which to me meant burnt. Not having made any measurements I could assume that the 68 system is passing more current through that circuit thus requiring a different thermistor. NPD was gracious in giving me a refund on one unit. Bill is correct, I had to use a 200 watt soldering iron to get them loose.
