I cannot get the fuel gauge on my 76 F100 to work. I've dropped the tank to check the float, and all seems fine there. No holes in the float, floats just fine in gas, and moves freely as well. What's the next thing to check here? The gauge or possibly the printed circuit? Possibly the IVR on the back of the instrument cluster? Would like to be able to test and diagnose this instead of just throwing parts at it. Thanks in advance for any help or tips you guys can provide.

 

When you have the fuel sender unit out and was checking the float, you could also work the float by hand and the fuel gauge should go from Min to max.

There's a variable resistor on the sender that controls the gauge as the float moves up/down.

 

I did try that with the sender/float removed from the tank, and the gas gauge needle did not move. So I take it the resistor is yet another possible culprit. Any way to test it other than that?

 

Do your other gauges work?? If yes, the IVR is good.

Ground the wire that goes to the sender. A good clean ground. Turn the key on. If the gauge goes to full, the gauge is good. Then you may have a bad sender or the ground wire coming off the sender is bad.

 

I'm guessing Lee meant to measure the resistance @ the sender, whilst moving the float from one extreme to the other. Yes that checks the sender resistor.

What you did was fine for a quick test, also that can tell you if the float may be gas-logged. But your test does not take into account the connections along the way or the possibility of a gage issue, or the cluster as others have mentioned.

 

I believe resistance between the two wires on the sender should vary from approximately 20-70 ohm if I recall?

 

Kind of a dumb question.. but how do you ground out the gauge. I'm having the same issue with mine. I'm a wood worker and not an electrician... trying to learn lol

 

Just take the wire that is connected to the positive terminal of the sender and touch it momentarily to a good ground. The gauge should sweep to FULL in a few seconds. Only perform this test long enough to prove the gauge is OK.

Don't ask me to make a 45° angle cut... lol

 

With a multimeter you will put one lead on each wire to check it. Grounding the lead: When the tank is still in the truck, all you need to do is slide under the truck at the front of the tank, pop the wire connector loose which is NOT the ground wire (the ground wire is screwed onto the frame there - not hard to tell which is which). Touch the harness-side connector to ground - have someone available to tell you if your gauge goes to Full. If it does, the dash gauge itself is working properly and your electrical path from there to the front of the truck is good. If the dash gauge does not, you have a problem somewhere between the tank and the dash. The electrical path this follows is pretty simple - current from the wiring harness goes through the gauge, to the back of the truck, through the sending unit, to ground. When you ground the wire you have little resistance at the back so your gauge reads full. The higher the resistance in the line, the lower the gauge reads. If your sending unit has broken, you will have an open (not complete) circuit and your gauge will just stay on E all the time. Hope that helps? Obviously, you need to look for where the 12V+ is interrupted between the gauge and the ground.

 

If you have dual tanks, it could be the dash switch - I have to wiggle it a little on 2 of my pickups at times. Not sure how common it is, but it's a 66.67% failure rate for me . The third one reads zero on both tanks all the time, but the other gauges are a little weird, too, heat runs way high and oil pressure runs way low (verified with mechanicals). I'm assuming it needs a regulator first off, and probably other stuff......

 

Nice write-up.

But, does the (IVR) regulator put out +12 or about a 5 volt pulse...?

 

5 v pulse, you can actually see this with an ohmeter if you connect it to the positive.
it will sweep/fluctuate from 0-5

 

ICVR (Instrument Cluster Voltage Regulator) affects the fuel gauge, water temperature gauge and the oil pressure gauge (if present).

1976/78 F100/350: TWO different printed circuit boards: With Oil/Amp GAUGES or with Oil/Amp WARNING LAMPS.

1975/79 F100/350 with dual fuel tanks has a fuel tank selector valve and solenoid assembly mounted on the cross member.

The fuel selector switch located in the heater control panel changes the tanks and the dash fuel gauge from one tank to the other.

 

did you check the fuse?

 

Thanks for all the info fellas. Ended up being the sending unit itself.