Okay, so I recently bought a '78 F-150. Love it!!!

Fuel gauge was working when I bought it, however the gauge lights were not. Found out that the printed circuit "board" on the back had the light sockets so worn there was no connection for them to light up.

Not having one handy, I put one from my '75 F-100 parts truck and that got the lights working, but the fuel gauge quit on me. (though it was cute when I'd turn on my high beams and the "brake" light would come on)

Fast forward a month --- bought a new printed circuit from LMC for the '78 and still the fuel gauge is not working.

Have I destroyed it? What else should I check?

Thanks so much for your wisdom and/or input.

 

Disconnect the sending unit lead and ground the lead coming from the truck to the same point the sending unit grounds on the crossmember. With the key in ACCY, the gauge should peg to FULL. If it does, then the sending unit is bad and everything else is good.

If it does not, try grounding the same lead against unpainted bed sheetmetal (good ground). If that makes it peg, then move the sending unit ground to this good ground, and see if the gauge works now. If it still does not, then the sending unit is bad.

If you can never get the gauge to peg to FULL no matter how good of a ground you can find, then the problem is upstream (no power to instrument cluster, bad ICVR, open ICVR input resistor, bad gauge). However, this is very rare. The problem is almost always the sending unit or its ground.

 

1975 use different printed circuit boards than 1976.

1976/78 F100/350, Econoline & 1978 Bronco: There are TWO different printed circuit boards.

D6TZ-10K843-B .. Printed Circuit-Use with Oil/Amp GAUGES / Marked: D6TF-10C956-BA / Also same as 1979 F100/350, Bronco & 1979/91 Econoline.

D6TZ-10K843-C .. Printed Circuit-Use with Oil/Amp WARNING LAMPS / Marked: D6TF-10C956-CA
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F4UZ-13B765-A (replaced C8OZ-13711-A & B) .. Instrument Cluster Bulb Socket / Available from Ford

1970/89 F100/350 / 1970/2001 Econoline / 1978/89 Bronco / Misc 1968/79 FoMoCo Passenger Cars.

MSRP: $1.85 / FTE sponsor tousleyfordparts.com price: $1.22.
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TURN TO DUST before your very eyes! The POS composition cluster backs used on 1975/79 F100/350, 1975/80 Econolines & 1978/79 Bronco.

1981: FoMoCo came out with hard plastic cluster backs for Econolines and were the 'service part replacements' for the POS composition cluster backs.

Some of POS were replaced over the past 22 years, but some were not replaced.

E1PZ-10848-A .. Hard Plastic Cluster Back-Use with Oil/Amp GAUGES / Obsolete

1981/91 Econonline / Retrofit: 1975/79 F100/350, 1975/80 Econoline, 1978/79 Bronco.

CARPENTER NOS OBSOLETE PARTS in Concord NC has 450 = 800-476-9653.
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E1PZ-10848-B .. Hard Plastic Cluster Back-Use with Oil/Amp WARNING LAMPS / Obsolete

1981/86 Econoline / Retrofit: 1975/79 F100/350, 1975/80 Econoline / 1978/79 Bronco.

CARPENTER has 68.

 

You could also have a bad connection to the voltage regulator on the circuit board or it's bad. I tore the plastic just a tad where it snaps to it and my fuel gauge quit working. I soldered it and it was good to go. I had replaced the backing plate and circuit board and installed it in the truck. When I turned on a signal light that hold side of the panel lit up. As my luck would have it, I was sent a chinese piece of crap with very thin plastic that made it almost transparent. After I received my new one and removed the snap it tore. I had used dielectric grease but it was still stuck on there tight.

 

1979's with Oil/Amp WARNING LAMPS use a different printed circuit board than 1976/78's. It's the same as 1980/86 Econolines.