1975 D100 slant 6

New pickup in the tank that I tested before installing and saw the gas gauge go to full when raised completely. Once installed it barely reads about empty with 10gallons I it. Should be a 22g tank, I believe.

After driving around a bit I crawled under it and measure 62 ohms at the top of the pickup.

​​​​​​That should read about 1/3 tank.


​​​​​​Next I pulled the gauge cluster from the dash. I measured the resistance between the B terminal on the wiring harness (that goes to the gauge) and the wire connecting to the fuel pickup. 0 ohms. All good.

Measured the resistance between the D terminal and chassis - 0 ohms.

So the gauge is getting 62 ohms from the wiring.


Next, I measured the resistance on the printed circuit board across the fuel gauge. This measures about 15 ohms.

Not sure if this should be 0 here or not.

Any hope for me to get this gauge working correctly? The wiring seems not at fault.

 

Double check you ground. Reading full or even off the scale is usually a dead short in power to the fuel sending unit. Reading low is usually a ground issue. No ground would cause it to read empty. A poor ground will usually read low.

 

Any other gauges wonky?

 

The ammeter seems steady but occasionally jumps to the far right and the temp gauge works but it does seem to read lower than I would expect.

I'm suspicious of the voltage regulator. There's some resin on the firewall under the VR like it overheated at some point.

I'm going to test that today.

 

That's what i was thinking. Took some digging to find this but this place makes a solid state regulator. I knew i had the link someplace. They also make replacement tech boards.
https://www.rt-eng.com/index.php/RTE_limiter.html

 

Well the circuit board regulator wasn't it - nothing changed.

I finally took it and completely filled it up and to my surprise the needle moved to F.

It took 15g to fill it up, so it should've had 7 in the tank. It should be reading about 1/3 full but was at E. I guess the gauge just doesn't scale like I expect it to and I don't see any way to fix that. If it were the opposite I could add a resistor inline, but there's no way for me to take resistance out.

I'm considering dropping the tank and seeing if the float arm can be bent down. I really want E to mean E.

 

It might have been stuck and the upward pressure of a full tank might have broke it loose. Older style fuel floats can often be iffy.

 

None of 'em are all that accurate. On mine, the low fuel light comes on at 1/8th of tank. Fill it up, and it only takes 26 gallons..... leaving 8 still in the tank.... Granted, it gets terrible gas mileage, so, a bit of a 'buffer' isn't really a bad thing.....