Stovebolt.com Forums The Shop Area The Electrical Bay Faulty gas gauge, 53 3/4 ton

The gas tank is full?
The indicator on the dash should want about 30 ohms to show full
And if you are getting 35.x now that means the needle will peg to maximum
If your tank is actually filled to the top this situation maybe livable
But if actual full is 39 ohms then maybe that gonna be a lot off
If empty is still 0 at least you will know when your getting to the bottom of the tank

Anyway you could connect it up and should see the needle go to max
And as the level drops the needle should as well eventually

If that float hits the wall inside the tank that could also be a source of error
So if the needle doesn’t go down as the tank empties disconnect the sender and ohm it out
If you aren’t getting an actual 0 or near 0 ohm value at empty the problem will all be in the tank end

If you have a 30 ohm resistor you could put it on the indicator to ground to see how close it is to original full position
Better would be if you have a potentiometer (variable resistor) about a 0-100 ohm
Then you could adjust it on the indicator for empty and full position and use the ohmmeter to measure at what ohms the needle is where you want it
That would let you know if the indicator is also out somewhat but that’s more rare
-s

So my problem seems to be in the wiring? The instrument should be grounded to the cluster when it’s mounted ?
Otherwise it should have 12 volts wired to the E terminal from the key switch?
It should work!

The gauge indicator in the dash does need a ground but based on your results that seems to be likely okay
The dash gauge responded appropriately

Also based on your meter readings of the sender that also seems to indicate the circuit is at least correct

The open issues are:
Will you be happy with the possible mismatch in scale reading with a sender issuing 0-39 ohms possibly if your memory is correct, and a dash indicator likely expecting 0-30 ohms for full scale

Also the wire from the sender to the dash indicator,
Has it been carefully audited for connectivity?
We know how to anticipate an open circuit in this path, the needle will go to full scale.

All the individual pieces have been described sufficiently now
It’s a matter of poking at it with what we know about how they should work and building a model of what’s actually happening to debug further.

Let us know if you make more discoveries
-s

After retracing every wire & connections. I found nothing different. But now the gauge stays at 1/4 full, key off. Pings to beyond full, key on. Key off, the gauge drops to about 1/4 tank.
I honestly don’t know what to do!
If it just reads for me I’d be satisfied for now. The previous commercial gas gauge worked but did show above empty when the gas tank was empty.

Sounds like the dash indicator is sticky
Because it shouldn’t stop at 1/4 tank off
It should fall to the bottom

I think you are seeing two issues
1) the scale for full is clearly wrong because your sender is not 0-30 it’s too much resistance, 35 ohms will peg that needle if the dash indicator still expects 30 for full

2) the dash indicator is sticking so it does bad things like stoping at 1/4 tank, or sticking to max randomly so the needle has too much resistance to float freely to where the two magnets are trying to set it to

So if it was me
I would pull both parts out
And figure it all out on a work bench
Make that sender match full scale and free up that needle.

Best of luck
-s

Wow! Sometime one just hates to start over!

Short the wire that goes to the gauge at the sending unit to ground. No need to disconnect. If the gauge goes to empty, that tells you that the sending wire and the gauge are OK and the sending unit or the ground at the sending unit is the issue.

Perform the test and let us know the results.

I previously grounded the sender, the gauge went to empty with ignition on.
Let me slow down for more detail. The float unit sits on the tank with an old cork gasket. I used silicon over the gasket to make a good seal.
The sender is grounded with one of the 5 or 6 screws that hold it in (grounded to the chassis) with a wire.
I’m thinking I have 1/4 fuel in the tank.
The OMS previously read 35.6 in the tank. The unit’s plate has a positive continuity to the ground wire which is grounded to the chassis.

1/4 tank with an ohm reading of 35 ohms is absolutely broken

1/4 of 30 ohms is 7.5 ohms not 35

Sending unit is in need of attention
It should be 30 ohm absolutely full tank

We had been debating if 35 ohm on a full tank would be acceptable error
But 35 ohms on 1/4 tank that’s just broken and points to signs that the sender is not accurate or trustworthy at all

A good sender is 30 ohm float at top, and 0 at bottom.
I suggest you pull that sender and fix it, soak it in evaporust overnight and brush it clean with a horse hair brush under clean water air dry it see what you can do with it
-s

I agree with 2ManyTrucks. The sending unit is faulty. Clean it as suggested or replace it.

Silicone is not compatible with gas. I tried it on the same gasket you used it on and it turned it into slime like substance. Get a new gasket and if you must use sealer, use Permatex #2 non-hardening gasket sealer.