Can't Figure Out What's Wrong With My Gas Gauge

[SavageC20]

I have read every gas gauge thread that I could find, but I am still having trouble figuring out what is wrong with my gas gauge. The gauge reads approx. 1/3 tank when FULL. The needle will go down as the level in the tank gets lower...all the way to empty.

I have checked continuity on my sending unit wire and it is fine. I have checked the resistance of the sending unit and various gas levels in the tank. I know its supposed to be a 0-90 ohm scale, but I topped off the tank and it was reading 100 ohms?? Could be my cheap multimeter...not sure

I pulled the fuel gauge out of the cluster and tested the resistance across each stud and compared this to the readings given in this thread:

http://67-72chevytrucks.com/vboard/s...d.php?t=640615

My readings are shown in the picture below (sorry for bad diagram)

The other pictures are from the thread listed above

I also pulled the resistor off the back of the gauge and tried to check resistance, but I was unable to get a reading. Does this mean the resistor is bad?

The previous owner installed two saddle tanks on the truck and I don't know if they are wired into the gas gauge somehow. I don't believe they are because the wiring on the back of the gauge/cluster looks untouched. Both saddle tanks are currently bypassed and the electric transfer pump is completely disconnected.

Where do I start?

[Steeveedee]

Don't know what the resistances are supposed to be, but you could have a heavy float. My brass float sprung a leak and was half full (or so) with fuel, so it was hanging low. I put in a plastic float from a sender that I was going to use in my '68 Buick, but I ended up putting a '69 tank in it that had a recirc line. I then had to bend the arm a little to get it to read correctly. When it says "Empty", I have about 2 gallons left, which is good enough for me.

[SavageC20]

Originally Posted by Steeveedee

Don't know what the resistances are supposed to be, but you could have a heavy float. My brass float sprung a leak and was half full (or so) with fuel, so it was hanging low. I put in a plastic float from a sender that I was going to use in my '68 Buick, but I ended up putting a '69 tank in it that had a recirc line. I then had to bend the arm a little to get it to read correctly. When it says "Empty", I have about 2 gallons left, which is good enough for me.

The thing that's leads me to believe it's not my float is that the resistance of the sending unit increases with the level of fuel in the tank. I suppose I should still pull it out of the tank and inspect it.

[SavageC20]

I pulled the sending unit out of the tank and confirmed that it is working properly and the float does not have a leak. Any ideas on what might be the issue here? I am leaning towards resistor...

[Cap'n Hook]

Check your grounds

[SavageC20]

Originally Posted by Cap'n Hook

Check your grounds

Is there a ground from the tank to the cab besides the mounting bolts? Ill pull the cab to frame ground off and replace it.

[Sheepdip]

Originally Posted by Cap'n Hook

Check your grounds

^^^^^^^^^^^^^What he said^^^^^^^^^^^^

90% of electrical problems are grounds

[SavageC20]

Originally Posted by Sheepdip

^^^^^^^^^^^^^What he said^^^^^^^^^^^^

90% of electrical problems are grounds

I cleaned up the cab to frame ground and checked continuity, its fine. I checked the ground just below the cluster under the dash and that is fine. I also loosened the tank to cab bolts and tightened them back up. Still the same reading.

[LockDoc]

Originally Posted by SavageC20

Is there a ground from the tank to the cab besides the mounting bolts? Ill pull the cab to frame ground off and replace it.

Also make sure there is a ground strap from the engine to the frame.

LockDoc

[LockDoc]

-
You might take the drivers side sill plate off and make sure that the wires are not bad under it. It is a fairly common problem.

Have you removed the wire from the sending unit and grounded it to the tank?

This was posted in a thread by member VetteVet:

You can check the fuel gauge easily by just grounding the terminal in the fuse panel just below the hazard flasher. I think you know where it is. When you ground it with the key on you should see the gauge needle move to the left below empty. If you dont have a connection from the sending unit to this terminal and the fuel gauge has power it should read all the way to the right past full. If you get no reading then the gauge is bad or it's not getting a connection.

[RustyPile]

Originally Posted by SavageC20

...........<snip>.....I pulled the fuel gauge out of the cluster and tested the resistance across each stud and compared this to the readings given in this thread:

I also pulled the resistor off the back of the gauge and tried to check resistance, but I was unable to get a reading. Does this mean the resistor is bad? Where do I start?

A bad resistor will upset the "calibration" of the gauge.. With the resistor removed from the gauge, you should read around 85-87 ohms.. If your reading is correct, the resistor is bad.. If you have a known good resistor, install it on the gauge.. If you don't have one, a common metal film 2 watt resistor will work.. Radio Shack, Mouser, Fry's, or any other electronic supply source will have them..

[SavageC20]

Originally Posted by RustyPile

A bad resistor will upset the "calibration" of the gauge.. With the resistor removed from the gauge, you should read around 85-87 ohms.. If your reading is correct, the resistor is bad.. If you have a known good resistor, install it on the gauge.. If you don't have one, a common metal film 2 watt resistor will work.. Radio Shack, Mouser, Fry's, or any other electronic supply source will have them..

My readings were with the resistor installed on the gauge. I am in the process of sourcing another resistor to put on the gauge and will update when I get it installed.

[SavageC20]

I installed a new (used) resistor on the gauge and it works as it should now. I also replaced my ground going from the battery to the passenger side fender and I installed a ground from the engine block to the frame for good measure.

[71CHEVYSHORTBED402]

Same symptoms in my C20, was told it's probably the resister. When the gauge shows an 1/8th of a tank I stop for another $20-30. 1/8th tank is more like a 1/4 or so. No intentions of fixing it either, I just like to drive it. No heat, no A/C, rust. If I'm lucky the windshield wipers work, other times just takes a boost

I saw it read right, once Very cold morning, one second is the there the next it was back to abnormal.

[SavageC20]

Originally Posted by 71CHEVYSHORTBED402

Same symptoms in my C20, was told it's probably the resister. When the gauge shows an 1/8th of a tank I stop for another $20-30. 1/8th tank is more like a 1/4 or so. No intentions of fixing it either, I just like to drive it. No heat, no A/C, rust. If I'm lucky the windshield wipers work, other times just takes a boost

I saw it read right, once Very cold morning, one second is the there the next it was back to abnormal.

Yeah I would do the same thing when mine got to 1/8 tank. It wasn't much of a problem because you still know approximately how much fuel you have left, but it started annoying me so I decided to fix it.

[Hammond68]

I bought a used cluster with tach. The fuel gauge in it would only show a quarter or less. I had to use my other gauge. I already have a new sending unit. I could tell when it was low also as when it got near the empty Mark it was low.