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· Premium Member
25 Posts
Discussion Starter · #1 ·
I'm looking to resolve the problem with several sending units I've had. Back in 2012, I decide to remove the original gas tank and inspect the interior. I found it to be free of rust but the sock filter had fallen apart. As a result, I installed a new sending unit and sock filter from Ground UP. The old sending unit worked perfectly for over 10 years that I owned the car and just before I removed it. Slightly above F when fuel and about 3 gallons left when on E. All the gauges were installed new around 2001 and still work perfectly except for the gas gage. When the tank if full, it reads 3/4 full and when on E, I have about 8 gallons left as I can only add about 10 or 11 gallons.

So, I replaced the sending unit again in 2012 with a new replacement unit from Ground Up and the same readings. Both units appear to be identical to the factory one but unfortunately I threw away that unit years ago (never throw out original part until it's proven the new one works).

So in 2020, I decided to replace the tank with a stainless steel one and use the sending unit again. Again, same bad readings. The ground and fittings are clean.

So now I'm completing a frame off restoration and I'm ready to re-install the tank.

Can someone provide me a goof prove of testing the sending unit out of the tank with an ohm meet and car battery, how to hook it up and read it?

It would be great I could get good answer by tomorrow as I would like to straighten this out once and for all before re-installing the tank.
Appreciate a clear answer.

· Registered
1,646 Posts
When I first started looking into testing the fuel gauge circuits on GM vehicles, I did the very basic 2 tests with grounding the sender wire in the trunk going towards the gauge and then not having it connected to anything to see if I could get the gas gauges needle to swing to empty when that wire was shorted to ground (which would be 0 ohms) in the back and then swing to above full (above 90 ohms) when it was connected to nothing (and you have to have the ignition key to the run position so the gauge does get power) but I found out these two tests are NOT enough. One needs to mimic the output resistance values from the sender to where one can safely say that from that test point forward in the trunk towards the gauge is working as it should.

SO, with that said, I've lucked out in that I have access to different value resistors and gathered some to mimic what he sender in the tank should have as far as resistance to get the gauge to read 1/4 full, 1/2 full, 3/4 full, and then full. Since the system works on a 0-90 ohm design, if a 22.5 ohm resistor is wired in a particular way,then if the gauge does read 1/4 full then this would be good BUT one needs to keep on going with taking that test resistor out and putting in a 45 ohm one to where the gauge should read 1/2 full and then if the resistance was changed to 67.5 ohms, then the gauge should read 3/4 full and then with a 90 ohm, full.

What people do not realize is a lot of factory gauges had shunt resistors across the back of them and if these resistors are missing or broken or not of the correct value, strange gauge readings car occur.

Maybe look at this video I put together when I was testing the gauge system in my 68 Nova:

How the different value resistors get the gauge to read:

If with the tests the gauge does no read right, then there may be an issue with the shunt resistor on the back of the gauge or if it's a reproduction gauge there may or may not be one external but built in internally.

This is how the gauge may read with testing resistors but now the gauges needle does not go to the proper spot or give the right reading:

If you verify from the test point forward all is working as it should, then the sender can be tested with an ohm meter.

Also too, check how well the sender is grounded to the car body with it's dedicated ground wire.

On an old sender of mine some connections went south but after cleaning and soldering them, the sender began working correctly once again. This wasn't the best soldering but it worked.


· Registered
1971 Chevrolet Chevelle Malibu
7,042 Posts
When I replaced my tank the sending unit looked brand new so I reused it. What I did was I tested it out of the tank but by moving the float to the top and then looking at the guage and then to the center and then to the bottom. The guage reacted correctly. One thing I found out was on a 71 the gas tank filler pipe is supposed to run down to the back of the tank for this reason the float lever has to be bent to the other side. I just checked NPD and they show a 71 & 72 float are bent the same way and a 70 is bent the other way. On Ground-Ups sight they show two sending units for 68 to 72 and they both show the float bent the same way as a 70 tank would take.
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