I have a newbie question, I have a 1970 cutlass. When I have a full tank of gas the gas gauge goes past the full line; however when I'm close to empty I'm at a quarter of a tank. Where should I start troubleshooting to correct this problem? I recently just installed a new gas tank. Any help would be much appreciated.

 

Many people would be thrilled to have the problem you're having. At least you have a working gas gauge. My recently-sold '73 Custom Cruiser did the exact same thing. It was basically shifted to the right by one hash mark. I just lived with it, figuring that any attempt to fix it would be more problem than it's worth, and it may not be possible to fix it without putting in a new sending unit. It was not hard to remember that full was past the F mark and that when I was down to half a tank as shown by the gauge, it was time to look for a gas station.

Has the gauge only recently started to do this? Right after you installed the new tank? Did you put in a new sending unit at the same time?

 

Basically the float is out of calibration. It will need to be adjusted to the gauge. In order to fix it the tank needs to be dropped.

 

Yeah, this is a mechanical problem. Adjust the arm. Do these bend?

 

The float rods are metal, so they likely bend.

But the main point is that, to get at the float rod requires draining and removing the tank, no small task. Once you do that, how much do you bend it? Is it possible to bend it with enough precision to actually make the gauge work better, or do you run the risk of bending it too far and making the problem worse?

And how do you test it? The only sure way is to put liquid in the tank to various levels to watch the gauge move as the float does. What liquid will you put in? Probably water. But whatever liquid you use, that means supporting the tank at the proper angle and running wiring from the sending unit to the dash gauge, and then trying to add and remove incremental amounts of water to see how the gauge moves as the level changes.

In short, this is a BIG, pain-in-the-*** effort to fix a minor problem that can be lived with easily. I would leave it alone until some other reason to take down the tank arises if that ever happens again during your ownership of the car.

 

This is a new occurrence and I didn't change the sending until. Looks like I will be dropping the tank. Thanks for help!

 

I'm one of them. I had to jump through all sorts of hoops to get my heavily rusted fuel sending unit back in action. Prior to de-rusting the sending unit, my gauge always read beyond full (infinite resistance). After de-reusting the sending unit, my gauge reads just below 1/4 tank with no fuel in the tank.
Bending the float arm to change calibration is the first thing that i would think of with a new sending unit, but with an old sending unit it might be possible that the sending unit may not be reaching zero ohms output for other reasons. This could be due to several factors, like: dirty connections in the wires that cause you to have a couple ohms of resistance in the circuit when there should be zero; corrosion on the sending unit rheostat that prevents it from reading zero ohms when it should; etc. The problem of having low ohms present in the circuit when you should have zero ohms is magnified on those cars that have the low Z (0-30R) sending units.

Before dropping the tank, I'd take some resistance measurements between the connection at the gauge and the connection at the sending unit. You might find that you've got a couple of ohms resistance at the connections, or at the front and rear body connectors in the wiring loom.

 

My fuel gauge frequently reads far beyond full, i have to tap on the fuel gauge to get it to start working again. I'm assuming bad connection at the back of the gauge since it responds to tapping. Anyway, other than that, I have the same problem you describe with not reading accurately, but I had to put in a 16 gallon tank when a shop that was working on my car let their scrap metal guy take my 20 gallon tank, and the 16 gallon was the only tank I could find locally available at the time (many many years ago), so I presume using the 20 gallon sending unit in the 16 gallon tank is throwing off my guage when it does operate. Someday I'll swap tanks for a new one and see if that solves it, but getting at the back of the gauge seems like more trouble than it's worth unless you are a smurf, way too tight in there.

 

I'd be worried that my 20-gallon tank ended up on ebay.

Assuming that the sending unit is properly located on the 16 gal tank, and the float arm is bent correctly so that it's calibrated properly for your tank, then it shouldn't matter if you have a 16 gal tank or a 20 gal tank. It should still read such that Empty is empty, half is half, full is full. That is to say that when installing a sender there's a mechanical issue that needs to be addressed, and once you've got the arm bent correctly, you're good to go... unless you also have an electrical problem.

The purpose of my previous post was to remind people to think about electrical problems in addition to mechanical problems, especially when the gauge appears to be working but you can't get it to go down to E.

I don't know what the resistance sweeps are on the newer cars, but on the old ones that use a 0-30 ohm sweep, measuring fuel level accurately is notoriously difficult because low resistance measurements are highly prone to error unless you're using a kelvin meter. What I'll probably end up doing with mine is to convert the sending unit into a modern, high impedance unit that's easier to read accurately. Then I'll have to put a black box between the sender and the fuel gauge to drive the dash gauge with a calibrated signal.

 

Its a matter of lowering the arm closer to the bottom of the tank so it reads E when near empty and F when full. Anything in between is a guess as the contour of the tank dictates the level of fuel through out the range.

 

By 1970 it was 0-90 ohms.

 

What do you do when the arm is all of the way to the bottom of it's travel stop and the gauge still reads 1/4? I can't think of a mechanical way to solve that problem.

 

You bend the arm up to allow the rod and rheostat to move lower .

 

I think you're missing my point. Yes, in theory bending the arm should solve the problem, especially if you have a brand new sending unit. But in practice it does not always work. The world has plenty of sending units whose rheostats will not go to zero ohms even after bending the arm. When that happens the gauge can't give a reading of E and you have to look elsewhere for the answer.

 

In most cases it does. Either way it needs to come out and be checked.

 

maybe this will also fix your problem buddy. I had my gas gauge rebuilt but had to send in my float for proper calibration. i found a rebuilder in the back of hemmings it was in minnesota and costed 40 plus shipping and of course it was painted detailed etc and i bought a new float with it.