My gas gauge always reads half-full

Question:

The gas gauge in my 1938 Oldsmobile shows half-full at all times. I have had the gas tank taken apart for cleaning and reconditioning along with the sending unit. I have cleaned the contacts on the gauge and ensured a good ground connection. The gauge goes to half-full when the key is turned on, so it would seem to be an internal problem with the gauge. Can you suggest a self-help method of taking the gauge apart to examine it, or maybe you know of a good source that can rebuild it?

Answer:

The rheostat in the sending unit located in the fuel tank controls the dash fuel gauge in your Oldsmobile. That is most likely where you will find the problem. But because it is a big job to pull down the tank and remove the sending unit, it would be prudent to test the dash gauge first. My good friend and fellow restorer Dennis McGillis, who is also my auto electronics guru, had this to say when I asked him about your gauge:

All GM fuel gauges from the early 1930s to the 1960s operate on a 30-ohm scale. At zero ohms from the sending unit, the dash gauge should read Empty; at 30 ohms it should read Full. You can check these readings using a multimeter. It is vital that the sending unit is matched to the dash gauge. If you are using an aftermarket fuel gauge in the dash, you must have a sending unit that has a matched ohm rating or it will not function properly.

The sending unit is merely completing the circuit for the dash gauge by grounding it to the chassis. Just like a light bulb must have a ground to operate, the fuel gauge must also be grounded to operate. As the float arm is raised, the resistance is increased, until (at the top) the resistance should be 30 ohms (Full). At the bottom, resistance is zero (or completely grounded) and should indicate Empty on the dash gauge.

If the gauge itself checks out with a multi-meter, the problem is in the sending unit. To get to that, drain the tank using a siphon hose, or if there is a bung in the bottom of the tank you can open it. Drain it into a metal container, work outdoors, and don’t smoke. You can dispose of the gas by putting it into another car, or by putting it in a properly vented gas can. Needless to say, a spark from any electrical device or a nearby water heater could have catastrophic consequences, so have a fire extinguisher close at hand.

Remove the sending unit from the car, and clean and dry it thoroughly. Check the up and down travel of the float to make sure it works unimpeded through its whole range. If it does not, clean and adjust it so it does.

Check the cork float to make sure it is in good shape and not saturated and soggy. Or if the tank has a metal or later plastic float, make sure it is not leaky and containing fuel. If your cork float is sound, you can dip it in shellac and let it dry thoroughly before reinstalling it to keep it from absorbing fuel. If the float is metal or plastic, make sure it has no fuel in it, and then hold it up to your ear and shake it to see if there is rust or contamination inside. If there is, replace the float.

Next, remove the small cover on the rheostat. Inside you will find a metal contact that rides on a series of bumps or nodes in order to gauge the depth of the fuel in the tank. Clean and brighten the contacts and check the rheostat using a multi-meter. Move the float through its travel to make sure it is reading properly. If it does not read consistently through its travel, replace it.

And if you do need to replace it, bend and adjust its wire so it will work through the entire range, given your tank’s dimensions.

Install the gauge using a new gasket and a little Permatex, and add a ground wire from the sending unit to a spot on the car’s frame, just to make sure the unit is grounded properly. If the sending unit is functioning correctly and grounded, your gauge should read as it is supposed to.

You will have to do a little detective work if you need a new fuel gauge, but universal tank sending units are available from most parts stores. Once again, though, make sure the ohms ratings are correct.