My gas gauge is stuck at 1/4 tank. I removed the grounding wire an it did not budge. I partially removed the dashboard cluster to get at the back of the gauge. When I disconnect at the back of the gauge level still reads 1/4 tank but if I manually push the needle to empty or full and connect the gauge again it returns back to 1/4 tank. Is this a float or sending unit issue or is the gauge bad? It's a 67 fastback dressed like an ss. Any help would be much appreciated.
If you are sure that the grounding wire at the sending unit is clean and secure at both terminals, then I would drop the tank and check the sender first as they are readily available. Check it with an ohmeter, it should read 0 to 90 ohms as you move the float up and down
If you ground the wire going to the sending unit the gauge should read "Full"; take the wire off and it should move to "Empty". If the needle doesn't move the gauge is broken.
The gauge is nothing more than an OHM meter that measures the voltage differential that is being dropped across a variable resister (the rheostat in the sending unit). There is also a resistive load (a heater) that is attached to gauge to keep the needle from bouncing around as fuel sloshes back and forth in the tank. It is this resistive load that usually corrodes away and breaks to cause a gauge to malfunction.
In the tank the sweep that runs across the windings of the rheostat can over time wear a hole in the varnish that insulates the wires. This short will cause a gauge to remain fixated about one point like yours is doing. You can replace the sending unit by dropping an empty gas tank and after grounding everything knocking the sending unit out of it is retainer ring with a brass punch. If replacing the sending unit upgrade to the big block version as the feed line is three eighths of an inch instead of the smaller base motor five sixteenths inch size.
Oh, I just removed the grounding wire that goes from the tank to the frame. I thought that would cause it to read empty. So I need to ground the wire going into the tank? I was trying to avoid dropping the tank since I had just filled up. Guess I'll have to fix the starter issue first and then run it out of gas to get to the top of the tank.
Gauge should move when you remove the ground wire or install it at the tank (or in the trunk where it is a lot easier to get access to than under the trunk on top of the gas tank).
I only recommend an empty tank because gas weights 6.28 pounds per gallon. An empty tank is easier to pick up and hold in place while removing or installing the straps. A full tank is safer; as gas doesn't burn, only gas fumes do.
Last edited by Big Dave; 07-14-2012 at 01:25 PM.
Reason: Spleling erors