Fuel gauge problem

Testing the sender is easy, getting to it might be the difficult part

1) determine which of the two fuel tanks the sender is located in
2) remove the rear wheel and splash panel on the side the sender is on
3) disconnect the two wires
4) remove the unit by unscrewing the 6 cap bolts (note the exact position of the mounting plate relative to the tank)
5) attach an ohmmeter to the two output leads
6) lift the float arm across the full range of movement and observe if the changing resistance is smooth and regular

These are small fuel tanks and the float arm will not extend in all directions before hitting the side.
My problem was that while the sending unit was OK, it had been mounted incorrectly. When some prior person had re-installed it they did not preserve the original position of the mounting plate relative to the tank. They had rotated it just one bolt position CCW and the float would then hit the side of the tank at around the "third fuill" mark.

 

Marc - While Scott's post is informative, I believe their are a few things to consider/do before pulling the sender unit.

That the gauge swings from zero to full but does not rest in between speaks for a loose electrical connection, most likely outside of the fuel tank. I would first check the connectors behind the gauge and the plastic terminal blocks connecting the instrument pod to the main wiring harness. It is possible that these wires have come loose, and the contact is intermittent.

Second, I would reach to the top of your sending unit (some are in the passenger-side tank, some in the driver side tank) and simply unplug the two wires. There may be a ground (third) wire fastened via a ring connector onto one of the 8 screws (maybe 6) clamping the sending unit rubber washer. Disregard the ground wire if present. With the two wires unpluged, the gauge should read zero. If you touch the two wires together, the gauge should read full. The gauge should swing smoothly between these extremes. If the gauge does respond as suggested above, then the integrity of the gauge, instrument pod connectors, and wiring loom has been verified. The problem may be in the sending unit itself.

What Scott was referring to was the internal construction of the tank. The sending unit is a long arm with a float at the end. It must swing as the fuel level rises and falls. If it is inserted in the incorrect orientation, then the arm will be obstructed, that is, prevented from swinging as it will come up against one of the internal baffles in the tank. However, this would not lead to your symptom. If inserted incorrectly, then the fuel level would not rise above a certain level, but it would fall when empty. Furthermore, the gauge would not swing between the extremes. Your symptom is due to a loose connection somewhere, perhaps in the sending unit itself. It can be fixed. It may need a bit of solder or may require rewinding with resistance wire. If you take it out, make sure that you check the Ohms resistance of the resistance wire before disassembly. Contact me if you get to that point and I will help you through it.

Jim S.

 

Might be a bad connection, bad gauge or a bad sending unit in the tank. There are two lines plus ground going to the tank and Jim, I think you are wrong on the wiring and test. I believe one wire is for the gauge and one wire for the red light. Both work against the ground wire. The light needs a switch to ground. The gauge needs a variable resistance to ground. High resistance at empty tank. Low resistance at full tank. If you can get a voltmeter on each of the two wires right at the tank, you can find out what is going on.
With full tank, red light is out and there should be battery voltage on that wire. Gauge should read full and voltage on that wire should be low. Don't know exact number but substantially below battery voltage. If that voltage varies widely, something is wrong. Disconnect wires and check resistance of sender to ground with an ohmeter. Jack one side of car to shift gas level. If it moves smoothly with gas level, it is OK. If it is noisy, THEN you have to pull the sender. Mark it's position first!
Good luck.
John

 

John - right you are. I must have passed a brain stone.

There are two wires...one for the idiot light and one for the gauge. Touching one wire to ground will illuminate the light, while the other touched to ground will swing the gauge to full (or empty).

Sorry for the hiccup. I didn't bother to look at the schematic...was writing from memory.

Jim S.