91 Fuel Sending Unit Problem

[Keyav8r]

Problem - erroneous fuel gauge readings.

Background - When I bought my MJ the fuel gauge was not working correctly. Even with a full tank the gauge would not go above 1/2 full. The low fuel level light works, so I know I have time to get to a station when it comes on. When I dropped the tank to replace the leaking fuel vents I reached in through the filler neck with the universal tool (straightened coat hanger) and moved the sending unit float arm through its full range and checked the readings with an ohmmeter. They were pretty close to 5 ohms at full (arm fully up) and 105 ohms at empty (arm fully down).

Today I checked the ohms at the tank connector against the ohms at the instrument cluster connector - both were right at 40 ohms. Gauge reading was between 1/4 and 3/8 tank on a partial tank.

I replaced the fuel gauge with one from a 93 XJ, plugged the connector back on the cluster and the "new" gauge read between 3/8 and 1/2 tank. Since 1/2 tank ohms are 33, the gauge reading seemed more accurate.

Then I drove up the gas station and filled her up. Bummer - gauge only came up to around 5/8 full.

Drove back home, crawled back under the truck with the ohmmeter, and got a 30 ohm reading at the tank connector. I checked the gauge side of the circuit by checking the ohms between the body side of the tank connector and ground and got continuity (0 ohms). Turning on the ignition with the connector un-coupled causes the gauge to go past empty and turns on the low fuel level light (greater than 105 ohms resistance). Putting a jumper between the body side of the connector and ground causes the gauge to go past full (less than 5 ohms resistance).

Based on this, I believe the gauge is reading correctly per the ohms in the system.

Having checked the sensor previously and getting good readings from it, the only thing I can think of (other than the sender having gone bad) that might cause this is a defective float on the sender arm. Has anyone experienced a bad float?

After I burn some of the fuel down I'll drop the tank again, pull the pump and sending unit and give it a full checkout.

Any ideas of what else could cause this problem?

[HOrnbrod]

Have to say after reading your troubleshooting process your logic seems sound and it certainly appears that the float has some negative buoyancy preventing it from floating on the surface of a full tank of fuel. I've seen this before in fuel sending units due to a cracked/liquid-logged float, but not on an MJ sender. Either that or the float arm is getting hung up inside the tank somehow and the arm can't fully rise to the top. Hopefully it becomes obvious when you yank the assembly out of the tank..........

[Keyav8r]

Noticed that when the truck is sitting still the fuel gauge will show the higher reading but as soon as I start driving the reading drops to around 1/4 tank or less. Low fuel light doesn't come on. If I sit at a traffic light long enough, the gauge reading will gradually come back up to the high level only to drop again when I start moving again.

Don't know if this adds anything to the diagnosis process, but it is another piece of the puzzle.