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Fuel Sending Unit polarity 98 XJ


NAXJA Forum User
Looking for some advice on an an issue with my fuel gauge/sending unit.

The fuel gauge on my 1998 XJ never worked since I’ve owned it. I always figured it was a bad sending unit since it was on its original or , at least, a very old, fuel pump. Recently I had to change the fuel pump and I got a replacement Carter pump. When I climbed under the back and poked around I found that the insulation on the sending unit wires had cracked and the wires were rotted out and broken off right at the connector.

I wasn’t able to find a pigtail that had the same connector as the OEM fuel pump so I cut off both sides of the connector and used a generic 4 pin waterproof pig tail. I dry fit it and after I saw the gauge worked and the jeep started, I soldered it up. Everything was nice and tight with heat drink tubing and wrap.

After I was done I went to the gas station and the gauge went from 1/3 of a tank to full but it took 15 gallons so there was no way it should have shown 1/3. 150 miles into the full tank the fuel gauge is still reading almost full. I’d expect it to be at 1/2.

Any thoughts? My biggest concern is that I’ve got the polarity swapped on the sending unit wires but I really
My new Carter pump is a bit weird too. It takes a bit to get off of full. 180 miles in i was down to 3/4 tank. I hit 200 miles in and was sweeping 1/2 tank mark. Currently at 250 miles in and pushing 3/8-1/4 tank. Fuel light came on with 4 gallons in the tank on last tank at about 275 miles in.

Im accepting it since its accurate enough i wont run out of gas again 😅

Prior, my gauge only worked but only on full and between half and empty and the light would come on just below half (original mopar pump from 98/99).

If it were a polarity issue in your case, i would expect the gauge to read empty when its full and vice versa. Is it possible you have a cold solder joint/bad splice? Ground short somewhere else along the wiring? Incorrectly positioned the float arm when installing pump?

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I think the Cherokee was the same as the YJ, where the 15-gallon tank was identical to the 20-gallon tank. It had an extension to the the fill hose that extended further into the tank to limit how full it got and the sender was different. I had this on my 4-cyl YJ, and you could pull out the fill tube extension and magically go from a 15 to a 20 gallon tank. The only down side was that it read full for the first 5 gallons.
Thank you for the replies. This was my first post and I really appreciate the feedback!

I'm confident that the solder joints are sound and pretty sure the float arm is pointing in the correct direction inside the tank (thats what you meant, woodswanderer?)

I'm 260 miles though the full tank and the fuel is reading a little less than 1/2. The needle seems to be moving steadily so I don't think its hung up on anything.

I did hold on to the old sending unit so maybe I can swap it onto the new pump to see if it changes anything.