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Fuel Sending Unit Problems

Fuel Sending Unit Problems

Uscgzulu

Jeeper
Posts
4
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Location
Jacksonville, FL
Vehicle(s)
1980 CJ-5
I have a 1980 CJ5 with a factory 15 gal. fuel tank. When I first bought the CJ, the fuel gauge registered at 1/2 a tank regardless of how much fuel was actually on board. When the ignition was turned off, the gauge would fall below empty, but when power was supplied it would always go straight to the 1/2 tank mark and stay there.

I recently dropped the tank and replaced the entire fuel sending unit. Now the needle on the fuel gauge shoots all the way above the "F" positon on the gauge and stays there regardless of how much fuel is in the tank.

I have checked the sending unit ground, and it appears to be fine.


Does anyone have any other suggestions or ideas before I pull the entire tank back down to reinspect the unit?
 
Maybe it is not the sending unit and the gauge that is faulty. If I remember correctly it is just a resistive measurement. So a Ohm meter might help in diagnosing the issue as Ohms are just a measure of resistance.
 
Ohm meter is probably a good idea. The more I get into the problem the more I am beginning to believe that it has been the gauge all along. It just seems odd to me that changing out the sending unit caused a change in the gauge measurement. Maybe the gauge is drawing more ohms from the new unit for some reason? I'm going to try to pull the unit out again this weekend and test it outside of the tank. Hopefully that will tell me if it's the unit or gauge once and for all.
 
the new sending unit may have hung up when you put the tank back in. If you are going to drop the tank it will just be a question of wiring the sending unit to the gauge and moving the float up and down to see if the guage responds.

If you considder dropping the tank as much fun as I do (sarcasm) you may want to check the guage with a suitable resistor to see if the gauge is working before you drop the tank.


Ohm meter is probably a good idea. The more I get into the problem the more I am beginning to believe that it has been the gauge all along. It just seems odd to me that changing out the sending unit caused a change in the gauge measurement. Maybe the gauge is drawing more ohms from the new unit for some reason? I'm going to try to pull the unit out again this weekend and test it outside of the tank. Hopefully that will tell me if it's the unit or gauge once and for all.
 
I think your original sending unit was bad.
When the tank is empty the sending unit should be 73 ohms. When the tank is full it should be only 10 ohms.
If your gauge now reads full (or past full) all the time I bet you have a short. When you put on the new sending unit one wire goes on a stud / terminal on the sending unit. You may have a short to ground there. Make sure that wire doesn't touch the tank or any other metal besides the stud it's supposed to touch. I just replaced my tank and sending unit and I found the replacement sticks up higher than the original. If that connection touches the bottom of your tub then it will short out and 0 ohms will peg your meter at full.
Follow IO's sugestion: if you put a resistor from the gauge to ground you can test your gauge. A 10 ohm resistor will show full and a 50 ohm resistor will show over half a tank.
To do this test you will have to remove the wire from the gauge. At that time you could also put a ohm meter on the wire to see it's resistance.
 

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