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Jeep Service
Jeep Service, Jeep Owner
Category: Jeep
Satisfied Customers: 4606
Experience:  14 years with AMC/Jeep Specializing with pre-1994 Jeep products.
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Wagoneer: I,m installing a new fuel tank in 1973 jeep wagoneer.

Customer Question

I,m installing a new fuel tank in 1973 jeep wagoneer. The fuel gauge currently has no wires connected. There are two terminals on the back of the gauge. with the key in the on position, the needle sweeps from empty to full when I ground one terminal. Before installing tank, I want to verify old gauge is working, and temporarily connect the sending unit to the gauge to prove the gauge will show the proper fuel level before I install tank.
Submitted: 7 months ago.
Category: Jeep
Expert:  Jeep Service replied 7 months ago.
Hello. Welcome to Just Answer. Please allow me to assist you. If you ground the terminal, the gauge should read full. The gauge principle is a variable resistance to ground. As the float in the tank raises, the variable resistor inside the sending unit transmits a full ground path to the gauge. At half-full, one half of the ground is allowed, and, at empty, there is no ground path at all. The only way to check the gauge is to hook the sending unit up to the wires at the fuel tank and manually move the float arm up and down; the gauge needle should move accordingly. Connecting the sending unit to the gauge is simple. Ground the exterioor of the sending unit, and attach the wire form the sending unit to the gauge (where you ground and the needle moves). Turn the key on and move the float arm.
Expert:  Jeep Service replied 7 months ago.
Please confirm that you have received my input. If you have any questions, please feel free to ask. Thanks.
Customer: replied 7 months ago.
your answer doesn't tell me how to verify that the old gauge is in working order, or verify that the new sending unit is in working order. if I follow you're advice, and it works, all is well. But if the system does not work, I will not be able to determine the problem. that is why I asked originally how to test the components.
Expert:  Jeep Service replied 7 months ago.
1) Ground the wire above the fuel tank (with the old tank off). Turn the key on. The gauge should read past full. If so, Gauge is good. 2) remove the sending unit from the tank. Arrach a ground wire to the exterior. Attach the wire form the sending unit to the wire above where the tank would go, Manually move the float arm up and down. The needle should move as you move the float arm. If the needle does not move, you either have a bad ground or bad sending unit.
Expert:  Jeep Service replied 7 months ago.
Do you understand these directions?
Customer: replied 7 months ago.
there is a way to test the gauge and there is a way to test the sending unit individually. that is what I want to know. I don't want to hook the entire system up in hopes that the components are good.
Expert:  Jeep Service replied 7 months ago.
There is no way to test each unit without each other. In order for the gauge to properly work, it needs to see a specific amount of resistance to ground. If you were to hook an ohm-meter up between the plate and the sending unit wire, and read ohms, the resistance should change as the float arm is moved up and down manually (this requires the sending unit to be removed from the tank). There is no test for the gauge other than described above. If you ground the gauge, and the needle moves, it can be considered working OK...By Grounding the gauge, you are simulating the resistance that the sending unit would be doing if the tank was full.
Customer: replied 7 months ago.
what setting would I use on my multimeter? under ohms it has 200,2k,20k,200k,2m,20m
Expert:  Jeep Service replied 7 months ago.
I cannot remember if the actual resistance of those older sending units.,Start with 200. If the needle (or readout display; whichever you have) on the meter does not change as you move the float arm, move to 2K. The necessaty resistance setting should be in that area... 200 to 2K
Customer: replied 7 months ago.
on 2k setting, sending unit tested zero with arm up and 7 with arm down. with system wired as you suggested, the gauge reads full when arm up, but 1/2 tank when arm down,. obviously there is something wrong, but you're saying I can't test further to pinpoint the problem
Expert:  Jeep Service replied 7 months ago.
I do not have the actual resistance values on this particular sending unit. If the gauge reads full with the arm up (OK), and only drops to 1/2 with the arm fully down, (not OK), the sending unit is faulty. What is happening. This would not be uncommon.
Expert:  Jeep Service replied 7 months ago.
if the gauge was bad it would not drop to zero when the power is removed (key off)
Expert:  Jeep Service replied 7 months ago.
All the gauge does is respond to what the sending unit is telling it what to do.. As the resistance changes on the sending unit (by moving the arm), the more or less resistance sent tot he gauge causes the needle to move proportionally.
Expert:  Jeep Service replied 7 months ago.
a reading of only 7 in the down position does not sound right; should be more than that