Fuel Gauge Problems

[JeffP]

For those having fuel gauge issues here are some facts.
Both the gauge and sending unit resistance range needs to match. (Ohms) Zero to low ohms for full, high resistance for empty.
Both the gauge and tank need to be grounded with at least 14 awg wire! If your new gauge reads full all the time, it’s either a bad ground on the tank body or bad ground on the gauge. Or... the sending unit wire is shorted to ground somewhere.
Removing the tank is a pain in the rear so when replacing a sending unit, test it with the gauge before installing it in the tank!

 

[O1LL1F3]

This is my next troubleshooting job. Did a little last week. Neither my temp or fuel gauge were working. Keep in mind i have the Mercedes diesel so i was unsure if the temp gauge was even supposed to be working. I checked ohms like i had seen on many threads and dont recall the readings exactly but i do remember my "sending unit i think" was up in the 150's. What i did find is that i wasn't getting 12 volts to the fuel gauge resistor. I just 12 volts over from another source and the temp gauge started working but the fuel gauge is still not working. I am also only getting about 10 gallons of use out of my tank. Another issue i have noticed is i have no dash lights at all when i turn my headlights on? I just use GPS to see how fast i am going.

 

[JeffP]

This is my next troubleshooting job. Did a little last week. Neither my temp or fuel gauge were working. Keep in mind i have the Mercedes diesel so i was unsure if the temp gauge was even supposed to be working. I checked ohms like i had seen on many threads and dont recall the readings exactly but i do remember my "sending unit i think" was up in the 150's. What i did find is that i wasn't getting 12 volts to the fuel gauge resistor. I just 12 volts over from another source and the temp gauge started working but the fuel gauge is still not working. I am also only getting about 10 gallons of use out of my tank. Another issue i have noticed is i have no dash lights at all when i turn my headlights on? I just use GPS to see how fast i am going.

This is easy... take a deep breath and we will do this one step at a time. You WILL have a working fuel gage when done!
First, what year and model do you have?
Factory tank & gauge?
If everything is original start with this... there should be three terminals on the back of the gauge. 12v positive, Negative or ground (looks like an old tv antenna drawing) and “S” or “T” for the “S”ending unit from the “T”ank.
Note that after each step, the gauge could start working and you are done.
Verify that you have at least 12v+ to the 12v terminal when the key is “on”. If not, find out why & fix it. Or... simply run a new wire.
Next make ansolutely certain the Negative or Ground terminal is properly grounded to the dash / chassis. Use 14awg size wire or larger!
As a test, key “on” temporarily ground the “S” or “T” terminal using a jumper wire. If the previous steps were done... successfully, when you ground that terminal the gauge should move all the way to “full”. This verifies that the gauge itself is good and working. If not... stop, replace the gauge & start over.
If it works, remove the temporary jumper. Go back to the fuel tank and make sure the sending unit wire (going to the gauge) is connected. This will be the terminal in the center of the sending unit on the tank.
Next check that the other terminal on the sending unit... the outermost one, is properly grounded. Again this must have a solid ground! No rust, paint or corrosion of any kind! Use 14awg wire or larger.
If your gauge still doesn’t work, test the wire to the gauge by grounding the signal wire at the tank. The gauge should read full again. If not, replace the wire.
If the wire proves good then test and or replace the sending unit.
That’s it, nothing else can be wrong. It’s not a big deal.

 

[O1LL1F3]

Thanks JeffP, I will have a week and a half left offshore. I will try that when i get home

 

[Torxhead]

Here is some additional info on the Jeep CJ gauges:The new Gauge Page

 

[Lumpy Grits]

If your tank does not read full. OR your temp gauge reads high.
It's a bad dash GND.
LG

 

[JeffP]

If your tank does not read full. OR your temp gauge reads high.
It's a bad dash GND.
LG

Or... some teen stole some of your gas the night before and you have a cooling system problem.
I know.... I’ll shut up now. LOL

 

[O1LL1F3]

So i guess it would be worth mentioning. The fuel gauge the day i test drove it and the day i picked it up was reading 1/4 of a tank. The guy i bought it from even made the comment. I have had this thing a year and I haven't even burned a whole tank of diesel. Well after the 6 hour trip pulling it home on a trailer in some of Mississippi's fine roads. There were a few things that were no longer working. Found a few wires that had come loose. So my my next step i guess would be checking grounds and wiring connections. But i am still only getting 10 gallons use out of my tank, which from what i read more than likely means the filter element fell off of the end of the fuel pick up. Which would require my changing the sending unit anyways.

 

[Lumpy Grits]

The fuel pickup should 'lay' about an inch off the bottom of the tank.
LG

 

[Project86]

If one so desires the sending unit can be accessed by making a hole in the bed and not dropping the tank. On my second rebuild I did this. Measurements are posted with pictures. See the "OK I'm a slow learner" thread. I covered the hole with aluminum diamond plate.