77 CJ7 Fuel gauge not working - confused by test results

Just purchased a 77 CJ7 that had been rewired by the PO and he also installed a new fuel tank and sender. The temp gauge had been replaced with a full gauge in the dash. The PO told me that the gas gauge had just quit working, right before the purchase. I did some quick checking for voltage at the gauge, which seemed OK, but decided since the temp senor had gone bad, maybe I just needed a new gauge. So purchased a gauge, removed the cluster and reinstalled. Still no movement.

Went back to the Web and looked at several articles. Did all the tests with a circuit tester and my ohm meter. Everything checks out. If I ground the sender terminal, the gauge goes to full. If I check the sender wire to ground, I get 48 ohms, which I think is close for the amount of fuel in the tank.

I checked the sender ground wire where it attaches to frame and that is a good grounding point. I then removed the sender ground wire from the frame and tried to take a reading between the sender wire and the disconnected sender ground wire. I get nothing. Like an open circuit.

I'm totally confused. I assume the sender is faulty and needs to be replaced. Why would I get what appears to be a good ohm reading when the sender wire is grounded to the frame and nothing when it is grounded to the sender ground wire? And in any case why am I not getting any movement from the gauge if the circuit - as indicated by the ohm reading - is being completed???

Any advice for more testing greatly appreciated.

Thanks
Barry
77 cj7 304V8 auto

 

There are many posts on the subject. I connected my gauge to the sender outside the dash, it was quite correct, then on the dash bad work.
This is my 3rd gauge so, I note on a paper when I fill the tank so I know approx when it becomes empty!

Next time, I buy a gauge sender for boat, I make a hole in the tank and I connect it to a good quality dash gauge which will be not mixed with the odometer.

I have already the temp gauge from a Willys army model, not electric, and the oil pressure same business, direct lecture.

I would have prefer to have all like on a new CJ5, but ...

You look like a clever guy, so maybe you can find the electric problem. All grounds must be checked.
But I am not confident in the link between the temp gauge and the fuel gauge, they work together and IMO it is a very very light construction.

 

From the factory, there is no sender ground wire. The ground goes through the lock ring, through the gas tank, and to ground via the chassis. Aftermarket replacement senders are supplied with a separate ground wire because some replacement tanks are plastic, and in this case you need to ground the sender to the chassis through a ground wire.

If you see something other than open (ie 48 ohms) between the chassis and the sender wire, that's good. If the "ground wire" on the sender is open to the chassis, there must be a break in that wire due to corrosion or whatever, or it's not the wire that you think it is.

You can substitute a fixed resistor for the sender between the pink sender wire and ground, and your fuel gauge should read whatever that resistance translates to (ie 48 should be half-full-ish).

 

Thanks for all the quick replies. First to your questions, I did measure with the sender wire disconnected. and I was getting 5v or so at the gauge. I was just using a test light and it dimmed as described in the instructions.

While I was checking the wiring at the tank I saw that the sender wire loom seemed to have melted or been burnt close to the attachment to the pink wire. Not sure how it happened, although the tail pipe is in the same area. The wire was routed away from the pipe, but may have dropped down at some point. The wire inside the loom was charred, but didn't appear broken. I thought it wise to replace the burnt section which I did. When I plugged everything back in and turn the ignition on, the needle moved!

I don't understand why I got the 48Ohm reading. I didn't measure again after the needle moved. Instead I drove to the nearest station and put in 12 gallons of gas. This adventure started with me running out of gas. I had put in about 3 gallons so I'm sure the tank is full. After the fill-up the gauge read between 3/4 and full. It showed E with the 3 gallon. I can live with that.

Thanks again,
Barry

 

Several post about this problem around here. My temp and fuel guages worked fine for years. About two years ago both stopped reading. OK it typically is the voltage regulator on the temp guage. I sort of crawled in there to check things out. They put the dashboard brace directly under the instrument cluster. I can't even see the back of it. maybe I could have done the contortions around 40 years ago. I run two gas tanks so I just switch between them and when the tank that still has a working guage gets close to empty, I put fuel in both sides. One friend has a late CJ5. His fuel guage never works unless he is on a Jeep trail and he has pulled everything out and checked all of the wiring and such.
The neighbors fuel and temp guages quit in his 83 CJ7 and a short time later his motor had no spark. He threw all new computers in it and finally went with a DUI dist and a Webber carb. It runs fine now. Those guages still don't work however.

 

Sometimes troubleshooting with an ohm meter can cause massive confusion when testing 12v dc circuits. A test light will show power where you are supposed to have it. Your confused reading could have been a resistive reading of a tail light bulb or something else that was shorted in the loom.

 

Agreed. I purchased a test light after I couldn't figure out what was going on with a multi-meter. Using the light, everything still passed. I think my real error was looking at the ohm reading and seeing that it was within the work "range". However with hindsight, I knew I only had about 3 gallons of gas in the tank, so it should have been higher. I'll remember that the next time.

Barry

 

The scale is not linear. It goes full-half-empty 10-23-73, so 48 is more close to empty.

The tank holds 14 gallons, so 3/14 = 0.21 or about 1/5 of a tank. Seems about right.