Electrical gauge mystery with clues...

[85Renegade]

Sorry it's long, but there is alot of information...

I have a real mystery on my hands... But I'm confident that with the immense knowledge on these boards someone can provide me an answer. Here are the clues..

When I first got the jeep a 1985 CJ7 Renegade , the temperature gauge as well as the fuel gauge were not working. No matter how warm the engine got the needle didn't move.

I removed the speedometer cluster, in an attempt to experiment I used a multimeter and it seemed that the gauges were working normally.

While the gauge was removed from the car I did a significant amount of work under the hood. Nutter, Team rush etc...
While doing this I accidentally removed the wire that was connected to the temperature sender, without realizing it.

A week ago I reinstalled the speedometer cluster and amazingly the fuel gauge was working...!!! However the temperature gauge was not. I soon realized it was because the sender wasn't connected.

Fast forward to today... Here is where the real clue is I think...
I ran a new wire going from the temperature sender to temperature gauge. I started up the car and as normal the fuel gauge went to half a tank. I let the car idle so it would warm up, to see if the needle would move. Then it happened the needle on the temperature gauge started climbing but at the same time the fuel gauge started to drop..!!! Eventually the temperature gauge sat at half temp and the fuel gauge sat at E... I then pulled the wire I just installed, put the wire back on when the fuel gauge was working, however the fuel gauge just sat on E and the temp went back to cold....

I know the two gauges share a power, but why would that cause the phenomenon I'm seeing. If anyone can provide any level of insight that would be great.... Thank you all.

 

[mstar]

Check and double check ALL of the grounds on your jeep.Bad grounds will do strange thing in a electrical circuit.The temp. and fuel gauge need 100% good circuit to be accurate. Mike

 

[85Renegade]

That's what I seem to be reading.... I'm going to run new grounds to everything tomorrow. Probably be good for everything...

 

[Torxhead]

Bad grounds are a CJs weak point, after that look up John Strenks CJ gauge page. This is something every late model CJ owner should have.

 

[85Renegade]

Update... The plot thickens.. I ran all new grounds throughout the car. I ran a 2 gauge wire from the block to the battery, from frame to battery, a 12 gauge from tub to battery and a 12 gauge from the cluster housing to battery..... Problem still persistes. I will turn on the car fuel gauge will climb to half tank then slowly drop back down and stop at zero.... I tried switching power and sender wire and it climbs to full..? Now the temp gauge appears to be reading all over the place.

Here is the kicker..... I went and got the jeep gased up and the needle went straight to full temp gauge seems to be closer to correct. It appears to me that once the gas drops below half a tank everything goes to ...... What does this mean? Is it the fuel sender?

 

[Torxhead]

You could do a resistance test from the battery ground to the instrument cluster to check and see if you need an additional ground wire there. This is what I use for gauge problems:
The new Gauge Page.
If you give up, the replacement gas and temp gauges come as a kit.

 

[85Renegade]

I'm headed in that direction, but only if I know that's the problem don't want to change them to find out its a sender somewhere....

 

[oss]

Did you check the ground on the fuel tank? I know it sounds weird, but it's a weird problem.

 

[thescot]

I spent a few hours recently chasing bad gauge lights. My wiring is pretty clean, I have extra grounds to the dash and cluster also. Even with the gauges mounted tightly into the dash panel they werent getting a good ground. I pulled them out and sanded the edge of the hole they sit in and now they work. Like many said, one cant stress enough the bad grounds issue on these things.

 

[danray48]

I'm for checking John Strenks CJ gauge page with a test light, it's great information. I had a problem similar with my fuel gauge. Bought a new gauge, and the same thing. Grounds, grounds and more grounds.

 

[Choppa]

After you have tested all the earths and you are satisfied that that is not the problem, take a multimeter and measure the output voltage from the temp gauge to the fuel gauge on the little bridge piece that runs between them.

I once had a issue that the voltage fluctuated and realized the converter inside the gauge did not stay on 5Volts when the load on the fuel gauge changes.

I had to replace both as a set afterwards as I burnt the fuel gauge not knowing the earth on top of the tank was disconnected and drew to much current from the temp gauge converter.