85 CJ7 Temperature Gauge Problem

[expertshooter22]

Maybe somebody here can help me figure this out... I have owned my 85 CJ7 for 3 years now and have never had the Temp Gauge working on it. I recently rewired the entire jeep with a painless harness (which worked out awesome by the way) and replaced the Temp gauge and sending unit at the same time. I know the gauge is working because when i ground out the signal terminal on it the gauge climbs to the hot position.... when i test the sending unit when the motor is cold it measures approximately 180 ohms. At normal operating temperature the sending unit measures somewhere around 45-50 ohms, which leads me to believe the sending unit is working as well. when i hook up the sending unit wire and test it under the dash it reads the same ohms as at the unit. But when i hook it all up and the motor is warm nothing happens to the gauge. I am starting to pull my hair out because of this, hoping someone will be able to give some advice, the only thing I am not sure if I am testing properly is the jumper wire that goes from the fuel gauge to the temperature gauge. When i test it the voltage meter goes crazy, anywhere from 1 to 7 volts fluctuating at the speed of a machine gun ( which i am told is what it is supposed to do but doesn't seem right) Anyway, any help would be greatly appreciated, i don't like getting my butt kicked by a temperature gauge.

 

[SteelHeadz]

Purple and white goes to the coolant temp. sending unit. The other terminal is jumped to the middle terminal of the fuel gauge. Looking at the gauge from the back, the pink wire goes to the signal terminal which is connected to the fuel level sending unit. The terminal to the far right should have the red wire connected to it. Check your connections.

 

[expertshooter22]

Yea thats how it is all hooked up, the fuel gauge is working, and all the wires are producing the current/ohms that they are supposed to produce. The only thing that I am thinking now is that the actual gauge itself gets its ground through contact with the speedometer cluster and maybe its contact isn't good enough for the gauge to be grounded. My other question is what should the voltage be coming into the back of the temp gauge from the fuel gauge, to the best of my knowledge it should be 5 volts, but should it be a constant 5 or should it skip around in between 3 and 7 volts.

 

[SteelHeadz]

The Jumper/Regulator Strap is composed of two strips of metal that continuously make and break contact, regulating the output to the Temp gauge to 5 volts. A volt meter applied to the A terminal on the Temp gauge should fluctuate (plus and minus) near 5 volts. A reading of 12 volts on the temperature side indicates a bad Jumper/Regulator Strap. 12 volts applied to the temperature gauge's A terminal will cook the gauge.S to A 19-21 ohms

 

[expertshooter22]

ok, with my jumper/regulator strap it fluctuates anywhere from 3 to 7 volts, but it does it at an incredible speed, so fast that my $100 digital multimeter can hardly keep up with it. does this seem normal or possibly also indicate a bad strap. And if the strap needs to be replaced where can i get one? I called the jeep dealership and they say there isn't a part # for it, and places like quadratec and jc whitney don't have it either. Thanks for your help with this.

 

[SteelHeadz]

Did you replace both the Temp. gauge and the fuel gauge?, i may have overlooked something here.

 

[expertshooter22]

no i didn't replace the fuel gauge

 

[SteelHeadz]

Almost all the one's i looked at on the catalogs has a special note on these gauges. It says that you must replace them as a pair. Why, i don't know. It might be worth checking out.

 

[expertshooter22]

yea now that you say that i have seen it too. Why you would have to do that is beyond me as well, but i would like to replace that jumper wire, any ideas where to get one? or is that a pretty common part that i can get at any auto parts store.

 

[SteelHeadz]

I tried searching for that jumper earlier when i was still at work, but i came up with nothing. I'm assuming that there's a difference in the way the old gauges Vs. the new ones are wired. I thought about looking for the one i pulled off my Jeep but i think i already gave it away a long time ago. I will still check though on what i have in the shop.

 

[expertshooter22]

yea that is what i am wondering too, even though i can't imagine how they would wire it differently, but screw it, i will buy a new fuel gauge, all the other gauges i have already replaced so why not, it will make all the orange needles the same color instead of having a faded fuel needle. if it gets me closer to my temperature gauge working i will try it.

 

[SteelHeadz]

I replaced all my gauges with autometer ultralight. I really like it.

 

[expertshooter22]

holy crap, looks like you have done your fair share of customization... looks good, i have never seen one set up like that. Ok, so i took the jumper/regulator strap off of the back of the gauges.... Everyone says that it is supposed to reduce from 12 volts to 5 volts. I took the insulator wrap off of it and all it is is a strip of metal! it has no resistance at all, so is the 12 to 5 volt resistor inside the fuel gauge? that would possibly explain why they say to replace the fuel gauge with the temp gauge.

 

[expertshooter22]

that thing is definately not 2 strips of metal.... and i know that it isn't something that has been changed from original because it looks exactly like every picture i have ever seen.

 

[SteelHeadz]

that thing is definately not 2 strips of metal.... and i know that it isn't something that has been changed from original because it looks exactly like every picture i have ever seen.

I'm thinking maybe the resistor is going to be inside the gauge instead of the jumper being the resistor and a path to a circuit. I could be wrong. when you call in your order for the gauge, ask them. Their technical support should know why that is.

 

[expertshooter22]

here is a really good link in case you ever run into this again... it says in here that the fuel gauge has an internal resistor to power the fuel and the temperature gauge.. I guess meaning that the fuel gauge doesn't operate on 12v either. which also would start to explain why they say to replace the fuel gauge and temp gauge at the same time. but being the type of person that i am i can't just do what is suggested, I have to know why. Hence the reason probably that i have had so much trouble with the thing. So now I know to test the output stud on the fuel gauge and see that it is putting out 5v and if it isn't then i know i need a new fuel gauge. This also explains why you can't but that jumper strap, because of the fact that it is just a piece of metal there is really no reason that it would ever break.

Basic Troubleshooting for CJ gauges

 

[Turbogus]

Holy Schneiglies SteelHeadz, and I thought I spent time on my dash~!