Hello questor and "WELCOME" to Jeepz.com,
I am not afraid to attempt a "help" on this problem. I survived helping to raise three kids to adulthood -- nothing scares me now!
questor, there isn't really a whole lot to the overhead console compass-thermometer thingy.
First, let me try to briefly describe how the compass-temp display system works.
If your setup is like most late model Chrysler products, there is one roughly 6 x 3-inch printed circuit board (PCB) inside of the overhead console. That PCB contains all the electronics for displaying compass heading, outside temp and the misc. DTE, gallons-to-empty, elapsed time, misc other eye-wash functions and the "Mode," "Step" and other push-buttons.
Here is a picture of a compass-temp-function PCB from a 99 Chrysler, Town and Country. The black box above the PCB is a Radio Shank "handi-box." The PCB will be installed in the handi-box and then the whole assembly will be installed above the inside rear-view mirror of my 85 CJ-7. A sort-of mini overhead console. It is not for nothing that my name is "Inspector Gadget."
Only the compass and temp functions will be operative in my CJ. Enabling the other PCB functions would require all of the sensors and the main computer of the Chrysler van from which the PCB was removed. I am not going there!
Note that this PCB is not the vehicle main computer: the overhead console PCB is an ancillary computer.
There is also a temperature sensor mounted in the front of your vehicle. The temp sensor is usually mounted in the grill area, behind a headlight, or sometimes behind the front bumper. The electrical resistance of the temp sensor varies in accordance with outside ambient temperature.
Unfortunately, I cannot supply a picture of the temp sensor. I have already mounted it in the grill area of my CJ, but the temp sensor is about 3-inches long, black in color and shaped like the back end of a fountain pen.
The overhead console PCB applies a nominal +6 volts to the temp sensor via a wire which runs from the PCB to the temp sensor. A second wire returns from the temp sensor to the overhead console PCB (chassis ground is not used for an electrical return in this circuit). In a round-about way, the PCB combinational and computational logic senses the temperature dependent resistance of the temp sensor and converts that resistance value to the temperature reading that is displayed in the overhead console.
There you have the "Reader's Digest" version of how your compass-temp display works. So, you really have three things to deal with:
(1) The outboard temp sensor. (About $20.00 at a dealer, about $5.00 at your local Pick-and-Pull.
(2) The PCB mounted in the overhead console. (You should be sitting down now -- up to a couple of hundred dollars at your dealer or much, much less at the Pick-and-Pull.)
(3) The wiring which interconnects (1) and (2).
A quick system test: locate and disconnect the plug and socket at the grill mounted temp sensor. The overhead console thermometer should then show the letters "O" and "C," OC meaning -- Open Circuit. (Who would ever think of that?!?!) If the temp reading displays "OC," then it is likely (BUT NOT CERTAIN) that the overhead console PCB electronics are functional. Replace the outboard temp sensor.
Here is a pic of the PCB in the handi-box, powered-up, but with the temp sensor disconnected. I know the display is a bit hard to discern in this picture, but the display is showing "NW" (Northwest) and "OC" (Open Circuit.)
Trouble shooting the PCB electronics is well beyond scope of anything I would go into here. But, if it turns out the PCB is the likely culprit, perhaps the easiest and least expensive way to deal with the problem would be to swap out the PCB with one from the local Pick-and-Pull. As I mentioned earlier, several late model Chrysler products use the same PCB. It would, of course, be best to obtain a PCB from a vehicle identical to your Jeep.
The fact that you have a changing temp reading, even though incorrect, implies that the PCB to temp sensor wiring is OK.
The PCB is electrically connected to the vehicle wiring via a plug and socket, so it is no big deal to disassemble the overhead console to gain access to and remove the PCB.
NB: The PCB is electrostatically sensitive. Observe all the usual handling precautions when messing around with the PCB.
I hope the above helps and feel free to ask for additional information.
Regards,
Gadget
PS: I was going to mention what follows earlier, but didn't for fear of coming off as a smart-a$$: For the sake of completeness, check to be sure the display is set for Fahrenheit and not Celsius. (Tap the "Mode" buttton to cycle between degrees "F" and "C."
