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Started today, the temp guage would drop like the electric was off, but would jump back up. happened off and on. sometimes for a couple mins, sometimes for just a few seconds. happened when i put on the gas, happened when i was just cruising.
sounds like an intermittent connection. What year is your XJ? If it's RENIX I don't know for sure where the gauge temp sensor is, I think it's on the side or back of the head. If it's OBD I or a 96 I think it should be at the back of the head. If it's a 97+ I seem to recall there only being one temp sensor, since the gauge cluster is computer controlled, and it's the one on the thermostat housing.
All or some of this info may be blatantly incorrect... except for the intermittent connection bit. I'm quite certain that's what the problem is.
instantaneous. figured it was electrical, but not sure if behind the dash or at the sensor/sender/whatever. have a chilton, but its cold outside. looking for info before i dig into it.
Pull the plug off the temp sender on the thermostat housing and clean the contacts on the temp sender. Same thing happened to my buddy's 98...even got to the point one day where he started it up after changing the oil and while he was letting it run it spazzed out so much it made the e-fan come on and the motor shut off (threw a code at this time for the temp sender). After cleaning the contacts, no more problems.
Mine ('97) was doing that and it turned out to be a short to ground on one of the sensor wires at the back of the head. If it's an intermittent problem then it's probably not the sensor. Inspect the harness for rubbing and see if there's an issue.
Drive it around with the gauge pegged. If the gauge suddenly drops you know the problem is an intermittent open in the circuit. If the gauge remains pegged, replace the sender and don't use a ton sealant--it grounds through the body to the head and too much sealant will screw up its readings.
OK--you have a 97. On 97 + my suggestion isn't going to work. You have a combination PCM coolant sensor and temperature gauge sender in the thermostat housing. The PCM controls the gauge if I understand the other posts I have read on --new fangled doo-dads you young whippersnappers get yourselves mixed up with!
I would check with Winterbeater, Kastein, 5-90, ehall, Tim_MN, or BirchlakeXJ on your issue. Send one or all of them a PM.
The resistance values for the sensor across a range of temps have been posted on here before. A quick google didn't find them, but they're around. Someone may be nice and post the values from the FSM- if not, I'll try to find them later- I know I've got it around somewhere. I'd clean the contacts, and check the wiring where it's visible as a start- intermittent function would lead me to think it's a physical issue with the wiring/terminals.
on a 97+ I'm not sure, I'll poke around in the parts catalog/FSM and see if I can find any info. You are right on the CCD bus bit for sure, the 97+ dash is mostly fed via that.
On a 96 down to sometime in the early 90s, it's a 1 wire sender at the back of the head.
EDIT: add MJR to that list, either he remembers way too much XJ tech or works at a dealership, I forget which.
EDIT2: from the 97 FSM -
The instrument cluster circuitry calculates the proper gauge pointer position based on an engine coolant temperature message received from the Powertrain Control Module (PCM) on the Chrysler Collision Detection (CCD) data bus. The PCM uses an input from the engine coolant temperature sensor and internal programming to decide what engine coolant temperature message is required. The PCM then sends the proper message to the instrument cluster on the CCD data bus.
...
The engine coolant temperature sensor is installed in a threaded hole that penetrates a coolant passage of the engine. Refer to Group 14 - Fuel Systems for more information on the PCM and the coolant temperature sensor.
Looks exactly like the table in my FSM. However, I am pretty sure posting a snippet that size (and the size of the quote I posted) is well within fair use guidelines, so I'm pretty sure you are in the clear.
In 2000 FSM, wiring diagram shows a tan/black wire going from the PCM to the temp sensor. Ground is through a brown/yellow wire going back to the PCM. That is a common sensor ground for many sensors. You could put a variable resistor in the plug in place of the temp sensor, adjust it to put the gage in the middle and drive it around a little to see if it still jumps around. You can also measure the resistance if your variable resistor (with an ohmmeter) at the temp points you want to compare in the resistance table. An old volume control from a radio or TV would work fine for a variable resistor.