Temperature Gauge for 98 XJ Limited

A few weeks ago during a massive rainstorm I drove through some standing water on the road and dunked my alternator. My check gauges light came on for 3 seconds as my voltmeter dropped. After 3 seconds it came back to normal and the check gauges light went off. I searched the forum at that time and found it had happened to others. Since then, it has not repeated and the gauge reports normal charging.

Now, around this time during the same rain storm (2 days later) I noticed my temperature gauge was fluctuating. It normally reads just below halfway at about 200 degree and stays dead steady. On this day is dropped to about 125 degrees and stayed there, only to return to 200 a few moments later. The next day (storm finally over) the problem did not repeat and the XJ went about it's business for weeks. Over the last couple of days the same thing has happened twice now (no rain involved). I'll be driving and the engine will warm up naturally (I drive 20 minutes on the freeway twice a day). The temp gauge will fluctuate between normal, and around 125 degrees, or a quarter of the way up the gauge. What could be causing this:

Grounding issue in the cluster
Temp sending unit or temperature sensor
thermostat issue

The radiator and the thermostat are one year old and were replaced when a leak started in the radiator (XJ never overheated)

possibly got a ground dirty, could have blown the resistor in the sensor/sender as well

what year?

I would start at the battery, cleaning connections. Same with the alternator, clean and add dielectric grease for waterproofing. Got an air hose? Blow out the alternator, although it is probably pretty dry by now. Keep going on cleaning connectors and adding dielectric. Underside of the PDC may need some TLC too. Just keep hitting the connectors, it should straighten up.

Must be a 98 thing ...

Had the same issue a while back ... cleaning as mentioned above fixed it.

In my case ... a clean of all the scale off the temp sensor probe and a spray of the connector plugs with cleaner and water displacer was all that was nescessary.

No more random, half temp readings.

Thanks for the great advice everyone. I'll clean as mentioned above and monitor. I'm glad to hear it's nothing too serious. My XJ has 148K and I need it to hold together for at least another few years.