Fan not turning on when hot- fan works!

[Reddleman]

95 XJ 4.0, auto.



I don't drive this much when salt is on the road but use it mostly for hauling and local work. I was starting to get hot readings when pushing it (like going up a long hill). Gauge would read between 210 and the next hash mark, sometimes touch it. Never pushed it into the red since this started happening.

I checked with an infrared thermometer and it read 232 at the heat sensor on the thermostat, and in the 220s before and after the thermostat, so it's definitely running hot. The electric fan never came on. It does come on when I use the defrost or A/C, and the temps start to drop on the instant read (below 212) within a minute, but the gauge still reads high.



I'm going to refresh the whole system (thermostat, water pump, fan clutch, radiator, hoses, flush heater core, etc.) when my rockauto order comes in, but I want to make sure the electric fan works as a backup.



What am I missing?

 

[Rob Mayercik]

Sounds like the sensor that's supposed to activate the fan may not be working.

Try measuring resistance across that sensor on the tstat housing when cold and then again when at operating temperature. If you get the same reading both times (identical, or within a couple ohms), the sensor's probably gone bad.

 

[JMA_2001XJ]

Sounds like the sensor that's supposed to activate the fan may not be working.

Agreed, most simple explanation.