1991 Jeep Cherokee Engine Cooling Fan Only Works With Heate

Tiny
FLYR
  • MEMBER
  • 1991 JEEP CHEROKEE
  • 6 CYL
  • 4WD
  • AUTOMATIC
  • 175,000 MILES
My son's Cherokee is a recent purchase "fixer upper". It runs strong and is in good mechanical shape now but for one thing. The electric engine cooling fan only turns on and off if the heat or vent positions on the cars interior heater are selected. It runs about 195-210 on the gauge when driving, but if allowed to sit and idle it will begin to overflow through the coolant tank. If I turn the heater on at that point the electric fan comes on and it immediately starts to cool and soon stops overflowing. We live where the outside temperature is usually in the nineties, and would really like to get this worked out. Any help is greatly appreciated.
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Friday, July 17th, 2009 AT 10:46 AM

5 Replies

Tiny
JDL
  • EXPERT
Hello, you might check the coolant temp sensor, maybe in the thermostat housing? Wiring colors show to be black with lite blue tracer and tan with black tracer.

Can you check for codes?
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Friday, July 17th, 2009 AT 11:27 AM
Tiny
FLYR
  • MEMBER
I can't check for codes. I do have the sensor in the thermostat housing with the wires you identified. If that is malfunctioning wouldn't my temp gauge be inoperable also, or does the guge info come from a different sensor? Thanks
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Friday, July 17th, 2009 AT 11:51 AM
Tiny
FLYR
  • MEMBER
I can't check for codes. I do have the sensor in the thermostat housing with the wires you identified. If that is malfunctioning wouldn't my temp gauge be inoperable also, or does the gauge info come from a different sensor? Thanks
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Friday, July 17th, 2009 AT 11:53 AM
Tiny
JDL
  • EXPERT
Previous to 94, my wiring diagrams are lacking, I don't have a wiring diagram for your temp gage. I did look at an online partsbook for your year--make--model for temp switch and temp sensor, it showed two different parts?

The codes probably don't matter, but, did you try cycling the ignition switch? On--off, on--off, on--off, on. You turn the key till the dash lites come on, no further, then the mil starts flashing, you count the flashes to get the codes.
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Friday, July 17th, 2009 AT 1:37 PM
Tiny
FLYR
  • MEMBER
No luck with reading the codes via ignition switching method. I did however experiment with the temp sensor at the thermostat housing. It does NOT seem to affect the temp gauge when disconnected. With the car running if I unplug the sensor, after three seconds the cooling fan comes on and runs continually. Upon plugging the sensor back in the fan quits again after three seconds. I have ordered a new sensor and will try that next. I will update on progress or lack of progress as soon as I know more. I have a "heat reading gun" that reads temps of whatever it's aimed at. If you know the temp the fan should come on at I can check and see how hot it really is when it boils over. THANKS
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Friday, July 17th, 2009 AT 7:17 PM

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