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1994 XJ AUX Fan circuitry

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Old 07-13-2018, 09:52 PM
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Default 1994 XJ AUX Fan circuitry

Found multiple write ups of the aux fan. However quick question bc mine is not kicking on...

if engine is running and I disconnect the coolant temp sensor connector and it kicks the fan on is that pretty much conclusive evidence that the temp sensor is bad?
I would think so but I just donít know the circuitry as in is the sensor supposed to be closed and opens when it reaches the specific temp. Just curious before I spend $30 on the sensor. Thanks
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Old 07-14-2018, 08:47 AM
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IIRC, it's supposed to do that.
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Old 07-14-2018, 09:21 AM
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So if my xj runs hot (nearly overheating) and the electric fan never kicks on but will turn on if I disconnect the coolant temp sensor...it is then obvisously the coolant temp sensor because disconnecting that is testing that the rest of the circuit works properly, correct.

Im not saying that the electric fan issue is the root cause of my overheat problem but trying to check each issue off the list one at a time.
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Old 07-14-2018, 01:33 PM
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The PCM doesnt know what the engine temps are with the temp sensor disconnected, so it (the PCM) turns the fan on as a fail safe to prevent the engine from overheating.
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Old 07-14-2018, 01:48 PM
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Okay thanks. So I have replaced the relay in the fuse/relay box in the passenger side of the engine bay. Checked every fuse, verified the electric fan works and the previous owner repacked the coolant temp sensor on the thermostat housing as well as the temp sensor on the rear of the cylinder head.

Electric fan still does not turn on as temp rises near redline of overheating?

whats next to check or through at it?

I would rather have it work as advertised than put a switch in line to turn it on.
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Old 07-14-2018, 03:49 PM
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My newly acquired 94 is the same way. Well over 220* and no efan. I did not try disconnecting coolant temp sensor, I am gonna try that. Did check everything else as you did.
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Old 07-14-2018, 04:54 PM
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Iíll let you know if I figure out what is going on with mine...it just might be the same thing.

as far as running hot Iím thinking I may have a bad head gasket and/or a cracked head because everything else is now replaced.
Also thinking of removing the clutch fan alltogether and going with the three way electric fans.

this overheat issue is the only thing going on the rest of the jeep is in great shape so I hope I figure this out soon.
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Old 07-14-2018, 06:30 PM
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So I unplugged the cts and fan came on. Fan also comes on with a/c. I'm starting to think my actual temp gauge is bad. Jeep isn't running as hot as the gauge says. I did replace the cts in rear of head.
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Old 07-14-2018, 08:53 PM
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Jdriscoll, have you disconnected your fan and applied 12 directly to the fan? Don't worry about polarity - if you get it wrong it will just run backwards. You'll know it's not the fan, and it won't hurt it at all.
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Old 07-14-2018, 09:00 PM
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Jdriscoll, have you disconnected your fan and applied 12 directly to the fan? Don't worry about polarity - if you get it wrong it will just run backwards. It won't hurt it at all. This test will tell you if the problem is the fan or not.
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Old 07-14-2018, 09:03 PM
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Yes the fan works. Powering through fan directly was the first thing I did. Fan works. I changed the relay just cause it was cheap and figured it was an easy way to rule it out. Both the thermostat and cylinder head temp sensors are brand new. Also as I said before when I unplug the CTS connector from the thermostat the fan runs...I guess I should replace the sensor again just to rule out the sensor because all seems fine except it doesnít turn on as it should.
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Old 07-21-2018, 11:13 AM
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The aux fan is turned on by the temp sensor in the thermostat housing the gauge is run by a tiny sensor left side of block near back the fan should turn on at 118-225. I just did a calibration test on mine put the sensor in pot of boiling water at the same time I read the resistance of it pin to pin it was 675 ohm. I plugged in 675 to the connector with a decade box and the needle was right at 210 line get your temp up to210 and read the resistance of sensor if radically above 675 it is bad you can do the boiling water test too
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