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  #1  
Old September 9th, 2020, 19:29
Leakyforlife Leakyforlife is offline
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Electric fan problems

1991 laredo 4x4 4.0L. So my stock electric fan is not coming on and I've been checking everything! Tonight I went through the harness with my continuity meter and have some questions for all yall wiring and schematic gurus.

So the first thing I did was test the fan to make sure it was actually working. I bridged at the relay, pin 30 and 87 and the fan came on full power. I checked the resistance of the temp sensor in the thermostat housing at room temp and then with the jeep slightly warm. 8.8k ohm at around 80 degrees and 4.5k with engine warm but not hot. It is a brand new thermostat and the old thermostat at room air measured close at 8.9 ohms.
Next I traced the continuity of the temp sensor back to the main pins on the ECM and found it on the top row left side pin 2 and 4. I'm not sure of the exact pin out so I'm trying the describe what I'm seeing. I had continuity from the sensor wires to the ecm plug.
Then I traced pin 85 on the fan relay in the PDC to the ECM harness and found it on the middle row, right side as pin 1 from left to right with continuity.
The last pin I checked was the fan relay pin 86, which I assumed would be a ground for the relay coil. The interesting thing I found was that this pin goes with continuity to what I think is the chrystler OBD1 port near the ECM and nothing is plugged into it at that connector. The connector has six slots but only 4 terminals installed and a standard auto relay does not fit. One of the pins in this connector goes to ground.
To me it seems this is the missing connection (ungrounded coil from pin 86) and why my fan relay in the PDC is not energizing the coil to turn the fan on. But in my research the connector i am looking at is only for analyzing the ECM. Can anyone confirm this or help point me in the right direction to get my fan to come on when the temp is >218 or what ever the set point is.
I'm sure the engine in this 100 degree weather has gotten that high.
I also know it is somehow wired into the AC system and I do not have the AC compressor hooked up or the dryer for that matter. I have tested trying to get the fan on with the AC on but no luck.
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Old September 10th, 2020, 00:50
tempest411 tempest411 is offline
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Re: Electric fan problems

Unless you see evidence that the wiring has been modified, I'm sure you're not missing any connections. Have you found a schematic for your year model? I have a Renix-era XJ, so my fan circuit is a stand-alone system. Have you tried simply changing the fan relay? Or at least checking the trigger wire from the ECU with an LED test light to see if the ECU is sending a signal when it's good and hot? However, one thing mine and yours would have in common is the A/C override that should turn on the fan. This part won't work though if your system isn't charged up enough to close the pressure switch. You can bypass this by making a jumper for the pressure switch connector. Make sure you have the wire to A/C clutch itself disconnected so it doesn't come on. Without refrigerant and oil going through it you'll trash it pretty quick.
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Old September 10th, 2020, 06:06
Leakyforlife Leakyforlife is offline
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Re: Electric fan problems

Good idea, ill check the ECM output today. Do you know if the output from the ecm is a 5 or 12v signal to turn the relay on?
I attempted bridging the AC pressure switch but it did not make the fan come on.
Is the ECM switching the relay through grounding or positive voltage? I think I read it was grounding the relay coil but I could be mixed up.
I still don't understand why I have continuity from the relationship pin 86 to the OBD1 connector and when I checked the wire colors they are the same white with blue strip wire.
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Old September 10th, 2020, 19:30
Leakyforlife Leakyforlife is offline
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Re: Electric fan problems

I bridged the AC pressure switch, no fan.
Bridged the temp sender, no fan.
Took the ECM connector off and ran 12v through pin 31(output for relay coil), no fan.
Grounded the OBD1 blue/white stripe wire, Fan came on...
Idk what is going on, it makes sense but why would the ground for the fan relay coil be going to the OBD1?!
Left ac and temp sensor bridged and attempted to get the engine to 218 but even with the rad blocked off couldn't get it hot enough in this cool night air.
I was not ever able to get any output from the ECM pin 31 no matter what was bridged. I even swapped to another working ECM and no output to the relay coil.
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Old September 11th, 2020, 09:55
Jim Malcolm Jim Malcolm is offline
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Re: Electric fan problems

Everything else appears to be working as it should. If you apply ground at the ECM (I believe pin 31 from your previous), the fan should turn on, not power. At operating temperature, the resistance of the temp sensor should be less than 1k ohm; at 4.5k, the ECM thinks it's ~110F. I suspect the wiring is brought out to the OBD connector make it easy to test the wiring without needing to warm up the engine.
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  #6  
Old September 12th, 2020, 06:52
Leakyforlife Leakyforlife is offline
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Re: Electric fan problems

Yeah I'm worried the after market temp sensor is off. Headed to the JY today and I'm gonna pull a couple and hopefully get a factory oem one to try out. I'll test grounding pin 31 to see if it kicks the fan on, but i know 12v at pin 31 with the obd1 grounded at the blue/white wire kicked the fan on.
If that doesn't work, screw it and ill just put in an external fan regulator as I don't really want to have an interior switch unless its just for backup secondary to regulator failure.
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  #7  
Old September 12th, 2020, 08:20
89Mock 89Mock is offline
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Re: Electric fan problems

You bridged the air conditioning and you didn’t have a fan come on? I think that’s a problem itself. My air doesn’t work so my fan doesn’t come on when I turn the air conditioning on. It DOES come on when I bridge the connection though.
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