Hey guys, new forum member here. I recently purchased a 2000 Jeep GC Laredo 4.0L (WJ). I am having an issue with my cooling fan. The fan does not turn on - when I hotwired it to the battery it still did not turn on so I know that the old fan is bad. I decided to go ahead and purchase a new fan since it was only $50. Of course still, the fan doesn't run. I checked all of my fuses (good) and the fan relay under the headlamp. When I checked the fan relay using this guide, the fan relay showed about 8.5v when the A/C was off and showed 12v when the A/C was turned on. As the guide says the relay should be showing 0v with the A/C off, I am not quite sure if the relay is bad. I also checked the voltage at the cooling fan harness (with the relay connected) and was getting the expected 0v when the A/C was off but only ~0.2v when the A/C was turned on. I am wondering if I should try replacing the relay, check for a wiring issue in between the relay and the fan, or something else. Thanks for your help and suggestions.
Old fart with a wrench
The fan relay primary is a digital level 5 volts supplied by the computer and is pulsed to control fan speeds. If you have the normal 2wire fan used on the 2000 4.0L, the fan will not run at all until the engine reaches 210* at which time it comes on at high speed and runs until the temp drops to 195*, then it runs on low speed continually until either the temp raises again or you turn off the engine. HOWEVER, it runs on high speed whenever the A/C clutch is engaged and cycles with it.
Because of the pulsed input, the relay gets extremely hot and that's why it's mounted out in the airstream under the headlight. My brother used computer chip bedding compound under mine when we replaced it and it's lasted over 100K miles so far.
Because of the pulsed input, the relay gets extremely hot and that's why it's mounted out in the airstream under the headlight. My brother used computer chip bedding compound under mine when we replaced it and it's lasted over 100K miles so far.
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Because of the pulsed input, the relay gets extremely hot and that's why it's mounted out in the airstream under the headlight. My brother used computer chip bedding compound under mine when we replaced it and it's lasted over 100K miles so far.
Hey, thanks for your response, but this didn't quite answer my question. As I said, I tried running the AC to see if the new fan would come on. When the AC cycles on, the fan's wiring harness is pushing only 0.2v to the fan, but the PWM seems to be outputting 12v. I'm guessing bad wiring is the cause of the voltage drop?Originally Posted by dave1123
The fan relay primary is a digital level 5 volts supplied by the computer and is pulsed to control fan speeds. If you have the normal 2wire fan used on the 2000 4.0L, the fan will not run at all until the engine reaches 210* at which time it comes on at high speed and runs until the temp drops to 195*, then it runs on low speed continually until either the temp raises again or you turn off the engine. HOWEVER, it runs on high speed whenever the A/C clutch is engaged and cycles with it.Because of the pulsed input, the relay gets extremely hot and that's why it's mounted out in the airstream under the headlight. My brother used computer chip bedding compound under mine when we replaced it and it's lasted over 100K miles so far.
Old fart with a wrench
D'oh! Yeah, that seems to be obvious. Have you tried running a jumper to bypass the harness? If it was the fan ground circuit, you would still see 12v at the fan connector. Are you seeing 12v signal to the fan when the A/C isn't on?
Checking my FSM, it's showing the relay control signal originates in the PCM so maybe for some reason the A/C clutch circuit isn't tripping the engine fan relay to come on with the clutch. I'm grasping at straws here.
Checking my FSM, it's showing the relay control signal originates in the PCM so maybe for some reason the A/C clutch circuit isn't tripping the engine fan relay to come on with the clutch. I'm grasping at straws here.