juan Posted March 9, 2006 Share Posted March 9, 2006 The auxiliary fan in my 88 comanche 4.0 5 speed is not coming on. I have checked all the connections and ground and all checked out. I diconected the fan and "Jumped" it and it worked so I checked the next thing down the line which is the relay on the side of the air filter - the relay checked out fine with a voltimeter next I checked for power at the relay and I think here is my problem because I do not have a constant 12V at the relay (red wire) so I follow the red wire all the way to the other side of the engine and it terminates at the a/c relay - do not understand because as far as I know it is supposed to be a source of direct 12 volts. The book refers to a fusible link at the end of this red wire but I could not find it! where is the power coming from to the aux.fan? anybody trobleshoot this before? any help is apricieted. Is there a fuse I'm missing here? Link to comment Share on other sites More sharing options...
ROADLESS Posted March 9, 2006 Share Posted March 9, 2006 Yea, i'm doing the same thing with my 89 MJ. If you trace one of the red wires from the aux. fan relay, it should have a green fusible link on the end of it where it terminates (or begins power wise) and that is at the starter solenoid by the front passenger area of engine compartment (along with a birds nest of other fusible links). The FSM has a good procedure for checking it :? Link to comment Share on other sites More sharing options...
holeski Posted March 9, 2006 Share Posted March 9, 2006 The relay is connected to the motor temp sensor. When that temp sensor hits it’s happy point it will either send power to the relay or ground the relay witch in return sends power to the fan. The older 4.0L have the temp sensor on the top-rear driver’s side of the block. Newer ones are on the thermostat housing. The older one is known for doing stuff like that and sending false readings. I run both on our XJ the one on the block sends info to the computer and the one on the thermostat housing is connected to the fan relay. The fan works much better that way, plus I can hook a switch to it if I need to. Link to comment Share on other sites More sharing options...
juan Posted March 9, 2006 Author Share Posted March 9, 2006 Roadless - you are right and that is exacly how the book describe it but when I trace the red wire all the way to the passanger side of the engine - this red wire goes into the A/C relay - this confuses me because the AC relay does not connect with any fusible links - I mean I see all of them right next to the battery - I pulled on them but they do not seem to be broken, is there a way to test these fusible links one by one - I spent many Hrs. going around in circles last night :( Link to comment Share on other sites More sharing options...
ROADLESS Posted March 9, 2006 Share Posted March 9, 2006 juan, hopefully this will make a little more sense. The aux fan is run from two circuits, one is the sensor in the radiator (which sends a signal to the relay you already tested) and the other off the AC/defrost circuit. The fan will only run when the sensor gets to a certain temp, or whenever the AC/defrost is turned on (the other circuit). This is why the red wire ties into the AC relay... With the engine running, turn on the defrost or AC (thereby completing the circuit). If the fan does not start, listen if the AC relay is clicking on and off... If not you should check the AC relay as well. This will help determine what to check for next, should the relay turn out OK. Link to comment Share on other sites More sharing options...
juan Posted March 10, 2006 Author Share Posted March 10, 2006 That make sense.... thanks!!! :idea: Link to comment Share on other sites More sharing options...
CWLONGSHOT Posted March 10, 2006 Share Posted March 10, 2006 Juan, If you can, bring the MJ tonight. I'll throw in my electric tool box and we can test the circuts with the test lead and continuity. CW Link to comment Share on other sites More sharing options...
juan Posted March 11, 2006 Author Share Posted March 11, 2006 All done, thanks CW!!! We found out that the fan diode was bad and that the fan relay does not have a 12v constant source of power unless the truck is running - once we figure it out everithing checked out. I have to get a new diode and I'll be all set. Link to comment Share on other sites More sharing options...
ROADLESS Posted March 11, 2006 Share Posted March 11, 2006 All done, thanks CW!!! We found out that the fan diode was bad and that the fan relay does not have a 12v constant source of power unless the truck is running - once we figure it out everithing checked out. I have to get a new diode and I'll be all set. Glad to see your MJ problem fixed. BTW, how did you test the diode? I can't find how from the FSM. Link to comment Share on other sites More sharing options...
CWLONGSHOT Posted March 12, 2006 Share Posted March 12, 2006 Technically, the diode is still working. But the wires broke off, leaving too little to re-attach. I believe he had a fusable link that was toast...we just could not find it. We ran a test jumper and the bugger fired right up!!! I added one to my MJ today. I wired it is as if it was a set of lights or other accessory, you know with a relay, done right. then I added the temp sender to the rad, ran that as my trigger with a seperate 12V switched line so I can also run it when I want to . :) My MJ has no AC so I did not have the aux fan assmbly. I got all the parts out of a '90 XJ I parted out. CW Link to comment Share on other sites More sharing options...
ROADLESS Posted March 12, 2006 Share Posted March 12, 2006 Good idea, I'll probably wire similar. Its good to be able to manually operate the fan :-D Link to comment Share on other sites More sharing options...
Pete M Posted March 12, 2006 Share Posted March 12, 2006 Its good to be able to manually operate the fan :D Amen to that! I almost lost my engine to a fan that wouldn't turn on. Now I just flip the switch and the aux fan comes to life. Flip another switch and the hood vent fan comes on. I like switches! :D Jeep on! --Pete Link to comment Share on other sites More sharing options...
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