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arosier 11-10-2007 08:00 AM

2003 Jeep Grand Cherokee overheating
I have a 2003 WJ 4.0L L6 2wd with a problem. It started overheating one evening when I came to a stop. The cooling was boiling out of the overflow resivior. I changed out the thermostat the next day and it ran fine for the next 20 minutes or so of driving. As soon as I came to a stop and it started overheating again. I then checked to see if the cooling fan was operational and it would not ever turn on. I had the vehicle towed to a garage for a basic diagnostic test. It turned out that I had a bad cooling fan motor and cooling fan relay. I switched both of them out and the cooling fan was back in business. I let the vehicle sit idling for 20 minutes with the AC running and the fan working fine. Then the fan cut out again. I am running out of ideas on what is causing this. I plan on changing out the coolant temp sensor on the thermostat housing later today. Someone told me it could be cooling fan control unit. I have never heard of this part and the guy couldn't tell me where it was located on the vehicle. Any comments would be greatly appreciated. Thank you.


indept 11-10-2007 11:11 AM

Do a search, I've posted a lot on this. Check the fan relay electrical connector. I had mine burn up where the pins were corroded. Jeep sells a replacement for about $15 that you cut the old one off & put this one in. You have to crimp the pins on to the wires. It comes with 2 large gauge & 2 small gauge wires. Make sure you use the large for the fan circuit & the small for the control to the relay which doesn't carry as much current as the fan itself. Also check the fan connector for the same corroded issue. That connector is on the passenger side just under the top hose.:cheers2:

arosier 11-10-2007 09:05 PM

The relay that I switched out from under the right headlight looked like this:,APP1286327...ductDetail.htm

There are no wires sticking out of it; I just used the connector on it. Are we talking about the same device?


indept 11-10-2007 09:42 PM

You have the right thing. Take the connector off & look at it. Do the pins on the connector look clean or do they look corroded. The wires I spoke of come with the new replacement connector. The relay looks like it has the male pins so it may be difficult to see any corrosion. Try disconnecting the connector & reconnecting it then check & see if the fan starts working by running the engine. If so, let it cycle a few times, if the fan stops working, wiggle the relay connector & see if it starts again. That will prove it's the connector bad. Can you get the old connector from the mechanic to check the male pins on it?

arosier 11-11-2007 07:25 AM

I will definitely double check that. When I switched out the relay I did not look at the connector because I accessed it by removing the battery and the headlight. I might just remove the bumper this time for easier access. One more question: Does the fan motor receive 12v or does the relay step it down? Thanks so much. Having one vehicle is not very amusing. I appreciate it.

indept 11-11-2007 09:49 AM


Originally Posted by arosier
I will definitely double check that. When I switched out the relay I did not look at the connector because I accessed it by removing the battery and the headlight. I might just remove the bumper this time for easier access. One more question: Does the fan motor receive 12v or does the relay step it down? Thanks so much. Having one vehicle is not very amusing. I appreciate it.

It's 12v Pulse width modulated. It gets pulses going from 0 to 12v the wider the pulse the faster the fan turns. You can run the fan with straight 12v (as opposed to gay 12v:laugh: ). To get to the relay, just remove the headlight assembly & if it's not done already cut a hole in the plastic panel to get to it. That's what everyone does with these. No need to remove the battery or the bumper.

01black 11-20-2007 12:56 AM

okay guys im new here and im having the same problem. my 01 JGC is overheating, the problem is my fan not turning on. I rewired the fan straight to the battery to test it and fan worked fine. so i replaced the coolant temp sensor today and the fan is still NOT working.:brickwall This is the first time i hear about this relay thing and it sounds like its going to be my next move.
Hope u can help me. What is the name of this relay? Where is it located?
How much does it go for?

Thanx in advance.

Rica1981 04-06-2010 10:27 PM

Did you ever resolve your overheating issue Arosier username? My Jeep is doing the same thing with overheating. Please assist.


pjohn 05-25-2010 08:10 PM

I have a 2002 jeep grand cherokee and it's doing the same thing also.....any solutions?

Rica1981 05-26-2010 12:04 PM

I never heard from Arosier PJohn. I guess he resolved his issue. Mine has been acting okay since I got my thermostat replaced. I couldn't give you all the specifics as my buddy did it for me while I'm deployed here to Iraq. Hope you find your cause as well. Alot of these threads proved helpful as I was telling my buddy the possible solutions with his mechanics skills to resolve my issue.

thudson331 05-26-2010 09:04 PM

i had this happen and it wasnt a bad job replacing the relay. had to take off the bumper, light (rh) and grill assy. works perfect now:)

Pazdew01 06-26-2010 09:12 AM

I am working on replacing the fan relay right now on my 03 laredo and just thought I would share some information with you guys. I took it all apart and unscrewed the relay from the body... then when I opened the package containing the new relay straight from the dealership I found warnings all over it that state the relay is to be RIVETED ON not SCREWED ON or it will cause the relay to fail and the vehicle to overheat. This irritates me because the dealership replaced my fan and relay two years ago and the date on the instruction sheet containing that warning are from 1999. So I am waiting for the dealership to call me back so I can demand they reimburse me for the price of the relay and rivet it on for free, as I don't have a rivet gun.

indept 06-26-2010 05:09 PM

I don't care if you screw it , rivet it, or hold it in with an elephant sitting on it, it's going to fail where it is. That relay gets to well over 200 deg's. There is no where for the heat to go there on the fender well. I relocated mine to just in front of the AC condenser just behind the grill. I got an old Pentuim II heat sink & mounted it there with the heat sink fins facing up into the air flow. It went from over 200 deg's to around 100 deg's. Heat is the enemy of solid state devices & that IS a solid state relay. Besides, the elephant approach might work, but you have to feed them regularly, which can be a pain in the arse.

jgc86 01-31-2011 12:38 PM

Hey I have had overheating issues also for awhile now, the vehicle overheats at idle. I replaced the fan relay and the thermostat. Just found out that the connector for the fan relay is faulty and has a replacement at the dealership. It shorts out the relay cause the fan not to kick on. The replacement costs around $120. I havent bought it yet to try it but that is on my list. Good luck hope this helps. Oh the dealership guy said something about having to reverse polarity on the new harness to get it to work right.

sixmikey9 01-31-2011 11:06 PM

My problem after replacing fan and relay was my idle was waay too high, 2k rpm. Ended up being Iac motor wiring shorting to +voltage in the wiring harness (by the valve cover on the driver's side) Hope this helps!

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