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Unread 06-20-2013, 09:02 PM   #1
jeepmatrix
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1999 WJ 
 
Join Date: May 2012
Location: Hot Springs, AR
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Does the electric radiator fan run after engine is off?

I just found out my electric radiator fan motor is dead. I don't think it ever worked since getting this 1999 Grand Cherokee but engine temp has been okay until recently in hotter weather and heavy stop and go traffic. I know the fan motor is bad but before replacing it I'd like to know if the '99 WJ is like some other vehicles where the fan motor runs for a couple minutes after you shut off the engine. If it does I will add a separate relay and switch to kill power to it when the engine is off. Just 'cause .... I don't care to take chances of an engine not starting due to a run down battery. At a $100 or more a pop for batteries now I use 'em to the last watt

Also anyone who is up to speed on the relay for these please comment on my understanding of the PWM and Solid state relay. Knowing electronics fairly well I know about PWM and solid state relays but as far as I know there is no such thing as a solid state PWM relay as it's often called. To clarify the relay is a solid state circuit that acts like a relay but reacts to PWM sent by the computer. The WJ service manual calls it a pulse width modulated relay and I think because of that it has become known as a PWM relay but technically it's just a solid state relay used in a PWM circuit. Because the computer is putting out PWM a solid state relay is needed to keep up with the speed of the pulses. An electromechanical relay would likely burn out fairly fast trying to keep up with a PWM signal. I'm asking to clarify this because I think any solid state DC relay in this voltage range that can handle the current will work. In case it is bad I'd rather avoid the $80 or more dealer price for a relay I can get for around $5 to $10 if I don't already have one.


Last edited by jeepmatrix; 06-20-2013 at 09:39 PM..
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Unread 06-20-2013, 10:13 PM   #2
michiganmaveric
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No they do not run after the engine is shut off. As for the relay, not sure. It might work but the wiring would be the problem. You would have to rig something up as I am sure the connector is not the same. If you cut the connector off then it does not work, better make sure you have enough wire on the old connector to splice it back in. Look on E-Bay and you might find a cheaper OEM relay.
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Unread 06-20-2013, 11:35 PM   #3
jeepmatrix
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Thanks michiganmaveric - that's what I thought but wanted to make sure. I'm good on wiring and am sure I could hook a generic one up if I can match the specs. I know it does not need to be over 40 amps as that's what the fuse for the fan is rated if I recall correctly. But you are right about them having ones cheaper on eBay made just for the fan relay. I hadn't looked at that option yet so a big Thanks again! I'm seeing them around the $20 range so that is a lot more reasonable if I need one.

I think I did come across some comments on non OEM relays though failing fast. I suspect they don't have a diode or a proper one to prevent DC spikes hitting them from the fan motor when it's not on as it can become a generator when the belt driven fan behind it drives it to turn by the wind.
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Unread 06-21-2013, 03:11 AM   #4
cykaaro
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I'm not sure if that's exactly true though. Had an overheating issue a couple weeks ago, which turned out to be a bad connector at the fan, causing it not to come on, and the relay I recently put in was fine. While the engine was OFF, and no key in the igintion, I wiggled the wires, and the fan came on, which shouldn't be possible to do from my understanding. Of course I questioned what the hell was going on when getting the connector fixed. (and I'm since working on putting in a permanent override switch just in case that won't backfeed into the system). Supposedly, if the temperature is too high and above a certain point, the fan will run longer while the vehicle is off to bring the temperature down to normal operating temp and will turn itself off after a predetermined condition is met. Interesting as it's the first I've ever heard this, but then again, when I overheated before, like many run into, it's the relay so it's not possible for the fan to do this.
System was checked with a DRBIII scanner, and no other issues, so can not see any other probably explaination why the fan would out of the blue keep on running with everything off. First thought was something was short circuiting, but all is fine.

As for the relay, I refuse to pay dealer prices which are worse up here. I always have a few on hand I have from my trips to the wreckers, and it's surprising how many virtually new ones you can actually come across.
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