Radiator Fan Override Switch (The Easy Way)

fyrfytr1717 said:

kastein, I think you're thinking this is the delay timer thread...



This whole thread is about how to wire up an override switch. Just set up your "probe relay" in place of the override switch so that it triggers the fan circuit relay. If you want to do both, I don't see why you couldn't wire it up so that either your probe relay or the override switch would activate the fan circuit relay.

The best thing to do though would be to figure out why your fan isn't activating like it's supposed to and fix the problem rather than working around it. Then wire up an override switch if you still want/need it.

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crap, I probably am :dunce: I tend to get em confused.

Agreed that the best fix is to actually fix it, I have seen two different XJs now with horribly rednecked override switches that actually made things worse. On one the previous owner did it and never told the next owner (a friend of mine), the wiring quality was horrible and the fan was completely nonfunctional even if you could find the switch. We put it back to stock and installed a new temp sensor and it worked perfectly again :doh:

Did my 98 today and it worked out perfectly! The only trouble I has was locating the correct blue and white start run wire. Other then that it was smooth and painless. Thanks for the write up

Sweet, glad to hear it worked for you! Almost 10 months with this set up now and still no problems or CEL's. I'm confident it works as I intended.

So I tried this the simple way without a relay and just splicing into the DB/PK wire to a grounded switch and got a CEL in my 99. Funny thing was, I cleared the code with my scanner and the fan override stopped working, but I did not have a CEL. After unplugging the battery, the CEL came back and the override switch worked.

So, long story longer, I need to do a relay. I should be able to figure this out thanx to this thread. Seems like 99s don't like the quick an easy way.

Thanks for the confirmation that this won't work without the relay. At least I know for sure now that the extra work involved wasn't for nothing.

I have been a little busy, but I did finally finish this project. Works great!

Do you have to ground the relay in order for this to work? I noticed it worked before I hooked up the ground to #30.

Without 30 connected to ground your override switch will still work. However, your PCM won't be able to turn on your fan. It shouldn't throw a code, but it will definitely contribute to overheating if you don't turn your fan on manually when your engine starts getting hot.

So my Jeep is not a daily driver and I went wheelin the other day and noticed my CEL came on when I used the AC. Turns out it didn't like this setup. I have double checked everything including hooking up my ground source and to no avail a CEL everytime. So, I gave up put everything back the way it was. I will have to try something else to keep my jeep cool in the summer time.

Thank you for the write up and the follow-up help though!

Again my Jeep was a 99 Classic and I have seen elsewhere the 99s do not like this mod.

Hmmm, strange... Seeing as we both have '99 Classics I don't see why this wouldn't work for you if wired up correctly. The easiest mistake I could see being made is tapping into the the wrong Dark Blue/White wire. It took me a little trial and error to figure out which one I needed as it was burried deepest in the loom. The only other thing I can figure is maybe the relay I'm using is making the difference as it has an internal diode in it? I really didn't (and still don't) think the diode is/was necessary, but that is the only other variable I see. I guess if it you've already pulled it all out this isn't of much help to you, but maybe someone else can use the info. Thanks for the update.

IMPRZV1 said:

So my Jeep is not a daily driver and I went wheelin the other day and noticed my CEL came on when I used the AC. Turns out it didn't like this setup. I have double checked everything including hooking up my ground source and to no avail a CEL everytime. So, I gave up put everything back the way it was. I will have to try something else to keep my jeep cool in the summer time.

Thank you for the write up and the follow-up help though!

Again my Jeep was a 99 Classic and I have seen elsewhere the 99s do not like this mod.

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I actually worked through this and you can find the information here link

I've been running about 3 months with this setup and no CEL codes.

Hey Mudderoy, been awhile since I've seen you post up here. I imagine XJTalk takes up a majority of your forum time these days. I appreciate your taking the time to provide some feedback. Anyways, I was reading your version of my write up that you linked to in your post and had a couple questions.

I saw that your work around was hooking the relay coil up directly to the battery instead of using the DB/WT wire that I recommended. That should work too as you just need a 12V+ source to make the relay work, but my reason for using the DB/WT is that it is an ignition switched 12V+ source. If you have it hooked up to the battery instead, you could potentially forget to turn the override switch off and cause your battery to drain when you shut the vehicle off. Granted a relay coil doesn't draw much power, but the fact is that the relay coil would remain energized and it would be a small power draw that would eventually leave you with a dead battery. If you don't want to use the DB/WT wire, I'd suggest finding another source of ignition switched power or setting up an ignition switched bus bar for accessories that you don't want to accidentally leave on when the ignition is shut off.

Second, you recommended installing a 10 amp fuse between the switch and the ground. Why? A relay only draws about 0.16 amps so a 10 amp fuse would do very little for you. Additionally placing the fuse where you suggested offers no protection to anything except the short piece of wire connecting the fuse to the ground. You are however correct that you should install a fuse when using your suggestion of hooking the relay up directly to the battery, but it should be as close to the battery as possibly and only needs to be a 1 amp fuse to protect the relay and associated wiring. Using the DB/WT wire for relay power takes advantage of the fact that the DB/WT wire is already a fused circuit.

Again, thanks for the feedback. Maybe I'm just missing something and you can clarify for me?

fyrfytr1717 said:

Hey Mudderoy, been awhile since I've seen you post up here. I imagine XJTalk takes up a majority of your forum time these days. I appreciate your taking the time to provide some feedback. Anyways, I was reading your version of my write up that you linked to in your post and had a couple questions.

I saw that your work around was hooking the relay coil up directly to the battery instead of using the DB/WT wire that I recommended. That should work too as you just need a 12V+ source to make the relay work, but my reason for using the DB/WT is that it is an ignition switched 12V+ source. If you have it hooked up to the battery instead, you could potentially forget to turn the override switch off and cause your battery to drain when you shut the vehicle off. Granted a relay coil doesn't draw much power, but the fact is that the relay coil would remain energized and it would be a small power draw that would eventually leave you with a dead battery. If you don't want to use the DB/WT wire, I'd suggest finding another source of ignition switched power or setting up an ignition switched bus bar for accessories that you don't want to accidentally leave on when the ignition is shut off.

Second, you recommended installing a 10 amp fuse between the switch and the ground. Why? A relay only draws about 0.16 amps so a 10 amp fuse would do very little for you. Additionally placing the fuse where you suggested offers no protection to anything except the short piece of wire connecting the fuse to the ground. You are however correct that you should install a fuse when using your suggestion of hooking the relay up directly to the battery, but it should be as close to the battery as possibly and only needs to be a 1 amp fuse to protect the relay and associated wiring. Using the DB/WT wire for relay power takes advantage of the fact that the DB/WT wire is already a fused circuit.

Again, thanks for the feedback. Maybe I'm just missing something and you can clarify for me?

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Hey thanks for the reply. I purposely didn't mention my site as I didn't want to be doing FREE advertising on another Jeep XJ site.

The site pretty much runs itself, so I've been spending more time setting up for a live ustream show and podcast.

This is a great mod and it really helped me get off my a$$ and get her done. I had a problem with the check engine light initially and was really happy when I finally got that resolved.

I didn't want to hook the relay up to the 12 volts directly but having it connected to a 12volt source that was there and not there seemed to be telling the ECM that there was a problem, therefore the check engine light would come on. After a few weeks of staring at that light and thinking about what my OBD II scanner was telling me it dawned on me I needed a constant 12 volt source going to the ECM. I can only assume that I am correct since providing that constant source resolved the check engine light issue.

As for the 10 amp fuse, well since I have running the ground wire to the switch when I bypass the relay and turn the fan on, grounding it, I am running the full current of the electric fan through that ground wire and the switch.

I probably need to install another relay and let all the current be there under the hood. I was concerned about the wire running to my over head switch box becoming light bulb filament and glowing a bright white just prior to the headliner fire.

I'm doing all this by memory. I need to go back and look at the diagram and re-read my post.

Mudderoy said:

The site pretty much runs itself, so I've been spending more time setting up for a live ustream show and podcast.

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Wow, sounds like you're keeping busy with the online stuff. Great that you're doing so much to keep the XJ community alive. Figure it needs all the help it can get seeing as they've been discontinued for a decade now...

Mudderoy said:

As for the 10 amp fuse, well since I have running the ground wire to the switch when I bypass the relay and turn the fan on, grounding it, I am running the full current of the electric fan through that ground wire and the switch.

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I think you're a little confused as to what my write up (and your version of it) actually accomplishes... The whole point of this mod is to override the factory system. You are not activating the fan when you flip the switch, you are activating the factory relay which in turn activates the fan. That DB/PK wire does not connect to the fan, it connects to the relay that the PCM is supposed to trigger to turn the fan on. If it did, a 10amp fuse would blow every time the fan came on as the factory fan circuit uses a 40amp fuse. Regardless, you should always place a fuse as close to the power source as possible. It can only protect whatever components are downstream from it.

Mudderoy said:

I didn't want to hook the relay up to the 12 volts directly but having it connected to a 12volt source that was there and not there seemed to be telling the ECM that there was a problem, therefore the check engine light would come on. After a few weeks of staring at that light and thinking about what my OBD II scanner was telling me it dawned on me I needed a constant 12 volt source going to the ECM. I can only assume that I am correct since providing that constant source resolved the check engine light issue.

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The PCM can only activate the fan relay when the ignition is on. The DB/WT wire (so long as you tapped into the right one) is only on whenever the ignition is on. The PCM would never see it as "there and not". If you look at the FSM based wiring diagram I provided in the first post, you'll see that the factory relay is powered by a ST-RUN source. Using the DB/WT wire (which is also a ST-RUN source) exactly mimics the factory set up. Again, a constant 12V+ source will work fine, it just opens up the possibility of draining your battery if you forget to turn the switch off.

As for your fix solving the CEL issue, I can only figure that either you were using the wrong DB/WT wire (which incidentally is "there and not" while the ignition is on) or the integrated diode in my relay is preventing the CEL. I'll have to try putting in a normal relay and see if it causes a CEL to find out for sure. To make sure you were using the correct DB/WT wire, perform a continuity test with your DVM between it and Pin A2 on Connector 1 in the PCM as detailed in the write up.

Mudderoy said:

I probably need to install another relay and let all the current be there under the hood. I was concerned about the wire running to my over head switch box becoming light bulb filament and glowing a bright white just prior to the headliner fire.

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No need to worry about fireworks in your headliner, as I mentioned in my earlier post a standard Bosch style automotive relay only draws about 0.16amps. That plus power for the indicator light in your switch (if so equipped) is all the current that is running through that wire. Shoot, I'm only using 24g wire for my switch and that still provides a significant safety factor. Fortunately for you (and me), that whole circuit is already protected by the fuse protecting the factory fan relay. The 10amp fuse you have in there won't hurt anything, it just isn't going to do anything. You should still consider fusing the 12V+ side of your relay coil though.

fyrfytr1717 said:

Wow, sounds like you're keeping busy with the online stuff. Great that you're doing so much to keep the XJ community alive. Figure it needs all the help it can get seeing as they've been discontinued for a decade now...



I think you're a little confused as to what my write up (and your version of it) actually accomplishes... The whole point of this mod is to override the factory system. You are not activating the fan when you flip the switch, you are activating the factory relay which in turn activates the fan. That DB/PK wire does not connect to the fan, it connects to the relay that the PCM is supposed to trigger to turn the fan on. If it did, a 10amp fuse would blow every time the fan came on as the factory fan circuit uses a 40amp fuse. Regardless, you should always place a fuse as close to the power source as possible. It can only protect whatever components are downstream from it.



The PCM can only activate the fan relay when the ignition is on. The DB/WT wire (so long as you tapped into the right one) is only on whenever the ignition is on. The PCM would never see it as "there and not". If you look at the FSM based wiring diagram I provided in the first post, you'll see that the factory relay is powered by a ST-RUN source. Using the DB/WT wire (which is also a ST-RUN source) exactly mimics the factory set up. Again, a constant 12V+ source will work fine, it just opens up the possibility of draining your battery if you forget to turn the switch off.

As for your fix solving the CEL issue, I can only figure that either you were using the wrong DB/WT wire (which incidentally is "there and not" while the ignition is on) or the integrated diode in my relay is preventing the CEL. I'll have to try putting in a normal relay and see if it causes a CEL to find out for sure. To make sure you were using the correct DB/WT wire, perform a continuity test with your DVM between it and Pin A2 on Connector 1 in the PCM as detailed in the write up.



No need to worry about fireworks in your headliner, as I mentioned in my earlier post a standard Bosch style automotive relay only draws about 0.16amps. That plus power for the indicator light in your switch (if so equipped) is all the current that is running through that wire. Shoot, I'm only using 24g wire for my switch and that still provides a significant safety factor. Fortunately for you (and me), that whole circuit is already protected by the fuse protecting the factory fan relay. The 10amp fuse you have in there won't hurt anything, it just isn't going to do anything. You should still consider fusing the 12V+ side of your relay coil though.

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Jeep Cherokees and the people that have them are great fun. I do all the stuff I am doing because I enjoy it. Still it's great to have a pat on the back. The most recent thing is a ustream.tv "live" show and a audio only podcast for later listening. At it's heart it's a talk show about Jeep Cherokees and their owners. We had 50 people total watching the show last Wednesday night. We've only done 5 shows, so even though the Cherokee has been out of production for 10 years, it seems the fan base only continues to grow. Who knows maybe Chrysler will bring back the Cherokee like they did the Charger. {rolls eyes}

I love reading posts like "Just bought my first Cherokee and found your site..."

Knowing the switch is just activating a factory relay is great to know. Takes a weight off my mind. I read (somewhere) that the OEM fan draws 10 amps on start up, so that is where I got the 10 amp fuse idea. All to often I do instead of research something. I saw this post and got busy.

fyrfytr1717-- thanks for spending the time to make this thread and document it. Installed a fan override on my 2000 Cherokee today. This thread was the only one I could find anywhere that pointed out the blue/white wire for 12v+ ignition. That saved much time!!!

Awesome, that wire can be tricky to track down, even with it pointed out. I hoped it worked out for you!

I did this mod as origionally explained on my 00 xj and it did not work. Put the engine light on. All wiring was done correctly and i used the exact relay as stated. As soon as the wire which actuates the fan relay (blue and pink) is cut the light comes on. If i reconnect the control wire to itsself the light stays off. Has anyone else done this on a 00 or 01? I really would like this to work so any advice would really be great. Thank you!

Yeah Project00XJ, get this sorted out so you can wire up the fan on mine.

Bill

Project00XJ said:

I did this mod as origionally explained on my 00 xj and it did not work. Put the engine light on. All wiring was done correctly and i used the exact relay as stated. As soon as the wire which actuates the fan relay (blue and pink) is cut the light comes on. If i reconnect the control wire to itsself the light stays off. Has anyone else done this on a 00 or 01? I really would like this to work so any advice would really be great. Thank you!

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Did you check out the link on post #70? That is how I did my 1998 and it works perfect. (I had the same problem you did at first).

Ghosthemi your in business. I just figured it out. Most people would have quit 5 times by now but i really wanted to do this. So.. my problem was that the blue and white wire in my harness showed power in the run position and ground in the off position. With further testing it was not enough to open the relay. I ran a jumper to the 86 post and it worked perfect. No cel. Lol turns out my vehicle has 2 harnesses that run side by side by the hood hinge on the driver side. I figured my wire to the batt was good enough, so when i was about to be done and put the other harness ( which was pulled out of the way) i said what if, well is was. Turns out both harnesses have a blue whit wire and the other one did the job. I AM IN BUSINESS AND NEED ANOTHER BEER!