• NAXJA is having its 18th annual March Membership Drive!!!
    Everyone who joins or renews during March will be entered into a drawing!
    More Information - Join/Renew
  • Welcome to the new NAXJA Forum! If your password does not work, please use "Forgot your password?" link on the log-in page. Please feel free to reach out to [email protected] if we can provide any assistance.

Radiator Fan Override Switch (The Easy Way)

I tend to agree. It's fairly shortsighted, all it does is tell the ECU that its own components are working and theoretically connected to something. It's like target shooting by listening for the gunshot instead of seeing if you hit the target. But hey, it's what we have to work with, and I'm glad they set it up that way because it makes it easier to override with a switch without triggering a CEL.
 
well kastien if u find the wire colors in your 98 please lemme know! i split my loom and have the relay and splices ready to go!

EDIT: wait.. what harness is it in? the bigger one on the little one?
 
Last edited:
Ive looked over a few write ups on this issue, and I am just curious where the heck you get a relay, and where exactly do you put it if, just making he fan switched is all you want to do?
 
waytekwire, delcity, most parts stores...

Truckntractorhick, the wire colors are the same on my 97, so I'd venture a guess that they didn't change between 96 and 99. Same colors as in fyr's schematic, as far as I know.

You can access the blue/pink wire near the connector on the PCM- that's what I did while fooling around with a crude attempt at this. I'll have to check, but I believe it's in the third connector, the rear-most one.
 
I did this mod a couple nights ago because I felt like it (I have no need for it yet.) I did it by installing the relay in the PDC, though, so my findings will likely not help that much. It wasn't bad, just cutting and splicing a few wires and I was done.
 
Sorry, I only have a '99 FSM. I'd assume '98 and '99 would have the same colors, but it's entirely possible that they're different. My wires were pretty burried in that loom and required some considerable digging around with a dental pick to pull them out of the bunch.

I did not use a new fuse in my wiring set up. The old relay retains fuse protection via the original circuit and the new relay gets it's fuse protection through the ST-RUN circuit that you tap into.

I believe I used either a 30 or 40 amp fuse, however it really doesn't matter as there is very little load being passed through it. I used the same size and type of relay as many of my other accessory circuits use so that I can carry fewer spares.

As an update for anyone interested in this thread, 4+ months later and this set up is still working perfectly for me. The fan still cycles on and off with the A/C, turns on and off at the appropriate engine temps, turns on and off with a flip of the switch, does not trigger any CEL's, does not blow fuses, no problems at all...

Excellent write up and very clean wiring...
Did this same modification to my '98 almost 8 years ago and much like you it required research with the FSM and probing around with a VOM.
 
You're partially correct. The ECU does switch the relay on by grounding the terminal - but what you're missing is that the ECU will notice if you ground the same terminal by yourself. It'll throw a CEL saying that the fan relay is stuck low for some reason.

What this mod does is turn the relay on without forcing the ECU fan relay control wire to ground, so the ECU never realizes you're doing anything.

actually ...... ;)

I've got a 97/98 update .....

My over-ride for the e-fan is no more than a splice into the blue/pink wire at the PDC ... and a switch that grounds the spliced wire.

No relays / diodes or anything else ... and it works a treat if I ever need to turn it on.

Theres a writeup on jeepforum that confirms it ... altho many have the CEL as you mention - depending on year models.
 
Interesting. My 98 fsm tells me the ECU will trigger a CEL. Another strange bit of info - looking at my fsm wiring diagrams, my wiring harness matches that of a 2.5L. I have a 4.0. This only pertains to the fan wiring, I haven't checked for any other differences.
 
Interesting. My 98 fsm tells me the ECU will trigger a CEL. Another strange bit of info - looking at my fsm wiring diagrams, my wiring harness matches that of a 2.5L. I have a 4.0. This only pertains to the fan wiring, I haven't checked for any other differences.

Since Im not a wiring expert .... I was wondering wehether MY splice at the relay terminal ... rather than a splice point closer to the PCM may have averted the CEL .... but I wouldnt think so. ????
 
I tried the simple grounding of the wire, and it threw a code. I used a relay (NO/NC) to open the PCM side and ground the relay side, to prevent that, but it would still throw a code when the PCM wanted to run the fan. I gave up as the cold weather arrived. I've read numerous posts on other forums where guys said this mod (the simple grounding of the wire, not fyrfytr's variation) didn't throw a code on 97+ Jeeps, but that wasn't my experience.
 
well i did mine and it flat out didnt work :( heres some pics below 2 help out.....
this is the fan wire (100% sure its dark blue with pink tracer)
4783146062

here is the ST-RUN power its dark blue with white strip once again 100% sure of it
4783145996

and heres my jeep :)
4782512065


i just tryed wiring it 2 my fuse block and it worked!!! so i may just keep it like that.........

EDIT: pics dont work :mad: just go here http://www.flickr.com/photos/51977521@N06/?saved=1
 
I tried the simple grounding of the wire, and it threw a code. I used a relay (NO/NC) to open the PCM side and ground the relay side, to prevent that, but it would still throw a code when the PCM wanted to run the fan. I gave up as the cold weather arrived. I've read numerous posts on other forums where guys said this mod (the simple grounding of the wire, not fyrfytr's variation) didn't throw a code on 97+ Jeeps, but that wasn't my experience.

When I installed my relay I cut the blue ground wire at the fan and pigtailed a wire to terminal 87 and connected it to the other end of the cut blue wire going to the fan motor. Then attached another wire to terminal 87a and then to ground. Wired in the relay to actuate by grounding the relay coil with a toggle switch on my center console. this seup has worked flawlessly for many years with no codes thrown. When grounding the wire from ECM to the PDC relay this is something similar to what the Police Dept XJs have with the additional (4th) switch on the bezel beneath the climate controls in which there is also an additional relay to also actuate the IAC to increase RPMs during prolonged idling with the A/C on. I researched this modification many years ago and then consulted a well respected local Jeep dealer tech to verify my findings were correct.
 
So it doesn't throw a code when you have the fan forced on, and the AC on? I just assumed (don't know why I never bothered to check) that it watched the voltage/grounds of the entire fan circuit, not just the wiring between the PDC and PCM. I need to tear into the dash again soon- I might just try that.
 
So it doesn't throw a code when you have the fan forced on, and the AC on? I just assumed (don't know why I never bothered to check) that it watched the voltage/grounds of the entire fan circuit, not just the wiring between the PDC and PCM. I need to tear into the dash again soon- I might just try that.

This modification allows you control the fan independently of the A/C and the fan still works when the A/C is on.
 
When I installed my relay I cut the blue ground wire at the fan and pigtailed a wire to terminal 87 and connected it to the other end of the cut blue wire going to the fan motor. Then attached another wire to terminal 87a and then to ground. Wired in the relay to actuate by grounding the relay coil with a toggle switch on my center console. this seup has worked flawlessly for many years with no codes thrown. When grounding the wire from ECM to the PDC relay this is something similar to what the Police Dept XJs have with the additional (4th) switch on the bezel beneath the climate controls in which there is also an additional relay to also actuate the IAC to increase RPMs during prolonged idling with the A/C on. I researched this modification many years ago and then consulted a well respected local Jeep dealer tech to verify my findings were correct.
My info says this is wrong, extended idle is just a switch that grounds a pin on the ECU when the switch is turned on. The ECU uses this to change its idle RPM setpoint from 750 to 1000 and turn the fan on when required. Covered in section 14 and wiring schematic on page 8W-14, at least in the '98 FSM. If you ground pin A12 in C1 (connector 1) of the ECU wiring harness on any 4.0L XJ this should be enabled.
 
My info says this is wrong, extended idle is just a switch that grounds a pin on the ECU when the switch is turned on. The ECU uses this to change its idle RPM setpoint from 750 to 1000 and turn the fan on when required. Covered in section 14 and wiring schematic on page 8W-14, at least in the '98 FSM. If you ground pin A12 in C1 (connector 1) of the ECU wiring harness on any 4.0L XJ this should be enabled.

OK.... I have the same info in my '98 FSM.
I was just using that explanation as an example...

This idea of grounding out an ECU terminal is the wrong approach to manually turn the fan on....
The research (almost 7 years ago) I referenced in an earlier post showed this to be a mistake that could harm the ECU...
 
Back
Top