Radiator Fan Override Switch (The Easy Way)

[carves]

Originally Posted by kastein

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.

[kastein]

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.

[carves]

Originally Posted by kastein

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. ????

[hubs97xj]

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.

[truckntractorhick]

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)

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

and heres my jeep


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

EDIT: pics dont work > just go here http://www.flickr.com/photos/51977521@N06/?saved=1

[CharlesS]

Originally Posted by hubs97xj

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.

[hubs97xj]

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.

[CharlesS]

Originally Posted by hubs97xj

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.

[kastein]

Originally Posted by CharlesS

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.

[CharlesS]

Originally Posted by kastein

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...

[hubs97xj]

(apologies to fyrfytr for cluttering up the thread)
Okay, poking around with the meter, doing some googling. The light blue wire isn't ground, it's power. Ground is black, and unswitched. The relay in the PDC, when energized, puts 12V on the blue wire, fan runs. Taking an educated guess here, but is this how you actually wired it up, Charles? (and sure enough, the PCM doesn't monitor the fan, just the control ckt)



Switch off, stock power wire connected to 87a (NC) contact, wire from fan connected to 30, fan runs only when PDC energizes.
Switch on, battery power on 87, fan on 30, stock power wire held open, fan runs constantly.

Have I got it right?

[truckntractorhick]

^ that helps!! if it works....

[kastein]

That should work. The blue +12V line should be from an ignition-switched source unless you want to be able to leave your fan on when the jeep is not running.

[hubs97xj]

Noted and fixed (I think). You could also use a relay on the battery lead, controlled by a keyed source as well, but it would be a little more work. I like fyrfytr's approach in that it lets the PCM still run things, but I don't see any issues with this since the PCM doesn't care what is or is not connected to the fan wiring.

[CharlesS]

Originally Posted by hubs97xj

(apologies to fyrfytr for cluttering up the thread)
Okay, poking around with the meter, doing some googling. The light blue wire isn't ground, it's power. Ground is black, and unswitched. The relay in the PDC, when energized, puts 12V on the blue wire, fan runs. Taking an educated guess here, but is this how you actually wired it up, Charles? (and sure enough, the PCM doesn't monitor the fan, just the control ckt)



Switch off, stock power wire connected to 87a (NC) contact, wire from fan connected to 30, fan runs only when PDC energizes.
Switch on, battery power on 87, fan on 30, stock power wire held open, fan runs constantly.

Have I got it right?

On my '98 I cut the ground wire going to the fan motor an attached the incoming to term 30 and other end to term 87a and wired in an additional ground wire to term 87. 87a is N/C and 87 is N/O. 87a is used for normal use by the AC and 87 is used for manually running the fan for extra cooling of the engine. This setup has worked fine for over 7 years.