Radiator Fan Override Switch (The Easy Way)

[fyrfytr1717]

I know there are many, many write-ups out there regarding how to override the electric radiator fan, but so far as I've been able to find, this is the first that takes this particular approach. My hands on experience only involves the 1999 model year, but I'd assume this information should apply to all '97-'01 models and most likely (with some adaptation) to earlier years. Take the proper precautions when doing any wiring on your vehicle, especially when it comes to tapping into factory circuits or you may fry your PCM (like I almost did)...

I wanted the PCM to be able to activate the fan as designed, providing a ground signal to the Radiator Fan Relay when coolant temp reaches approximately 218* or A/C is requested. I also wanted to be able to make the fan run continuously. I did not however want to run an additional heavy gauge fused power wire to the fan to make this happen. I figured there had to be a way to take advantage of the factory wiring and fool the system into activating the fan.

I knew I was not alone in this pursuit as I had read many posts about trying this with the outcome being either a CEL or no fan function at all. I had spent way too much time reading through threads and staring at the FSM wiring diagrams and was close to throwing in the towel when a fellow user (MrJCRod) posted up the description of how he had made it work. I turned it into a wiring diagram, researched the location of all the necessary wiring, and decided it looked like it should work. Here's how I went about doing it...

Here's the wiring diagram. It is based upon the FSM wiring diagram, but I did a little cut and paste work on it to show how the wiring needs to be run. Essentially, the wire leading from the PCM (DB/PK) which supplies the ground signal to the Radiator Fan Relay is cut. This wire is re-routed to a New Relay, which in turn supplies a ground signal to activate the old relay. A switch which also provides a ground signal is connected to the old relay as well.

Here's the possible on/off situations that would be faced:

1. PCM off and switch off - new wiring is invisible to the PCM
2. PCM on and switch off - PCM activates new relay which in turn activates fan relay / PCM isolated by new relay
3. PCM off and switch on - switch activates fan relay / PCM isolated by new relay
4. PCM on and switch on - fan relay activated by both new relay and switch / PCM isolated by new relay


So having a wiring diagram is all fine and dandy, but I'm sure you'd rather know how to go about actually wiring this up...

First off, I wired up a switch for the fan override. I located it in my overhead console (along with a bunch of other switches) and routed the signal wire into the engine compartment. Something important to keep in mind here is that you need to set up your switch to provide a ground signal. Mine was originally set up to provide 12V+ so I had to do a little rewiring before I could get started...

Next up was to find a place to tap into the wiring. After researching the FSM I found that all the necessary wiring ran from the PCM, along the firewall, and into the PDC. My relays are all mounted on the driver's side of the firewall so I chose a convenient spot and removed a few inches of the ribbed wiring loom.

I needed the signal wire for the Radiator Fan Relay and a fused ignition switched 12V+ (ST-RUN) source to simulate the original circuit. The signal wire is Dark Blue w/ Pink tracer and is found in Pin C2 on Connector 3 in the PCM. The ST-RUN wire is Dark Blue w/ White tracer and is found in Pin A2 on Connector 1 in the PCM. I would suggest doing a continuity test between the PCM connectors and the wires you are about to cut/splice so you don't make the same mistakes I did. I found that under fluorescent lighting, the Dark Blue w/ Orange tracer looks an awful lot like the Dark Blue w/ Pink tracer. (This led me to the discovery that it is possible wire up an A/C Compressor Clutch override switch, if you really wanted to...:shhh I also thought that the DB/WT (one of my favorite wires for a number of other mods I have done) would be pretty straightforward. Unfortunately, as can be seen in the following pic, there is another DB/WT and a DB/GY that run through the harness in this spot which could be easily mistaken for the correct wire.

Next, cut the DB/PK wire as shown in the above picture. Attach extension wires onto both ends, each long enough to reach your New Relay. Tap into your (ST-RUN) wire and provide a long enough wire to reach your New Relay as well. You will also need a ground wire leading to your New Relay. Mine is daisy chained from the relay sitting next to it. Make sure your switch's signal wire also reaches the location of your New Relay. Mine is the small gauge white with tan tracer wire shown in the following pic.

Make the connections on your New Relay as follows:

Relay Pin 85 - DB/PK wire coming from your PCM
Relay Pin 86 - DB/WT (ST-RUN) wire
Relay Pin 30 - Ground wire
Relay Pin 87 - DB/PK wire going to the Fan Motor Relay (in the PDC) and the ground signal wire coming from your override switch
Relay Pin 87a - May or may not exist on your relay, but is unused regardless

That's it, that's all there is to it. I've been using mine for a few days now and can confirm that everything works as it should, and there's no CEL! Overall, I'm very satisfied with how this turned out. It takes advantage of the factory harness and requires no new fuses and very little additional wiring. One item to note, if you wire this up as I have detailed and use a lighted switch, the indicator light on the switch will come on every time the Radiator Fan Relay kicks on (regardless of the position of the switch). This could be easily prevented, but with my switch installed out of my line of sight it doesn't really bother me. If your switch is mounted on the dash, I imagine it would get pretty annoying (especially at night) when using your A/C as it continuously cycles the fan on and off.

For those of you who not only want to be able to turn your fan on at will, but also want to be able to prevent it from coming on for say water crossings or something, here is the wiring diagram for you. Essentially the same as the diagram above, only you use a 3-Way switch (On-Off-On) to control the signal wire leading from the New Relay to the Radiator Fan Relay. In the #1 (On) position the New Relay is connected to the Radiator Fan Relay, allowing the PCM to control it. In the #2 (Off) position there is no connection to the Radiator Fan Relay, preventing it from turning on. In the #3 (On) position, the Radiator Fan Relay is connected to ground, causing it to stay on. I have not tried this myself, but I see no reason it would not work.

I hope someone out there finds this useful! Keep cool out there... :greensmok

 

[gradon]

Good writeup. I already have a spare relay, so basically need to wire the st-run to make mine stop CELing. Now to figure out if the stock circuit can handle two fans(I want to use 10awg, and might have to bump up the fuse by 10 amps or so). If not, I can have the stock fan activate a relay for the second one when it turns on.

 

[kastein]

VERY neat wiring on those relays, I approve (as an OCD wire management type...)

Another note - remember to keep the relay you install the right way around if you use one with an integral EMF-shunt diode across the coil. I think that's the only thing you forgot to mention. The cathode (arrow in diode symbol points toward the cathode) should point towards positive voltage, away from ground.

 

[fyrfytr1717]

Thanks for the compliment kastein. I always appreciate your feedback given your electrical background. All the wiring will get even neater once the rest of the relays are wired up. Everything will be in loom and the exposed part of the harness will get covered back up again. I'm just waiting to get everything in place before I seal it all up.

As for the diode wiring, thanks for the clarification. I am in fact using a relay with an integral diode and I did have an issue with wiring it up the first time around. I assume you were alluding to that as I believe you posted up what ended up being the fix in my "Did I fry my ECM?" thread last week. I don't think it's absolutely necessary to use a diode protected relay for this mod, but I figured it's probably a good idea seeing as the PCM could theoretically be damaged by the voltage spike when the coil's magnetic field collapses upon deactivation.

My write up makes the assumption that all automotive style relays with an integral diode have the cathode connected to pin #86. Maybe this is not the case? This is however why I made sure to point out that the wire coming from the PCM (-) needs to be connected to Pin 85 and the ST-RUN wire (+) needs to be connected to Pin 86. I didn't provide the explanation as to why, I just figured if folks followed my instructions they would get it right without even having to think about it. If there are alternate configurations of this type of relay, it would be important for the installer to make sure they get the polarity right when hooking up their wiring. Here's a pic of the relay I used (Hella HL87104) with the schematic printed on the side of it.

As I mentioned earlier in my write up, it's not necessary to use a SPDT throw relay (87 & 87a Pins), I just bought my relays that way to leave myself some extra flexibility in wiring options.

 

[FlexdXJ]

Dude, that is a clean install! :cheers:

 

[kastein]

As for the diode wiring, thanks for the clarification. I am in fact using a relay with an integral diode and I did have an issue with wiring it up the first time around. I assume you were alluding to that as I believe you posted up what ended up being the fix in my "Did I fry my ECM?" thread last week. I don't think it's absolutely necessary to use a diode protected relay for this mod, but I figured it's probably a good idea seeing as the PCM could theoretically be damaged by the voltage spike when the coil's magnetic field collapses upon deactivation.

My write up makes the assumption that all automotive style relays with an integral diode have the cathode connected to pin #86. Maybe this is not the case? This is however why I made sure to point out that the wire coming from the PCM (-) needs to be connected to Pin 85 and the ST-RUN wire (+) needs to be connected to Pin 86. I didn't provide the explanation as to why, I just figured if folks followed my instructions they would get it right without even having to think about it. If there are alternate configurations of this type of relay, it would be important for the installer to make sure they get the polarity right when hooking up their wiring. Here's a pic of the relay I used (Hella HL87104) with the schematic printed on the side of it.

You're correct about the thread - I'm not sure if the diode is always connected the same way, it most likely is. I provided the warning as just following a schematic to wire up a relay is kinda a foreign concept to me - I don't look at the pin numbers when I wire them, I just pick whatever is right by name (coil/NC/NO/COM terminals are what I'm used to) and go with it. Relays with resistor EMF damping instead of diode EMF shunting will work either way, so you can be a bit sloppier with those and get away with it.

EDIT: IMO, this should be a FAQ/tutorial section writeup...

 

[fyrfytr1717]

Good writeup. I already have a spare relay, so basically need to wire the st-run to make mine stop CELing. Now to figure out if the stock circuit can handle two fans(I want to use 10awg, and might have to bump up the fuse by 10 amps or so). If not, I can have the stock fan activate a relay for the second one when it turns on.

Thanks! I think your idea of having the stock fan wiring (or the new relay) activate another relay for the second fan would probably be your best bet. I'm not sure what the stock Radiator Fan Relay is rated at, but I'd have to assume it is no more than 40 amps as it is just a standard automotive style relay. The fan motor is protected by a 40 amp fuse and only uses 12g wire which I think is probably a bit undersized to begin with.

Even if you replaced all the factory wiring with heavier gauge wiring and upped the fuse rating, I still don't think the stock relay would like being repeatedly subjected to the current draw of both fans starting up at the same time. Especially when you consider that with the A/C running, that fan cycles on and off about every 5-10 seconds or so. Even with just the single fan, I notice my dome lights dimming momentarily when the fan starts up. I think it would be a recipe for early relay failure.

 

[bdahlg]

EXCELLENT, clean install/writeup. Where did you get those awesome switches in the overhead console? I MUST get my hands on those! I hate my switches being unmarked and different colors :roll:

 

[kastein]

http://www.otrattw.com/ has em, among other places. They're made by Carling IIRC.

 

[tomcat]

http://www.otrattw.com/ has em, among other places. They're made by Carling IIRC.

love them as a dealer, great switches, i built my fitchbox with their parts.

i just started a project for my snowplow harness, and ordered up some of these skirted relays and harnesses:
http://order.waytekwire.com/productdetail2/M37/75601/SKIRTED MINI RELAY SPDT/
http://order.waytekwire.com/productdetail2/M37/75610/SEALED CONNECTOR 12" LEADS/
i am splicing it into the headlight harness with weatherpack connectors, for a very OEM look.
for my truck, each headlight, and high-beam/low-beam has a seperate relay (4 total) what a giant pain.

 

[fyrfytr1717]

I got all my switches from OTRATTW. They have the best selection of colors and labels by far, not to mention Jim's excellent customer service and technical support. He also likes to get feedback regarding his customers needs and uses it to get custom rockers made if he feels there is enough demand. For example, my "NAV LTS" switch has been replaced with a "ROCK LIGHTS" switch since the above picture was taken. He also has some labeled "WSHLD LIGHTS" available for the Wrangler folks (or the oddball XJ folks with lights on the A pillar). You won't find either of those rockers anywhere else.

 

[truckntractorhick]

i love this writeup! but..... i cant seem 2 find the wire in my 98 anyone know what color(s) it is?

EDIT: also what size fuse and how many amps on the relay did you use??

 

[fyrfytr1717]

i love this writeup! but..... i cant seem 2 find the wire in my 98 anyone know what color(s) it is?

EDIT: also what size fuse and how many amps on the relay did you use??

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...

 

[winkosmosis]

If this is easy I'd hate to see the hard way.

Why not just add a switched second line to the existing relay?

 

[motoxpete]

Man I wish I had these electric skills lol. I have ton's of electric stuff that needs to be done on my XJ. Including a tuarus fan that needs to be installed and this mod done.... I'm just lost when it comes to this kind of thing. Looks really good though man great job.

 

[kastein]

winkosmosis, depends on how you want to do it. You could splice a second relay into the heavy current wiring no problem but it's easier to splice into the low current side, requires thinner wire and less care.

I don't presently have an e-fan, because my '98 came without one installed. No CEL because the ECU only monitors whether the relay coil is open/shorted (as I expected) not whether the actual fan is operating. I'm going to my storage unit to pick the fan up and install it tonight, if I can find my set of OEM PDC terminals I'm going to get a head start on this mod.

 

[winkosmosis]

winkosmosis, depends on how you want to do it. You could splice a second relay into the heavy current wiring no problem but it's easier to splice into the low current side, requires thinner wire and less care.

I don't presently have an e-fan, because my '98 came without one installed. No CEL because the ECU only monitors whether the relay coil is open/shorted (as I expected) not whether the actual fan is operating. I'm going to my storage unit to pick the fan up and install it tonight, if I can find my set of OEM PDC terminals I'm going to get a head start on this mod.

But that's what I mean, splice a wire and switch to the wire that switches the relay on and off. According to the OP, the ECU or whatever triggers the relay by grounding it NOT by supplying power to it right? So you'd just need to splice in to that and add a switch with a ground on the other end. At any given time, either the ECU grounds the relay, or your switch grounds the relay, or both ground it.

If the ECU switched the relay by supplying power, then you could have a problem duplicating that with a switch, because the power from the switch would fight the ECU when both were switched on.

 

[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.

 

[winkosmosis]

Ah OK, makes sense.

How does the ECU know all this?

 

[kastein]

Hmm... want the electrical engineer nerdy version or the easy version?

Easy version - it watches the output wire and when it isn't doing the same thing it's telling it to, it screams about it.

EE nerdy version - The ECU monitors the ground-side-switched radiator fan relay drive line. When the voltage present on the line indicates it is grounded when the drive transistor is not turned on, or when the voltage present on the line indicates it is not grounded when the drive transistor is turned on, a fault code is registered - most likely P1491, Radiator Fan Control Relay Circuit (An open or shorted condition detected in the radiator fan control relay control circuit. This includes PWM solid state relays.)