johns91xj

I installed an override switch for my factory e fan and followed this write up: https://www.cherokeeforum.com/f51/el...switch-132465/

Just had one question. I noticed that it said there is no power running through the dark blue/ pink stripe wire. I took a light tester to it with the ignition on when the fan was off and it had power, then turned my ac on and tested the same wire and it had no power. I figured it tested out so I proceeded to follow the write up and everything works fine. Does the pcm switch the wire from power to ground? This was on a 96 XJ. Thanks!

BlueRidgeMark

That's correct.

nujeepguy

the voltage on bl/pk is thru the relay. the pcm grounds that wire so the 12v is across the relay, click!

johns91xj

Awesome! Thanks! I figured if it was working it was fine, but I also wanted to make sure I wasn't grounding out a power wire.

nujeepguy

you can use a light bulb to be safe,some resistance, no direct short. i always used a resistor when probing around transistor circuits, no shorts1

moparado

Just in case, to be clear.
One wire from the fan motor is permanently grounded to the chassis via the motor connector. The other wire from the motor connector gets Battery voltage via the oem fan relay.

If all you want to do is unconditionally turn on the fan, there's no need to fart around with the oem fan relay or pcm.
All you need is another 12volt SPST auxiliary relay capable of powering the fan, switch, in line fuse, fuse tap and wire. These automotive relays are dirt cheap. I got 5 of them from Amazon for $12. Same relays used to power aux. fog lights, running lights, etc.

Before doing anything pull apart the fan connector and with an ohm meter or continuity tester verify which wire is grounded to the chassis. This is the connector side that goes to the oem stuff not the fan motor.
The fan motor itself is likely isolated from ground. It gets its permanent ground from the cable connector.
I wouldn't go strictly by wire colors.

Basically the auxiliary relay contacts are in parallel with the oem relay contacts.
That way either the oem or auxiliary relay will turn on the fan.
If both relays are energized, no problem.

Here's how to wire up the auxiliary relay.
Fuses are a must!

-Run a heavy wire 18ga. min. from one of the relay's contacts to the (+) battery post with inline fuse near as possible to the + post. DO NOT WIRE WITHOUT THE INLINE FUSE!

-Run a wire from the other contact and splice into the NON-GROUND wire leading to the fan motor connector end that goes to the oem stuff.
This the 12 volt feed wire to turn on the fan. It also goes to the oem relay contact.

-Connect the (-) of the relay coil to a convenient ground.

-Connect a wire to the (+) of the relay coil, run it through the firewall and connect to one terminal of the switch. Connect the other switch terminal to a fuseable tap somewhere in the cabin fuse block. DONE.

I've done same with 2k but only with a delay circuit to keep the fan running for a while after engine shut off. Works great over a year now.

If you need a schematic, i can scribble something up and post it.

johns91xj

Thanks but all I did was tap into the blue/pink wire, run a ground wire to the same spot the factory fan gets grounded to on the driver side inner fender, then run the two wires through the hood latch cable rubber grommet, then run it to a switch next to the rear defroster. This way everything works like it should from the factory, but I can also override it if needed. Only thing that sucks about this way is the switch I used has a light on it and I don't think there is a way to get the light to work because of the way this modification works.

nujeepguy

im gonna do it same way just to have it ready.

johns91xj

Yea its nice to have it just in case. I also ran the wires in the loom with the rest of the harness and electrical tapped it like it was from the factory. All you see is the eylet for the ground and where the two wires go into the firewall I used a smaller loom from harness to firewall. Looks nice and factory. Except for the switch of course.

BlueRidgeMark

Don't do it this way.

The OP's method is good, but I prefer to completely isolate the PCM from the aux fan switch, by this method.

IJM

I did it the way you just described, and it's worked perfectly for nearly a year now. No issues with the PCM or CEL since its out of the loop.

moparado

WHAT!
The method i used 100% isolates from the PCM.
Where did you get your electrical learn'n.

In case you don't know, the oem relay 'contacts' are isolated from the from its coil thus the PCM also.
Just like every other oem relay in the vehicle that is energized by the PCM....they are all isolated from the PCM.
Control circuitry isolation from power circuitry is 'one' of the main benefits of using relays. DUH!

There is absolutely no reason to cut the hot wire then place the aux. contacts in series with it.

I like when my PCM decides to start my fan and when the air is turned on too!
Or when i see the temp rising, i can beat the PCM and turn on the fan manually.
And i use it a lot. Every time i shut that hot engine down, i turn on the fan with its 4 minute off-delay or when sitting in a long line at McD's on a hot day.

I'll stick with my method, going on 2 years with no issues.

johns91xj

I see I've started this argument back up. Lol. As long as the way I did it is fine and no one has had pcm issues then that's fine with me. Already had the CEL light on so that doesn't bother me and 91-96 don't get them with the two wire way.

nujeepguy

2nd thoughts. i have concerns about feeding into a computer,not knowing whats in there. if it used a relay contact id be ok but solid state might even have diode protection to complicate matters. i have relays. i might use . thinking again.

moparado

You asked for it by saying "Don't do this" to my previous post!
I would of let it all go if you didn't post another jab at me!

Take an introductory course in basic electricity and get back.
In order to have a flow of electrons to any OEM 'back circuitry' there needs to be a potential difference in addition to conductivity from the power circuit and the control circuit. Neither exist.
The OEM relay contacts are isolated from the PCM and any other 'back' circuitry.
This can't get any simpler than it is.

Besides cutting wires, your method has the fan current going needlessly through two sets of contacts (the OEM and the aux. relay) when the switch is off.
Not the best of designs.

I just scribbled up a coarse schematic of the way i wired up my fan switch with the exception of the delay circuit. No need to go cutting the fan's power wire.
Make sure the relay used has at least a 20 Amp contact rating.