Using PCM to control aftermarket electric radiator fan?

Interesting thread, and another interesting point on whether the circuitry actually exists. The 99-04 Jeep Grand Cherokee 4.0s with the same style PCM used an electric fan. Apparently the engineers felt it was more efficient there than a mechanical fan. Maybe it wasn’t offered on later model TJ 4.0s simply because they didn’t want to reinvent the wheel. When the 2.5 switched to a 2.4 an electric fan replaced the mechanical fan.

I would lean towards the circuitry not existing in the PCM. I have an 05 with a manual transmission and the plug for the optional automatic is void of any pins. My guess is they wanted to save pennies on unneeded parts, which adds up if you’re producing millions of PCMs across different platforms.

I have no dog in this fight, but I always like exploring deeper into things like @Steel City 06 has been doing here. I actually may test out the Ford fan mod just for fun. Carry on sir.
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So far as I can tell, the 2.4L TJ only provides two speeds, low via one relay, and high when both relays are activated. The way the factory motor is wired, there are two powered brushes, and two grounded brushes. When only one powered brush is supplied power (doesn't matter which one), only half the motor is "working". When both powered brushes are supplied, the whole motor is "working".

I'd be curious to know, if you can derive the "decision tree" from the ECU, the factory conditions that activate (or possibly, deactivate) each fan speed?
Just stumbled across this fwiw:


Well I had the chance to write some new tunes with various modes enabled, and to poke around the empty spots on connector C3.

I got a null result. I could not find any change in the pinout operations based on enabling each of the different fan modes.

So either the tuner isn't capable of enabling the PCM to do these tasks, or the PCM isn't capable of it.

Two things that came to me rereading the thread; what year is yours and have you measured how much room you have for the e-fan? I think ≈3.5" thickness is max that will fit and the spal you listed is ≈4.5".
I didn't read everything but why not just get an aftermarket electric fan controller and not worry about the TJ's computer?
Wow, that is really good info! I'll have to go really look at that pin. I have a 2006. It may be mapped to a different pin, but if it was available in 99, I doubt they would have removed it.

I'll have to look at the thickness. I haven't measured it yet.
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I didn't read everything but why not just get an aftermarket electric fan controller and not worry about the TJ's computer?
The computer takes into account a lot more variables than just the coolant temperature. For example, it can vary with A/C pressure as well. So on a hot day with the A/C blasting, it will spin the fan accordingly. It also varies with vehicle speed. There is little point to driving a fan at 80 mph.
So I hooked up a wire to C21 (gray connector) and played with the settings in HP tuners every which way I could think of. Nada. After looking at a bunch of PCM pinouts, it looks like C21 is used only for the high speed relay on Viper JTEC PCMs from what I can find. I do see several different XJ PCMs that use C2 for low speed relay. Might give this a shot soon.
Cool! Which pin is it? What's your setup?

Pin c2. Its a negative trigger. Set the high and low temp in hptuners to 199. Fan shuts off at 194. Changing dc fan to enable makes it not work. 2 speed fans doesnt seem to have any effect. One OS had the fan cycle on/off every second 6 or 7 times, then it would turn on. Repeated that when it shut off. Changed OS and the cycling went away. Runs as normal now. I set the ac temp to like 120, and the fan comes on when you switch the ac on.
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Thanks! I have a 99 XJ PCM I'm going to swap in and do this with. You are using HP Tuners I assume? Is it the Beta version?
I also just checked your profile and see you have a '98 as well. Did you rewire the oil pressure sensor to get it working properly?
Running vcm suite version 4.10.7 i think. The new version wont let you turn off dtcs. This jeep started off as a $1500 SE 4 cylinder with no axles and rotted arches at the rear axle, and i swapped in the 4.0 and ax15 from a 99, including the entire underhood harness.
I didn't know that about the dtcs codes. Thanks. If you get the chance, could you take a peek at the oil pressure sensor and tell me if it's a 2 wire or 3 wire? I really appreciate the info!
This was a very interesting thread. I'm not planning any electric fan conversions but still enjoyed reading and learning. Aside from the one poster shitting in the thread, the rest of the posts have been interesting. I did do the Ford fan/heavy duty fan clutch mod and love it.