XJ PCM vs TJ PCM

XCRN

XCRN

TJ Enthusiast
Original poster
Supporting Member
Ride of the Month Winner
Joined
Jun 1, 2020
Messages
592
Location
Springfield OH
I am working on wiring up the AC on my 2000 LS TJ and I want the Jeep PCM to control all of the AC related functions, one big thing is controlling the electric fan. Since the XJs use an electric fan for the AC I was thinking of swapping it in place. The only thing I care about still working after the PCM swap is the gauges and AC relay. So with that said, could I swap an XJ PCM in and it still run all the gauges and other features? Pinouts look identical so I am thinking yes. Since the PCMs look the same could the TJ PCM have the option to trigger an E fan with the use of HPTuners?
 
  • Like
Reactions: SkylinesSuck and Wranglerfix
I snagged a 99 XJ PCM to swap into my 98 TJ for the PCM fan control. Haven't gotten around to it yet but I plan to. I was told I'm going to have to use a newer style oil pressure sensor and add a 5v wire to the harness to make it play nice with my dash (which you likely won't have to deal with) fwiw.
 
  • Like
Reactions: freedom_in_4low
You'll probably need to turn off a bunch of auto tranny related codes. But you'd want to do that with pretty much all dtc's once it gets used solely as a gauge driver
 
  • Like
Reactions: SkylinesSuck
Rickyd said:
You'll probably need to turn off a bunch of auto tranny related codes. But you'd want to do that with pretty much all dtc's once it gets used solely as a gauge driver
Click to expand...

I just plucked the CEL bulb out. Would not work correctly even if I turned off all DTCs. Though if somehow these DTCs are turning on my Airbag light, I might consider spending the $50 to credit the PCM to turn them off.
 
  • Like
Reactions: Rickyd
So to follow up, I did try the XJ PCM, and not only did it run my gauges fine, it turned the electric fan on with the AC request. It was fairly simple and could probably work with 4.0 TJ if you wanted to add it. Still need to find the leak in the AC system to see how well it all works.

I only had to mess around with the C3 grey connector. I added a wire going from cavity 2 to the ground trigger of the fan relay, and I added a wire that goes from cavity 13 and is spliced into the wire that goes to the AC switch sense blue/white wire that goes into cavity 22.

On the TJ harness, cavities 2 and 13 are blind holes that I used an 1/8” drill bit to drill out. The pins in the Jeep PCM connectors are the same (or close enough) as Gen 3 LS PCM connector pins so which is easily sourced.

IMG_9019.jpeg


IMG_9020.jpeg


IMG_9016.jpeg
 
Last edited:
  • Like
  • Love
Reactions: Jamison C, SkylinesSuck and SSTJ
XCRN said:
So to follow up, I did try the XJ PCM, and not only did it run my gauges fine, it turned the electric fan on with the AC request. It was fairly simple and could probably work with 4.0 TJ if you wanted to add it. Still need to find the leak in the AC system to see how well it all works.

I only had to mess around with the C3 grey connector. I added a wire going from cavity 2 to the ground trigger of the fan relay, and I added a wire that goes from cavity 13 and is spliced into the wire that goes to the AC switch sense blue/white wire that goes into cavity 22.

On the TJ harness, cavities 2 and 13 are blind holes that I used an 1/8” drill bit to drill out. The pins in the Jeep PCM connectors are the same (or close enough) as Gen 3 LS PCM connector pins so which is easily sourced.

View attachment 534856

View attachment 534857

View attachment 534862
Click to expand...

Is the only difference with the xj pcm vs tj that it turns on an auxiliary fan with the ac?

How are you planning to use that on a tj? Two fans?
 
Apparition said:
Member @Longhorn84 put this up

Click to expand...

Basically cut the tj pcm out of the ground path(let the ac panel and hi/lo switches control it) and splice the 12+ coming out of the ac relay into the LS harness lead going to the gm compressor clutch? Sounds simple like some of the old refrigeration systems i work on.

I wonder why the oems have the pcm in the loop at all? Unless its for some kind of limp mode control?
 
Rickyd said:
Basically cut the tj pcm out of the ground path(let the ac panel and hi/lo switches control it) and splice the 12+ coming out of the ac relay into the LS harness lead going to the gm compressor clutch? Sounds simple like some of the old refrigeration systems i work on.

I wonder why the oems have the pcm in the loop at all? Unless its for some kind of limp mode control?
Click to expand...

The reason I wanted to use the Jeep PCM to control the fan only for the AC was to prevent the GM PCM from turning on the AC clutch when turning the fan on for coolant temp. I know I could have added some relay somewhere to prevent the back feed of the GM PCM turning on the AC clutch, but I felt this was the simplest way to do it and would “future-proof” my self going forward if (more likely when) I swap to a Gen 4 or 5 engine. I did consider getting a small pusher fan on the condenser and then using the AC request ground to also trigger that secondary fan relay, but I had an XJ PCM lying around and only took running a wire halfway across the fire wall and pinning in another 3in long wire.
 
  • Like
Reactions: Rickyd and SkylinesSuck
Rickyd said:
Basically cut the tj pcm out of the ground path(let the ac panel and hi/lo switches control it) and splice the 12+ coming out of the ac relay into the LS harness lead going to the gm compressor clutch? Sounds simple like some of the old refrigeration systems i work on.

I wonder why the oems have the pcm in the loop at all? Unless its for some kind of limp mode control?
Click to expand...

I believe most PCMs kill the AC clutch either at WOT or above an rpm threshold.
 
  • Like
Reactions: Rickyd
freedom_in_4low said:
I believe most PCMs kill the AC clutch either at WOT or above an rpm threshold.
Click to expand...

I know the gen 3 GM PCM shuts off based on throttle position and engine RPM and so does the Holley Terminator with the preset AC clutch control output function. I really wonder if the Jeep PCM does the same. I can probably check for any RPM shutoff easily by disconnecting the AC and plugging a light in its place. Though I can no longer check any shutoff based on throttle with out a Jeep TPS hooked up.
 
XCRN said:
The reason I wanted to use the Jeep PCM to control the fan only for the AC was to prevent the GM PCM from turning on the AC clutch when turning the fan on for coolant temp. I know I could have added some relay somewhere to prevent the back feed of the GM PCM turning on the AC clutch, but I felt this was the simplest way to do it and would “future-proof” my self going forward if (more likely when) I swap to a Gen 4 or 5 engine. I did consider getting a small pusher fan on the condenser and then using the AC request ground to also trigger that secondary fan relay, but I had an XJ PCM lying around and only took running a wire halfway across the fire wall and pinning in another 3in long wire.
Click to expand...

I see. Two separate ways of turning on your main cooling fan.
freedom_in_4low said:
I believe most PCMs kill the AC clutch either at WOT or above an rpm threshold.
Click to expand...

Ok,now it makes perfect sense. Gotta safeguard the compressor from those pesky car motors and allow them to have full power when needed. Tps could be a tough one but there are rpm signal converters. Easier to just let the LS pcm handle it though
 
  • Like
Reactions: Apparition
Rickyd said:
I see. Two separate ways of turning on your main cooling fan.


Ok,now it makes perfect sense. Gotta safeguard the compressor from those pesky car motors and allow them to have full power when needed. Tps could be a tough one but there are rpm signal converters. Easier to just let the LS pcm handle it though
Click to expand...

I don’t know if it would be easier or not with a gen 3 LS, but still easilly doable having the GM PCM control the AC. From what I remember looking at the GM wiring diagram, there were separate inputs for both high and low pressure sensors so they need to be wired in differently than the Jeep. Then the GM PCM would send a 12v+ signal to a relay to trigger the clutch. So you would need to wire a new relay because the Jeep relay is ground triggered. Again not a hard task, but I do not think is easier of the 2 options in my opinion.

Since practically all the wiring was left there and its mostly Jeep components on my AC, I kept it Jeep wiring and management. Just needed something for the fan since no more mechanical fan.
 
Top Bottom