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AC Compressor Relay Problems


NAXJA Forum User
Sherman, TX.
I'm setting aside my pride & asking for some help; been fighting an electrical issue for two weeks now. This post may not be in the right place so someone let me know if so. My Cherokee is a 1998 Classic 4wd. It is not stock; I've transplanted an LH8 / 4L60E into this thing, but I've left the entire engine harness in place for now, so technically that part is not modified.
Brief history: I've owned this XJ since it was 2yrs old. My goal was to get to 500,000 miles on the 4.0L; I made it to 498,500 when it started to ring the death bell. The XJ is still in really nice shape & a couple of years ago I decided to freshen it up. Everything to do with suspension & steering has been replaced. OME 3" lift to fit 31" tires, rebuilt the transfer case, all new stuff in the 8.25 rear end, added disk brakes to the rear. It's nice underneath. So when the 4.0 decided it was tired of life, I figured why add a little more modern power. The LH8 is an aluminum block truck engine, gen 4, from an H3 Hummer, but it has no afm or vvt.
Got that all done and put it back on the road last November. I used a stand-alone harness from Speartech so the engine/trans are operating independently from the XJ wiring. it has its own obd port. I did use a Dakota Digital instrument cluster as it simply plugs in & gets its info through the obd. I've tapped into the XJ harness for some indicator light functions on the instrument panel; fuel level, signal lights, etc.. Otherwise, the XJ harness has been left alone. The original wiring is still in nice shape; it's never been cut up and this XJ has spent 90% of its life on the highway so no corroded connectors.
Looks like about a third of my content made it to the post. I don't what happened, I'll have to try to type up all the specifics & questions again.
Sorry if I missed it in your post, but what exactly is your issue?
I spent a good hour or so typing up all of the relevant information & questions I had, and what you see is all that made it when I posted it. In short, I have really weird issues with the ac compressor & aux fan relays working. All of it still running through the original harness. I'll start working on it all again & try to get it reposted.
The rest of the story, I hope:
Here's a link to the page in a schematic I downloaded. It's the one I've stared at the most. I can't figure out how to insert an image here.

1998 XJ AC Schematic

Pay no attention to my scribbling, just making notes as I went along.
Picking up where things got left off, the wiring in my XJ is still in really good shape overall. I never had any electrical issues and never had any ac system issues on the electrical side of it. I used an accessory drive system from Kwik performance so there's a new Sanden SD7H15 compressor in place. New condenser, drier, orifice tube line, & flushed the existing evaporator. I work at a John Deere dealership in their service dpt, so I used our machine to pull a good vacuum on the system & then charge it with 1.25lbs. I connected the XJ harness compressor trigger wire to the new compressor & got nothing.

I have a triple electric fan setup on the radiator. Two fans are wired & controlled completely independently form the XJ & GM harnesses. The controller gets ect info from the obd system on the GM harness. I want to control the third fan using the XJ harness/pcm so it runs with the ac on or when ect gets above a certain point. I have the original 4.0l ect sensor installed on the LH8 so the XJ pcm has a temp input.

After the ac failed to work, I pulled the compressor relay & noticed when I put it back in it clicked on then back off very quickly, but the ac was switched on. Started checking things with a meter & found I have 12v on the Lt grn wire at pin C23 of the pcm, at the low pressure switch, & at the ac/heater control, with key on and ac off. If I turn the ac on the 12v disappears. It seems backwards to me.

I've verified power & ground to the pcm through connector A. All of the original grounds are in place, clean & tight. The grounds that were connected to the 4.0 I kept & grounded to the chassis. I checked continuity on every wire that seems like it had anything to do with the ac, checked for high resistance on the ground paths, I checked voltage on fuse 11 in the junction block; never lose it & no voltage drop. I opened the pdc & checked the the wiring & terminals underneath; they all look good. No signs of heat. No crust or corrosion. Same at the ac/heater control panel; pulled it out last night. The big connector there is tight, clean, no signs of heat.

I ohm checked the new compressor clutch windings; just over 2ohms. Jumped battery power to the clutch, it engages good & no excessive amp draw.

I don't know if the Lt grn wire is supposed to have 12v on it with the key on & ac off. I don't really understand the flow & sequence of events on that wire.

Does the ac/heater control panel output a signal on the c90 Lt grn wire to the pcm, pin C23 (ac select signal) & to the low pressure switch? If the system is charged properly (should be) and the high/low pressure switches are good (or jumped), does that signal then go to pin C22 (ac request signal) and tell the pcm to output grounds to the compressor & aux fan relay coils?

If that's the flow of things then it seems like the first problem is that circuit having 12v on it when the ac is off. I hooked up two meters, one on pin C1 & one on pinC2 at the pcm to watch the ground outputs. There's a ground output on them for a split second then it's gone.

I tried jumping both pressure switches & nothing changes. While I'm doing all this I don't have the compressor plugged in, just a meter connected to its feed to watch for voltage.

Could the original instrument panel no longer being connected have any bearing on this? I've gone through the schematic I have & don't see how it could. I did read that the CCD bus system does flow through it, but could an interruption on the bus stop the ac command at the pcm? I actually removed my top dash panel & managed, barely, to reconnect the original instrument panel. It made no difference. I did take some of the indicator lights from the C1 connector at the inst panel to use for inputs to the new inst panel, but it seems like that shouldn't affect anything.

I can't figure it out. It seems like the pcm is being told not to output grounds to the relay coils for some reason or it is flaked out on me. I don't know what could have damaged it though, if it is the issue. I didn't have any Oh Sh** moments when wiring in the LH8 cabling for the charging system. I did do some welding to modify the battery compartment, but there was no battery in it at the time.

I don't know what else to check. I haven't pulled out the junction block from the interior, but I don't have any voltage drops through any of the fuses there, and everything it's responsible for works. Well, the power door locks are flaky, but that started before this engine swap. It's the only other oddity.

I've been a long time browser of this forum & have gleaned a lot of good useful info here. If anyone has any ideas or can correct any of my thoughts if they're wrong, I would love to hear it. Thanks, Andy Houser.
Got home after work & pulled connector C from the pcm. Key on, ac switch on or off, I have 12V on both pins C22 & C23, the ac select & ac request signals. Key off, nothing. Makes no sense at all to me...