PCM going bad MAYBE
 

running a 2002 trans am LS1. Been driving for 2 years no issues. Cooling fans connected to the PCM Dark Green Wire and dark blue wire, these are both negative signal wires connected to 2 separate relays. Pretty sure set at 195 and 205. I decided to hook up my A/C factory retro and ran a totally separate fan wiring system with its own relays and triggers off the compressor power. When running A/C the fans come on always and temp sits at 190. BUT now when I don't run A/C the fans don't come on. I ran it up to 217 and no fans. I took a ground and touch the dark green wire between the relay and PCM and I get fan same with the dark blue wire. This tells me the problem is the pcm it is not working sending the negative signal at temp. Is this a sign the PCM is going bad. I assume my temp sensor is working as the Snap on scanning is getting a reading, Your thoughts??? I am going to check the wiring from the relay back to the PCM next.

Re: PCM going bad MAYBE
 

So here is how my fans with A/C is wired using a trinary switch. I do not use the PCM as I have an PMW external controller. Regardless of that, the fans only kick on when the A/C pressure is above a certain PSI as detected by the trinary switch.

Re: PCM going bad MAYBE
 

my fans work as designed with the a/c on. My challenge is when I am not running the a.c the pcm is supposed to turn on the fans at certain preset temps in the PCM. Mine being 195 and 205. They are not coming on. If I don't run the a/c I will overheat.

Re: PCM going bad MAYBE
 

You may have back-fed the PCM and cooked it. Best to let the PCM run the a/c portion of the fans as well.
Are there any codes for that circuit?
Can you use the scanner to actuate the fans?

Re: PCM going bad MAYBE
 

Attachment 2301279

I may have back feed the pcm. If so is the rest of it still going to function. I can rewire the fans to come on when I start the truck. I will have to get my mechanic to re scan it. when I wired the a/c fans I spiced into the fan wires after the relays so I don't see how I could have back feed the pcm as it would go through the relays on the hot power sides 12volts and not gone through the relay switch side which the pcm ground wire (dark green and blue) is connected to. here is my wiring. I know not easy to read but I am old. LOL

Re: PCM going bad MAYBE
 

Interesting issue, I'm running the same setup 2002 LS1 and Vintage Air unit.
I'm in the final stages of plumbing, but I haven't yet completed the AC install so I can't speak directly to your problem.

What I did, after consulting a friend that does lots of LS swaps, is to wire the ground from the trinary switch to the blue relay trigger wire from the PCM so that it will engage the primary fan either at the programmed temp or at the request of the trinary switch.

This is how my friend says that he wires them.
I'm not sure that it could back feed to the PCM since it's just a ground?

Re: PCM going bad MAYBE
 

Interesting. I still haven't hooked mine up, but don't want to cook anything. Following along here.

Re: PCM going bad MAYBE
 

If 12V power is back feed to the ECM ground it should blow the 12v circuit fuse.

Re: PCM going bad MAYBE
 

update. Changed the temp sensor in the head. No change still fans won't come on. Going to go test the fuse, is this the one that comes on the new harness or one I installed on relay?
would everything else work with the blown fuse.

Re: PCM going bad MAYBE
 

replaced my pcm today and disconnected my a/c wiring set up and started the truck. At 195 the fan came on then after cooling went off, works perfectly. So I conclude my pcm fan function on my old pcm went bad. Not sure what caused it but going to drive it with no a/c for a week or two and see how it reacts. then I will decide how to wire my a/c fan function back into it.

Re: PCM going bad MAYBE
 

How's that truck running?

Re: PCM going bad MAYBE
 

truck is running great. still haven't hooked up a.c wiring still deciding how to do it but leaning towards adding diodes on the fan power wire so no chance for any power to back feed to the PCM although pretty sure the back feed was self-inflicted by my mechanic after talking to him. This newest PCM has my correct rear end ratio programed in it and I can tell the difference, now I just need to get a way to read my RPMS at highway speed to make sure the overdrive is working. (don't want to buy a tach as I don't care to have a tach I just want a one time way of seeing what the rpms are.) Also if anyone needs a PCM replaced I have a guy I found on Ebay that did mine and I am impressed. Quick shipping very reasonable price. I am going to order a second as a back up so if or when this one goes out I have aback up. It was plug and play without any issues.

Re: PCM going bad MAYBE
 

not sure why you would want to change the fan wiring that was working correctly before to a separate system? i would use the scanner and do a component activation of the fans along with checking the relays.
clint