I put the OEM fans on my 01 Denali. I used the OEM harness. The ac control works perfect, but I cant get the low speed fans to turn on. I have 2 custom tunes and neither one works. The harness has been tested both on the truck and on the bench, and all works fine. I tested the pin at the ECM and can trigger the low speed fan like it should. I replaced the ECM pin just in case, but that didn't work either. I have gone through the wiring from ECM to the relays. I checked the lead from pin #42 with a test light, and it won't trigger at any temp. I'm out of ideas.
The tunes are coming from the guys at Diablo.
Any thoughts?

 

Did you try a new relay to see if that works?

 

I can trigger the low speed fan at the ECM harness, so all of my relays are good. I just can't get the ecm to trigger it.
I just wonder if I'm missing something.

The OEM harness only has 2 wires to trigger the fans, one is for the AC trigger and the other is the low speed trigger. it all works perfect on the bench. The ac trigger works perfect in the truck, just no luck with the low speed trigger from the ECM.

 

Faulty PCM driver or faulty tune.

 

I thought the PCM might be the issue, but the guys at Diablo said unlikely.
As far as the tune goes, you have 2 choices, it's either 1 or 2, 0 is off.
And I have both tunes.

 

If you know you have the right pin and you know the tune is set to enable that pin (under any condition) then you have a defective PCM.

 

Is it wired up like this thread? https://www.performancetrucks.net/fo...-chevy-484892/

Just want to make sure your signal's are set up correctly.

The PCM sends a ground signal not a power signal which is why the test light won't work. And just an FYI, you shouldn't be using a test light on the PCM any ways. As it completes a circuit it could cause damage to the PCM.

Per ScottyBoy's later post. "Green turns both fans on low. Blue turns only the right fan on high. If green and blue are both grounded at the same time, then both fans turn on high."

 

Yes sir, just like that.
Maybe I should try to find another ecm and start over.

 

I did ad some info to my original post.

How ever if you are sure that the signals are set up correctly then maybe your PCM is damaged. But you need to verify everything is set up correctly since the tune could be the issue.

 

I am 100% certain that my wiring is correct.
My truck runs great, I have a hard time thinking it could be the ecm, but stranger things have happened.