Greeting! I have an 88 Firebird with a 5.3 and truck PCM from a 2002 Yukon. I adapted the 3rd Gen AC setup to my swap and wired everything but nothing is powering on. I followed this wiring set up that Lsxmatt on youtube. But yea nothing powers on. Ive been searching around and found a post saying the truck pcms dont need the high pressure switch, which is kinda concerning on how the system would work without the high pressure switch. Im just looking for some clarification to get the ac system to start working.

 

Is the PCM seeing the AC request (12v on green with af on at control head)? Have you verified that you have power to the relay? If so, is the relay clicking when you turn the AC on? There's a lot of preliminary checks you can do based on that wiring diagram. If the PCM is looking for the pressure transducer input and it doesn't see it, it may acknowledge the AC request but not activate the compressor. You have a way of viewing data?

 

So i know im getting 12v from the ignition on the relay. If i bypass the relay and connect it directly to the compressor the compressor will power on. Wire 43 going to the relay is supposed to be a ground provided by the pcm. If i connect a ground from the chassis to the relay itll power on the compressor. On the switch, i am getting a 5v reference on wire 45 for the switch. Thats as far as what ive diagnosed. How would i see if the pcm is getting the request from the control unit? And how would i know the HP switch is working as it should?

 

Check for 12v at pin 17 when you command AC on at the control head. What's listed there isn't a switch but a pressure sensor. It sends a variable voltage to the PCM based on high side pressure. Your signal wire should be between.5 and 4.5 vs.
If you can see data with a scanner, you should be able to see "AC request" and "AC clutch"

 

So i have also found on my scanner where to see if the pcm is getting the request signal. It is not. I have the car running and im not getting a request from the pcm. Also HP sensor says its getting 0 volts.

So my bad i had the sensor disconnected. Its now reading 0.84v

 

So next step is to verify the 12v at pin 17 green.

 

Pin 17 is getting 12v.

 

Let me confirm that is the AC request pin on my diagrams. Is your PCM tuned with HP tuners? If so, is it possible AC has been turned off?

 

Confirmed.


So either the PCM doesn't know to look for it, or the pcm isn't recognizing it for some reason

 

I have no idea tbh. Is there any way to check or would have have to get HP tuners? I noticed that when I was checking the ac request with my scanner that for a second it said yes to ac request and then quickly changed to no.

 

It saw it for a second. It's possible you have a pin tension issue. Does your scanner just read data? Or can you perform functional tests? If you can, see if you can actuate the compressor clutch with the scanner. All you're listening for is the click of the relay and/or the clutch. Key on engine off

 

My scanner just reads data. No actuation tests. Ill see if i can find one that does.

 

Ok.

 

So i got a scanner with actuation tests. I can engage the clutch with the scanner so something must be up with the ac request wire.

 

Do you have a standard, incandescent light bulb style test light? If it can light that bulb at pin 17, it has enough current to send the AC request to the pcm. Not to say the pin has tension, but that's unlikely.

 

I got one and checked and the light comes on the tester. I started playing around with the new scanner i got and this is saying that the pcm is getting the request. Ive been switching it on and off and its reading getting the requests. This new scanner the Gearwrench bydirectional scanner

 

So there it is. The PCM isnt commanding the clutch on, but it's receiving the request. It appears it's looking for a low pressure switch input that is open. I'm not at work today, so I can't pull up the diagram for the 02 Tahoe, but it appears the ECM needs another input to complete the command

 

.87 volts is good static pressure btw, so that wouldn't be the reason (unless the PCM isn't seeing the signal)

 

Yea ive started looking around on this low pressure switch.
Thanks how the help btw. I really appreciate.

 

This is from an 03 I found on Google, same body style so I'm guessing it applies. It seems like the 02 Tahoe looks for a serial data signal from the control head to determine if low side pressure is low. There may be a way to turn that request off in HP tuners, I'm not sure. I don't know if there's a way to simulate that signal, as it's not strictly voltage

 

https://forum.hptuners.com/showthrea...901#post620901

This post seems to be dealing with the exact same issue.

 

Ok so what im understanding is that the computer is looking for a ground when connected to the low side pressure switch. If so i can probably use the 3rd gen low pressure switch and hook it up to the computer on pin 55 and lead it to a ground.
I was looking at this diagram that i found from someone with a 2002 Silverado and it seems that the low pressure switch shares a ground with the AC clutch and gets sent to red pin 55

 

That is so much better than the 03 I pulled up. If that's the case, that's exactly what you can do. In the meantime, to test the theory, apply ground to that pin running and see if the PCM activates the compressor

 

Hooked up a pin to red 55. I applied ground to it and AC clutch kicks on. Even hooked it up to the low pressure switch and it kicks on. Scanner is reading everything good. AC is now working!

 

Hell yeah man! Well done, glad it was something easy to resolve.

 

Im glad it turned out to be simple. All thanks to you for guiding me through it. I really appreciate it.

 

Happy to help! To be fair, I'm in the middle of building my wiring harness for my 0411/TPI, So this gave me valuable insight into my own project as well.

 

If you need help shoot me a PM. Ill be happy to help!

 

Very much appreciated!