so my 1994 Z28 LT1, Automatic a/c compressor will run all of the time and not cycle. Its been replaced 2x's and the problem appears to be with the computer. how close do I need to get a used computer to specs? ie z28, automatic, year etc. Also does it have to be reflashed with my VIN once it gets installed?
Thanks

 

So you are saying you have power going to the clutch of the compressor all the time? Why did you change the compressor twice?

Have you looked at the schematic for your year? 1994 was the first year for the PCM. I do not believe it controlled any accessories. In 1996 GM added the body control module and that connected to the PCM, and that controlled AC/heat. So even in the more modern cars it would not be a PCM issue.

Yes anytime you replace the PCM in a GM car with anti theft you will need to flashing your vin number.

Manuals are linked down in the manual section.

 

The 94 PCM does control the A/C compressor clutch relay. It uses two sensors to determine how to control it. One is the A/C refrigerant pressure sensor. If that indicates that the pressure is too low, the PCM will not turn on the compressor clutch relay.
The second sensor is the evaporator temperature sensor. It's purpose is to keep the evaporator from freezing up with ice. If the sensor indicates the temperature in the evaporator to be near freezing, it will cause the PCM to shut off the compressor clutch relay until it indicates a higher temperature. That makes the cycling.
I would check the evaporator temperature sensor, since it's failure (indicating a high temp) would prevent the compressor from cycling. Does the evaporator freeze up with ice?
My PC scan app for that car or a Tech II will show you the reading of that sensor.
https://camaroforums.com/forum/compu...-95-lt1-77363/

I don't have a chart that shows the voltages and temp readings for the sensor. But if you measure the output (dark blue wire going to PCM blue connector D, terminal 44) going to the PCM and is is either 0v DC or 5v DC, then it is probably defective or disconnected.

Another possibility is that the A/C refrigerant charge is too low to make the temp low enough to get to freezing.

Edit:
I just realized I can give you some readings from my PCM test setup. They are approximate:
1.0 VDC = 90 degrees F
2.0 VDC = 55 degrees F
2.7 VDC = 32 degrees F
3.0 VDC = 25 degrees F

 

Thanks these are helpful. I'll report back soon on findings

 

Are you saying that the compressor runs all the time regardless of the A/C control switch? Or does it just fail to cycle off after the A/C is switched on? If it is really always on, did the mechanic check the compressor clutch relay by swapping it with another one?

If it is not running with the A/C control switch off:

This is something that may help. The trouble code for the evaporator temperature sensor is 71. That code does not cause the Service Engine Soon lamp to come on. You would have to use a scan tool to see if that code and others for the A/C system are present. Once again, my pc scan app can help.

 

That is the question that I have. If the compressor is engaged at all times, regardless of the position on the AC/Vent control ****, then it is more than likely a wiring issue.

To the OP:
How long have you owned the car? Is this a new problem, or one that has always been there?

Replacing the compressor is just a waste of time and money if the power signal to the AC Clutch is always present.

It could be present because of a bad computer - not very likely, put possible; or someone could have hacked the wiring to apply a constant 12V and ground to the AC Clutch with the key on - not hard to do and very possible if a previous owner wanted to try and "fix" a non functional AC system.