Hello everyone,
ive decided to add AC to my swap. I have everything i need but i cant figure out the wiring for the R4 compressor. I wanted to keep everything with the AC system stock but i didnt know the compressor was wired into the ECM. The car was a roller when i bought it and that ECM and wiring were gone. Is there a way i can wire in the compressor to the hvac harness without the ecm? I know ecm kicks the radiator fans on when the ac is turned on but that wont be an issue since my fans are on continuously.

 

The ac system is standalone. What engine controller are you using? I know the factory ecm standalones include a ac request wire to help adjust the idle. Terminator x has the same, There are also add on harnesses that allow tps% cutout and turn the fans on through the ecm. If the Ac is on you need to keep the fans on.

 

Ahhh ok. Im using a stock GM pcm from an 02 Yukon. I was looking at the wiring sheet for the harness and i do have a ac request wire. But im a little confused as to how it be wired in. I was able to identify all the plugs on my new ac heater harness (the one on my car was ripped out) except for this one plug. Id assume its for the compressor but i dont know how itll be wired in.

 

'88 Firebird service manual is here, https://www.thirdgen.org/forums/tech...ce-manual.html
Go to page 1347 to see the AC system wiring schematics.

There is a harness from the HVAC head unit that goes thru the firewall directly behind engine. Inside that bundle of wires is a LT GREEN wire that gets +12V continuous when the HVAC head unit is placed in AC position. That wire passes through the AC Low-Pressure and High-Pressure switches and powers the AC compressor. The pressure switches cycle the AC compressor on/off when fluid pressures are in proper range. Wire changes to a LT BLUE of sorts exiting the low-pressure switch, and changes again to DK GREEN of sorts exiting the high pressure switch on it's way to the AC compressor.

That's the bare bones AC control. Different cars had different layers of sophistication beyond that:

• First layer of sophistication is to turn on cooling fan when AC compressor is running. Engine ECM does this with '88 Firebird, and your '02 ECM will do it too. You can see the voltage sense in the schematics that tells the ECM when AC compressor is running. When the ECM witnesses voltage going high (+12V, AC on), the ECM will change the fan strategy to run the radiator fans more aggressively.
• Second layer of sophistication is to interrupt power to the AC compressor (turn off) when engine is at high throttle or high rpm. This extends the life of the AC compressor and keeps it from operating above high speed limits, and gives you a little extra power with the loud pedal down. Again, your '02 ECM will do this. You can see the interrupt relay in the schematics. It has ability to cut off the AC compressor whenever the engine ECM wants. Smart people use the interrupt relay.
• Third layer of sophistication is the ECM needs to be able to sense when the compressor clutch is cycled ON so that engine doesn't stutter and stall at idle. Carburetors don't struggle with this because the idle circuit adjusts itself very quickly with increased engine load. But the idle air control valve in a fuel injected engine can't react fast enough to prevent engine stumbling when AC compressor suddenly kicks on. The ECM needs a little bit of a heads up what is about to happen and prepares the engine for the load of the AC compressor turning on. This becomes more important with rowdier engines that have less stable idle quality.

​​FYI - it's not smart to run fans all the time, you're over cooling the radiator, especially in cooler weather. Let the ECM control the fans. There are fan strategies built into the software designed to help the engine warm up quickly and keep it at a good operating temperature.

 

^^^^
Excellent Post above!