I have nearly completed my swap from a 2.8 to 5.3 LS in my 89 Firebird. One of the last things I need to get working is the air conditioning. I have verified the wiring from the heater/ac fan switch all the way to the compressor. Where I can't get a handle on what is happening is where the ECM is controlling the AC compressor control relay. What is the ECM requiring to energize the compressor control relay? Is the power steering switch a part of the answer? I have tried shorting the power steering pressure switch and leaving open with no results. I can make the compressor run by grounding wire number 905 so I know all is good except for where the ECU is supposed to energize the AC compressor control relay. I am toying with the idea of going around the compressor control relay by connecting wire 59 to 959 and letting the control head switch turn on the compressor directly. The wire is fused before the AC control head and is sized to run the compressor so I don't see a real down side other than whatever the ECU is controlling. Thoughts?

 

Your schematics are way too small to be legible. As for activating the A/C compressor clutch, 99% of the time, the ECM provides a ground to the relay coil.

The P/S pressure switch is usually an input to the ECM to let the ECM know to cut out the compressor. This is done so that in tight parking the added P/S load and A/C compressor together don't stall the engine.

RBob.

 

I just helped a friend with a LS swap get his A/C hooked up to the LS PCM. His is (I believe) a 92, but my guess is the HVAC wiring is probably the same... though I'd strongly advise to verify prior to doing the below.

We used this schematic as a guide...

Note that you need to get the pressure sensor and pigtail from a LS Camaro. It's a 3-wire sensor that replaces the existing 2-wire sensor on the high side line. One thing I also did was take terminal 30 on the relay and run it directly to the battery with it's own 10 amp fuse. That way when the relay is energized, the clutch pulls power from it's own dedicated 12V source. I didn't want to hook the A/C clutch directly to the main switched ignition circuit.

The sensor that's installed in the drier appears to be not essential to the A/C operation, so it's evidently deleted from this circuit.

The reason they tie into the IAT circuit is to use the low side of the 5V reference (essentially the ground side of the 5V circuit). The pressure sensor is also 0-5V, so I guess it was convenient to tie into the 5V circuit on the IAT.

If you're retaining the R4 compressor, then put the switch on the back of the compressor in series between the 87 terminal and the clutch. Terminal 87 off the relay sends the 12V back to the PCM to let it know the A/C clutch has actually been activated.

The light green wire on the back of the HVAC control panel sends out the switched 12V for the PCM, to tell the PCM you want the A/C activated.

The PCM activates the relay by grounding terminal 85.