How do I wire the A/C compressor to work with the LS ECM on an 87 swap?
Wire the '87 dash A/C request as a 12V+ signal into the PCM's A/C request input, and let the PCM ground-switch the compressor clutch relay. On red/blue P01 PCMs: A/C request is 12V+ on C2 pin 17, the low-side cycling switch provides ground on C2 pin 55, and the A/C clutch relay control is a ground-only output on C2 pin 43 that drives a relay — never the clutch directly. The tune must be programmed for your system type (cycling switch vs pressure transducer), and only certain 2003-2007 blue/green PCMs support the physical request wire at all.
Last updated
Who this applies to
- Computers: P01, P59
- Engines: Gen III LS/Vortec
- Transmissions: all
- Years: 1999-2007
- Not for: 2003-2007 blue/green PCMs whose service number is not on the supported list (physical A/C request input will not work)
1999-2002 red/blue P01 PCM (physical request supported on all)
- Run the dash A/C switch output as 12V+ to PCM C2 pin 17 (A/C request). Route it through the high-pressure cutout switch if you want factory-style protection. C2-17, 12V+ request
- Wire the low-side cycling switch to provide ground on PCM C2 pin 55. C2-55, ground from cycling switch
- Wire PCM C2 pin 43 to the ground side of a clutch relay coil; the relay's fused 12V feed drives the compressor clutch (green wire positive, black ground on GM compressors). C2-43, ground-only relay control
- Program the PCM for the cycling-switch system. tune: cycling-switch A/C type
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2003-2007 blue/green P59 PCM
- Check the PCM service number against the supported list: 12576106, 12581565, 12586243, 12589462, 12602801. Any other number will not accept the physical request wire. 5 supported service numbers
- Program the PCM for the pressure-transducer system and wire the transducer's 5V reference, low reference, and 0-5V signal back to the PCM. transducer system in tune
Careful
- C2-43 is a low-current ground driver for a relay coil only — wiring the compressor clutch directly to it will damage the PCM.
- On 2003-2007 PCMs, confirm the service number before wiring; an unsupported PCM makes the physical request wire dead on arrival.
Go deeper
Check this first
- With A/C commanded on, meter 12V at C2-17 (request) and ground at C2-55 (cycling switch).
- Check whether the PCM pulls C2-43 to ground with A/C requested — no ground = tune/wiring mismatch, not a bad compressor.
- Confirm the tune's A/C type matches the wiring (analog cycling vs pressure transducer).
Try this, then this
- If C2-43 never goes to ground, re-check the tune's A/C system type against your wiring. — A PCM programmed for a pressure transducer will ignore a cycling-switch input and never engage the relay. Worked if: Relay pulls in and the clutch engages.
- On a 2003-2007 PCM, verify the service number is on the supported list. — Unsupported blue/green PCMs physically cannot read the 12V request wire. Worked if: Request input works, or you switch to a supported PCM.
- Jumper the clutch relay manually to confirm the compressor, clutch air gap, and refrigerant side are healthy. — Separates the control-side problem from the mechanical/refrigerant side. Worked if: Clutch engages and the system cools when jumped.
How you know it's fixed
- Clutch engages with A/C on; PCM bumps idle, runs the fans, and cuts the clutch at wide-open throttle and during cranking.
Key numbers
| What | Value |
|---|---|
| A/C request input (Gen III) | 12V+ on C2 pin 17 |
| Cycling switch input | ground on C2 pin 55 |
| A/C clutch relay control | ground-only, C2 pin 43 |
| Supported 2003-2007 service numbers | 12576106, 12581565, 12586243, 12589462, 12602801 |
| GM compressor wiring | green positive, black ground |
Every option we know of
| Maker / product | Part # | Fits | Price | Where | Status / notes |
|---|---|---|---|---|---|
| PSI White A/C request wire (included in standalone harnesses) | HAR-1016 family | Gen III DBW harness swaps | included in harness | psiconversion.com | In production Installer wires the dash side |
| Vintage Air Trinary switch (fan control + high-pressure cutout when A/C is engaged) | Aftermarket A/C on LS swaps | vintageair.com | In production Does not replace the PCM request input | ||
| GM 3-wire A/C pressure transducer (2003+ truck) | 2003-2007 P59 transducer-type A/C | varies | GM dealers / salvage | In production Requires the matching tune setting |
Prices are approximate as of September 2026. We list every option we know of, including small shops and used parts we can't link to. Confirm fitment with the seller.
Time and cost: Relay, fuse, and wiring: 2-3 hours plus a tune update for the A/C system type.