How do you wire the A/C system when running Holley Terminator X EFI on an LS swap?
- Holley Terminator X provides an A/C Kick input on ECU pin A12 (connector pin A, white/blue) and an A/C Shutdown output on ECU pin B10 (connector pin G, gray/black) that deactivates the clutch at a configured throttle position; the kick input is a ground-activated signal. (applies to Holley Terminator X; GM LS)
- Holley Terminator X has an IAC Kick feature that automatically adds IAC steps so the engine does not experience a momentary drop in idle when the A/C compressor kicks on. (applies to Holley Terminator X; GM LS)
- Holley Terminator X fan outputs are Fan 1 on ECU pin B12 (connector E, gray/yellow) and Fan 2 on ECU pin B11 (connector F, gray/red); these are ground outputs that trigger fan relays and must never be connected directly to the fan motor. (applies to Holley Terminator X; GM LS)
- Holley Terminator X provides a fuel pump relay output on J1A connector pin A2 that supplies +12 volts at up to 10 amps maximum. (applies to Holley Terminator X; GM LS)
- Published figures differ: The GM 0411, P59, E38 and E67 work the same way: they send +12 V to the fuel pump relay coil. The fan outputs are the ones that ground their relays.
- The Current Performance Holley Terminator A/C integration module operates the A/C compressor clutch, interfaces with the Holley ECU for A/C shutdown and IAC kick, and signals the ECU to request electric fan operation while the A/C runs; it includes binary pressure-switch leads, an A/C relay, and a 10-amp inline fuse. (applies to Holley Terminator X; GM LS)
- Holley Terminator systems provide outputs for up to two electric cooling fans plus an A/C shutdown output that deactivates the A/C at higher throttle settings. (applies to Holley Terminator; GM LS)
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Confirm against the instructions for your exact parts before wiring. More: Other wiring in our tuning guide.
We've seen a Terminator X handle A/C in a third-gen with three wires. First, the 12V request signal from the third-gen dash controls is the A/C and idle-kickup command. Second, the Holley grounds the compressor relay through the Fan 2 output (gray/red stripe wire), with Fan 2 set to 300F on and 299F off so normal fan operation never triggers the A/C. Third, a cutout relay wired through its 87a (normally closed) terminal lets the Holley cut power to the compressor relay at WOT or under any other condition you program. For pressure switching, run the factory third-gen low-pressure switch and a universal 2-wire high-pressure sensor (both normally closed) in the ignition feed to the compressor relay. We don't recommend the 4th-gen 3-wire pressure sensor here, because it didn't work with the Holley in the build we saw.
Check this first:
- Fan 2 is not used for an actual fan
- Fan 2 temps are set to 300F on / 299F off
- Both pressure switches are normally closed and wired in the relay's ignition feed
Careful: Keep both the low- and high-pressure switches in the compressor circuit, since running the compressor without pressure protection can damage the A/C system.
Based on what we've seen work across our own builds, our friends' builds and the wider LS community, reviewed by our lead shop tuner (over 20 years of LS and LT tuning experience). Every combination is different, so use at your own risk and check our safe tuning limits before any wide-open pass.
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| Maker / product | Part # | Fits | Price | Where | Status / notes |
|---|---|---|---|---|---|
| Holley Terminator X MPFI | 550-931 (24x/1x) | Gen III LS | about $1,200+ | Holley, Summit, Jegs | Current production Fan outputs are ground triggers for relays only |
| Vintage Air Trinary safety switch | 11086-VUS | A/C pressure protection | about $60-90 | Buy vintageair.com, Summit | Current production Fan engagement at 254 psi |
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.