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How do I wire a Vintage Air trinary switch into a 3-relay dual electric fan setup with the LS PCM?

Here is what the documentation says. Each point links to its source; details depend on your exact computer, harness and transmission.

Other wiring

  • The Vintage Air trinary switch has three pressure functions: high-pressure cutoff at 406 psi, low-pressure cutoff at 30 psi, and electric-fan engagement at 254 psi. (Vintage Air)
  • The Vintage Air trinary switch installs in the high-pressure line between the condenser and drier (or in the drier itself if it has a provision) using a #6 splicer with service port; if the system is already charged it must be evacuated before removing the port plug. (Vintage Air)
  • LSx/Vortec: Current Performance's heavy-duty dual-fan add-on module uses three sealed relays and two fuses to run two fans in series at half speed first, then full speed; it has 50-amp relays, one 40-amp JCase fuse per fan, 12 AWG fan outputs, an 8 AWG main feed, and ground-triggered control wires meant to interface with PCM fan outputs. (Current Performance Wiring)

Harness and power

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  • E38 ECM; Chevrolet Performance NA LS control kit 19171935; LS2/LS3/LS376/LS7/LSX454; 58x/4x; 2006+: ECM-controlled fan output turns on at 97 C (207 F). (Chevrolet Performance)
  • E67 ECM; Chevrolet Performance LSA E-Rod control system 19299067; LSA: LSA fan requires vehicle speed before it turns off after activation. (Chevrolet Performance)
  • Universal; A/C-equipped LS swaps: Trinary switch pressure points: 406 psi cutoff, 30 psi cutoff, 254 psi fan engagement. (Vintage Air)
  • Universal; A/C-equipped LS swaps: Trinary-switched fan relay fed from BAT through a 40-amp breaker. (Vintage Air)
  • 0411 PCM (GM service #9354896 or 12200411); 1999-2007 Vortec DBC (LM4/LM7/LQ4/LQ9/L33/L59); T56 or non-electronic automatic: Fan trigger wires are ground-side controls for customer relays. (PSI Conversion)
  • 0411 PCM (GM service #9354896 or 12200411); 1999-2007 Vortec DBC (LM4/LM7/LQ4/LQ9/L33/L59); T56 or non-electronic automatic: White A/C-request wire is 12 V when the A/C is on. (PSI Conversion)

Confirm against the instructions for your exact parts before wiring. More: Other wiring in our tuning guide.

What we recommend

What we've seen work is wiring the Vintage Air trinary switch into the fan relays through diodes and resistors, so it can trigger the fans at its pressure setpoint without back-feeding ground into the PCM and setting codes. The Gen 3 PCM grounds the low-speed wire (green, C1 pin 42) at about 190 F for half speed with the fans in series at about 6V, and grounds the high-speed wire (dark blue, C2 pin 33) at about 205 F for full 12V. Wiring the trinary to the high-speed side only has held up for well over 100k miles in builds we've seen, since the low-speed circuit is usually already on once the engine is warm. If you want the switch alone to guarantee both fans on high, wire it to both sides.

Check this first:

  • Confirm the low (C1 pin 42) and high (C2 pin 33) wire colors and pins on your PCM
  • Use three 5-pin relays for the series/parallel setup
  • Check for PCM fan codes after wiring the trinary in

Careful: With the trinary on the high side only, a cold engine running A/C will get only fan 2 at full speed, so wire both sides if you need full airflow from a cold start.

Based on what we've seen work across our own builds, our friends' builds and the wider LS community, reviewed by our lead tuner (20 years of LS tuning). 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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