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How should electric fans be wired with a Terminator X / LS swap ECU?

Most aftermarket EFI systems have cooling fan outputs. Use them to trigger relays, never to power the fans directly.

Last updated

Recommended dual-fan setup:

  • Two relays, one per fan. Each fan's positive goes to its relay's output. Relay coils get switched power; the relay power feeds get fused constant power.
  • Trigger: the ECU's fan output (or a thermostatic switch) grounds the relay trigger side.
  • Don't run both fans off one standard relay. Two fans starting at once can blow the ~30 A fuse on a standard relay. A single high-current relay works if you're short on space, but costs more.
  • Staging: you can run one fan on temperature and the second on A/C compressor engagement or a manual switch.
What we recommend

We recommend giving each fan its own relay with its own fused power lead, and never twisting both fans onto a single ECU fan output. If one fan overloads the driver (a worn fan bushing is enough), you lose both fans at once. Ideally, use two different ECU triggers on the Terminator X so each fan comes on at a different temperature. The same relay-and-fuse discipline applies to the fuel pump output.

Check this first:

  • Confirm which ECU outputs are available as fan triggers
  • Size fuses and relays for each fan's draw

Careful: Overloading an ECU output can take out both fans at once and lead to overheating.

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.

Go deeper

Check this first

  1. Verify each fan has its own relay with a fused power feed; ECU only grounds the relay coil

Try this, then this

  1. Wire ECU fan outputs to relay coils, fused battery power to fans — manual. Worked if: fans run

How you know it's fixed

  • Fans cycle on/off at set temps; no relay chatter or blown fuses

Still not fixed?

  • Check relay coil ground wiring and fan current draw vs relay rating

Key numbers

WhatValue
Fan outputs are ground triggerswire to relay coil, not the fan
On/off temperature spreadat least 5 degrees

Every option we know of

Maker / productPart #FitsPriceWhereStatus / notes
Holley
Terminator X ECU outputs		n/aLS swapsECU ~$1,100Holley dealersIn production
outputs are ground-only, relay required
DIY
dual 40A relay harness with fuses		n/aany e-fanabout $40parts storeDIY
size fuses to fan draw
generic
40A automotive relay per fan		n/a

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: Wiring: 2-3 hours

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