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RH-03 LSX LTX EFI Dual Speed Fan Harness

RH-03 LSX LTX EFI Dual Speed Fan Harness

Regular price $180.00 USD
Regular price Sale price $180.00 USD
Sale Sold out
SKU: RH-03
Category: Fan Wiring
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After lots of requests from customers, it's finally here. Out plug-and-play dual speed fan harness.

How does this magic harness work you ask? Great question. It creates a low speed by running both fans in series at the low speed operation. If we have two identical fans and run them in series, the result is each fan will get exactly 1/2" of the applied voltage.

In a standard 13.8v automotive system, that means each gets 6.9v. Hence, it runs at about 30% of the max fan RPM at low speed. Then when full voltage is applied, it runs at full speed. Hence giving you two speeds.

I would like to take credit for this ingenious design, but it's been used in the automotive world going back to the 70s. The most popular usage of this series/parallel fan control is on the 4th gen fbody and C5 Corvettes with the LSX motors. 

We've packaged this together into a very easy to use plug-and-play wiring harness. It has two fan triggers that are activated by grounding the trigger wires. It can either be used with two temperature switches with two different temperature activation points, such as 175F for low speed and 185F for high speed. Though it's not recommended.

This harness is best suited for use with OEM PCMs like LSX, LTX, Holly, FAST, etc that have dual fan control built it. And control the fans by providing a ground switched control. This harness is activated by grounding the two triggers. 


  • Tripled sealed 40 AMP OEM grade relays
  • Dual 40 amp and a single 5 amp sealed fuse folders
  • Metripack sealed connectors used throughout
  • Lifetime warranty on everything
  • Heavy duty 10 gauge OEM grade automotive wiring
  • Installation instructions
  • Use with aftermarket A/C such as Vintage Air may require the RAC-1 relay. This will force the fans on HIGH when the A/C is activated.
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