ECUs & Tuning

Holley Terminator X is the most common standalone in the Gen Coupe LS community.

Picking the Right Terminator X Kit

  • Crank reluctor: Holley sells separate kits for 24x engines (LS1/LS6 and early trucks) and 58x engines (most Gen IV). Example: 550-903T is the 24x LS1/LS6 kit with EV1 injector connectors, about $1,450.
  • Terminator X vs X Max: get the Max if you want drive-by-wire throttle or automatic transmission control. Holley's DBW harnesses are listed for X Max and Dominator.
  • Inputs/outputs: the Terminator X has 4 programmable inputs/outputs (12V, ground, 5V, frequency inputs), which fill up fast with fans, A/C, alternator and VSS. Plan which pins go where before wiring.
  • Display: some kits ship with a USB/CAN tuning cable instead of the handheld. The 7" digital dash and 6.86" Pro Dash are sold separately.

12V Power While Cranking

The Holley needs 12V while cranking: driver's-side fuse box, 2nd row, third down, labeled ECU power. Photo and details on the Wiring page.

Holley Dash

I made a simple 90° L bracket to mount the Holley dash. With this setup I can tint the OEM bezels to dim the backlights and still see the fuel level gauge without rewiring it into the dash. Pushing the steering wheel up holds the dash in place nicely. Cut a hole in the base panel right behind the dash so you can route the harness down behind the dashboard.

Dash on the L bracket.
Interior view.
From the driver's seat.
OEM fuel gauge still visible.

Adding Speedo to the Holley Dash (T56)

This only works with the Terminator X Max. Update: on the new V3 software you don't need to do this — there's a dropdown setting specifically for the T56 Magnum.

To see vehicle speed on the Holley dash or feed it to the ECU, either run the Holley GPS add-on (554-140) or wire the trans VSS to the ECU.

To wire in the VSS, wire a 2-pin VSS connector to pins B14 (ground) and B18 (signal) of a J4 connector. Then add the Transmission ICF in Holley and set:

  • Select 4L80 trans
  • Enter custom trans gear ratios
  • Enter tire size
  • Enter 1.17 as the rear end ratio for the T56 Magnum XL

VSS Conversion Math

4L80 VSS teeth40
Tremec Magnum XL VSS teeth12
40 / 123.333
Diff ratio / 3.333 (e.g. 3.91 / 3.333)1.173 — enter as the new diff ratio
J4 connector with VSS pigtail.
Transmission ICF speed setup.
Holley forum note on VSS input.

Fan Control

Fan relay wiring, pin map, and key-on fan settings are on the Cooling page.

Community & Parts Links

Sources & further reading

Prices and part numbers were checked in October 2026 and can change. Always confirm with the vendor before ordering.