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LS fuel pump and electric fan relay wiring
How to wire the fuel pump relay and the electric fan relays to a GM 0411, P59 or E38 computer in a swap: which pin, which relay terminal, and what the computer does with it.
- Disconnect the battery before you splice or de-pin anything.
- Go by pin number and circuit, not color: GM reused colors, and harnesses differ by year and options.
- Never probe computer pins with a test light; it can damage the driver. Use a digital multimeter.
- The computer only ever switches the relay coil. The pump and fans run through the relay contacts and their own fuse, never straight off a computer pin.
How the relay is wired
A standard 4-pin automotive relay has two circuits. The coil (terminals 86 and 85) is the small side the computer controls. The switch (terminals 30 and 87) carries the pump or fan current.
| Terminal | Goes to |
|---|---|
| 30 | Battery positive, through a fuse sized to the pump or fan's draw |
| 87 | The fuel pump or fan positive. The other side of the pump or fan goes to ground. |
| 86 | Key-on 12 V (fused) |
| 85 | The computer's relay-control pin. The computer switches the ground side of the coil. |
Fuel pump relay and prime
GM lists the fuel pump relay control pin as a low-side driver on the 0411, P59 and E38: the computer grounds the relay coil to run the pump. At key-on it runs the pump for about 2 seconds to prime the system, then shuts it off if it doesn't see the engine turning. Once the crank signal arrives, the pump runs as long as the engine turns.
- Quick check: key on, and the relay's load terminal should have power for the first two seconds. No click and no prime? Check the key-on feed to 86 and the computer pin on 85.
- Fuse and wire: size them to the pump's draw, with a solid ground at the pump. Rail pressure on a factory-style system is about 55–60 psi.
- Secondary pump pin (circuit 1465) is for dual-tank trucks. Leave it unused on a single pump.
Electric fan relays
The computer drives each fan relay the same way: it grounds the relay coil. Never connect a fan straight to the computer pin. With two fans, the tune turns fan 1 on first and fan 2 at a higher temperature; GM's own three-relay setup runs both fans slow in series first, then both on high.
- Temperatures are tune settings. A typical tuned setup grounds the low-speed fan around 190 °F and high speed around 205 °F.
- A/C: some computers bring on fan 2 from A/C pressure. With a trinary switch instead, its fan contacts can ground the fan relay directly.
- No computer fan control? A thermostatic fan controller works too: adjustable ones run one or two fans, and ready-made relay kits come set at fixed temperatures such as 205/190 °F or 185/175 °F.
P01 / 0411 (1999–2002)
| Job | Computer pin | Wire | Relay terminal |
|---|---|---|---|
| Fuel pump relay | C2 (red) pin 9 | DK GRN/WHT, circuit 465 | 85 |
| Fan 1 (low speed) relay | C1 (blue) pin 42 | varies by harness | 85 |
| Fan 2 (high speed) relay | C2 (red) pin 33 | DK BLU, 473 | 85 |
Fan 2 (high speed) wires the same way: a second relay, with 85 on C2 (red) pin 33.
These fan outputs are wired on car harnesses. On 1999–2002 truck harnesses pin C1-42 is empty and C2-33 runs the HVAC recirculation door, so check yours, and make sure the tune has the fan outputs and temperatures set.
P59 (2003–2007)
| Job | Computer pin | Wire | Relay terminal |
|---|---|---|---|
| Fuel pump relay | C2 (green) pin 9 | DK GRN/WHT, circuit 465 | 85 |
| Fan 1 (low speed) relay | C1 (blue) pin 42 | DK GRN, 335 | 85 |
| Fan 2 (high speed) relay | C2 (green) pin 33 | DK BLU, 473 | 85 |
Fan 2 (high speed) wires the same way: a second relay, with 85 on C2 (green) pin 33.
Some 2005 and later harnesses have both fan wires from the factory. On most P59 harnesses pins C1-42 and C2-33 are empty cavities: add terminals there. Make sure the tune has the fan outputs and temperatures set.
E38 (Gen IV)
| Job | Computer pin | Wire | Relay terminal |
|---|---|---|---|
| Fuel pump relay | X1 pin 50 | DK GRN/WHT, circuit 465 | 85 |
| Fan 1 (low speed) relay | X1 pin 58 | DK GRN, 335 | 85 |
| Fan 2 (high speed) relay | X1 pin 17 | DK BLU/WHT, 473 | 85 |
Fan 2 (high speed) wires the same way: a second relay, with 85 on X1 pin 17.
The fan pins aren't populated on every E38 harness (it depends on the vehicle's options), so check your connector before you count on them.
Let us set the fans and pump up in the tune. A Start-up File ($79) sets the fans, injector data, idle and conservative fuel and spark, built on a read of your computer.