Fuel pump · relay · ECU control
Electric fuel pump wiring: relay, fuse and ECU control
The fuel pump is one of the biggest continuous loads in an injected car and it sits at the far end — in the tank. It runs through a relay the ECU controls, with its own fuse and a wire thick enough for the long run.
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The circuit
- Battery + → fuse near the battery → fuel pump relay 30.
- Relay 87 → pump + ; pump − → ground near the tank, on a wire as thick as the feed.
- Coil: 86 from switched ignition +, 85 to the ECU fuel pump output.
- The ECU primes the pump for 2–3 seconds at ignition on, then runs it only while it sees a crank signal — so the pump stops if the engine stalls.
Typical loads on 12 V
| Load | Power | Wire length | Current | Fuse | Wire |
|---|---|---|---|---|---|
| Parking lights | 10 W | 4 m | 0.83 A | 2 A | 0.35 mm² (22 AWG) |
| Headlight bulb H7 | 55 W | 3 m | 4.58 A | 7.5 A | 0.75 mm² (18 AWG) |
| Pair of headlights | 110 W | 3 m | 9.17 A | 15 A | 1.5 mm² (16 AWG) |
| Horn | 60 W | 2 m | 5 A | 7.5 A | 0.5 mm² (20 AWG) |
| Wiper motor | 120 W | 2 m | 10 A | 15 A | 1 mm² (17 AWG) |
| Radiator fan | 250 W | 2 m | 20.83 A | 30 A | 4 mm² (12 AWG) |
| LED light bar | 300 W | 4 m | 25 A | 35 A | 6 mm² (10 AWG) |
| Audio amplifier | 1000 W | 5 m | 83.33 A | 125 A | 35 mm² (2 AWG) |
Wire size for the long run
A 15 A pump at the back of the car, 5 m from the fuse, takes a 20 A fuse. 2.5 mm² would carry the current but loses over 0.5 V on that length; 4 mm² keeps the drop under 3 %. The pump’s flow falls with its voltage, so the drop matters here more than on most loads.
Single vs dual pump
| Single pump | Dual pump | |
|---|---|---|
| Relays | One | One per pump |
| Fuses | One | One per pump |
| Control | ECU fuel pump output | Pump 1 always; pump 2 from a second ECU output by load or boost |
| Feed | One dedicated wire | Separate wires, or one sized for both |
Build it in WireForge
- Open the “ECU fuel injection” demo or place an ECU module, a Battery, a Fuse and a Relay.
- Add the pump as a Cooling Fan Motor device renamed “Fuel pump” and set its power, e.g. 180 W.
- Wire 30 and 87 to the pump, 86 to ignition +, 85 to the ECU, and give the pump wire its length.
- Simulate and click the pump wire to read current and voltage drop; change the cross-section until there is no warning.
Questions
Why not run the pump from the ignition switch?
The pump would keep running after a crash or a stall, and the ignition switch contacts would carry its full current. The ECU-controlled relay stops it when the engine stops.
What does the inertia switch do?
Many cars have one in the relay coil or the pump feed: a hard impact opens it and stops the pump. Keep it if the car has one.
Can I take the pump ground from the body near the tank?
Yes, if it is clean bare metal and the body is well grounded to the battery. A separate ground wire back to the battery is more reliable on high-current pumps.