Injectors · main relay · low-side outputs

Fuel injector wiring diagram

Every injector has two wires: one is +12 V, the other goes to the ECU, which connects it to ground for exactly as long as fuel should flow. Here is how the circuit is built and what to check when an injector does not click.

Open the editor · ▶ Run the live demo

The circuit

Main relay → injector fuse → +12 V to all injectors in parallel.
Each injector’s second pin → its own ECU injector output.
The ECU switches that output to ground (low-side) to open the injector, in firing order, only while it sees a crank signal.

High and low impedance

Most modern injectors are high impedance, about 12–16 Ω: they draw roughly 1 A each and any standard ECU output drives them. Low impedance injectors, about 2–3 Ω, draw 4–6 A and need a peak-and-hold driver or series resistors; on a plain output they overheat it.

Watch the injectors fire

Open the “ECU fuel injection” demo: six injectors on an ECUMASTER EMU Classic.
At 0 rpm nothing fires — there is no crank signal.
Drag the engine speed up: the injectors fire one after another and pulses run along their wires.
Remove the injector fuse in the editor and see the ECU outputs go dead.

Questions

Which injector wire is power?

One pin is +12 V and is common to all injectors; it measures 12 V with the ignition on. The other is the ECU control wire.

Why does one injector not click?

Check the +12 V at its connector, the coil resistance of the injector, and the control wire to the ECU for a break.

Can I use low impedance injectors on any ECU?

Only if the ECU has peak-and-hold drivers, or with a resistor box in series.

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