Start button · starter relay · interlock
Push button start wiring: ignition, ECU and starter
A key switch does two jobs: it turns the ignition on and it cranks the engine. A push-button start splits them: one circuit switches ignition power through a relay, a separate one runs the starter — and only when it is safe to.
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A typical layout
- Ignition relay: its coil is switched by an ignition toggle (or the first press of the button); its contacts feed the ECU, the fuel pump relay and the dash through their fuses.
- Starter relay: battery + → fuse → 30; 87 → starter S terminal. Its coil gets power only from the start button.
- Interlock in series with the starter relay coil: clutch pedal switch on a manual, park/neutral switch on an automatic.
- Immobiliser or ECU permission, if used, also sits in the starter relay coil circuit.
Preventing unintended or repeated cranking
- Crank only while the button is held (a momentary button), never from a latching switch.
- Cut the starter when the engine runs: an ECU output or an RPM switch opens the starter relay coil above cranking speed.
- A start controller can add a crank time limit (e.g. 10 s) and refuse to crank with the engine already running.
- Never feed the starter solenoid straight from the button — its current (tens of amps on S) would weld the button.
Build it in WireForge
- Open the “Starting and charging” demo: battery, starter and generator are already wired.
- Add a latching Switch for ignition driving one Relay, and a momentary Switch (hold) for start driving a second Relay to the starter S terminal.
- Put a second latching Switch (the clutch switch) in series with the start relay coil.
- Simulate: the starter turns only with ignition on, the clutch switch closed and the start button held.
Questions
Do aftermarket push-start kits work with any car?
They work on simple wiring, but on cars with immobilisers, keyless entry or steering locks they need the car’s own permission signals. Check before cutting the ignition switch wiring.
Why does the starter need its own relay?
The solenoid’s S terminal draws tens of amps. A relay keeps that current off the button and its thin wires.
Should the button be lit only with ignition on?
It is a good cue and easy: feed the button lamp from the ignition relay output.