What tells fuel pump to run when engine has started?

The fuel pump doesn't directly get its signal to run from the engine starting. Instead, it's controlled by a relay that's activated by the ignition switch or engine control module (ECM). Here's a breakdown of how it works:

1. Ignition Switch: When you turn the key to the "ON" position, power is supplied to the fuel pump relay.

2. Fuel Pump Relay: This relay acts as a switch, allowing electrical current to flow to the fuel pump motor.

3. Engine Control Module (ECM): In modern vehicles, the ECM monitors various engine parameters and can also control the fuel pump relay. The ECM may activate the relay:

* During cranking: This ensures fuel is being delivered while the engine is starting.

* After the engine starts: The ECM may keep the fuel pump running continuously to maintain fuel pressure.

* Based on engine load and speed: The ECM may adjust the fuel pump operation to ensure optimal fuel delivery.

4. Fuel Pump: Once the relay is activated, power flows to the fuel pump motor, causing it to operate and deliver fuel to the engine.

In summary:

* Key turn to "ON": Fuel pump relay is energized, allowing power to flow to the fuel pump.

* ECM: The ECM can further control the relay based on engine conditions.

Note: Some older vehicles may have a mechanical fuel pump directly driven by the engine camshaft. These systems don't rely on electrical relays and operate continuously while the engine is running.