Here's why:
* The Relay's Job: The fuel pump relay acts as a switch, controlled by the engine control unit (ECU). It completes the circuit, allowing power to flow to the fuel pump when the engine is cranking or running.
* Relay Failure Modes: A relay can fail in several ways:
* Stuck Open: This is unlikely to cause a constantly running pump. It would simply mean the pump never receives power.
* Stuck Closed: This is the more dangerous scenario. If the relay is stuck closed, the fuel pump will run continuously even when the ignition is off, draining the battery and potentially causing a fire.
* Intermittent Failure: The relay might not make or break the connection reliably, leading to the pump starting and stopping intermittently or not at all.
* Constantly Running Pump - Other Causes: A constantly running fuel pump is usually caused by problems elsewhere in the system, such as:
* Faulty fuel pump itself: The pump's internal switch might be malfunctioning.
* ECU problem: The ECU might be sending the wrong signals to the relay.
* Wiring issues: A short circuit in the wiring can cause the pump to receive power even when it shouldn't.
* Faulty inertia switch: (Some vehicles have an inertia switch that cuts power to the fuel pump in a collision; if this is stuck in the wrong position, it could cause problems.)
In short, while a malfunctioning relay *could* theoretically lead to the pump staying on all the time (stuck closed), it's far more probable that a faulty relay will prevent the pump from working at all. If your fuel pump is running constantly, look for problems beyond the relay itself.
