A fuel pump relay takes control of the fuel pump during engine startup, but shuts itself off after the engine is running. An oil pressure mechanism resumes control of the fuel pump after the fuel pump relay shuts off and the vehicle's engine is running. The oil pressure mechanism also relays oil pressure data to the gauges on the inside of the driver's dashboard. The vehicle will not start, however, if there isn't enough oil pressure to generate the electricity needed to run the fuel pump. When the oil is warm, it becomes thinner, making it difficult to pump into the engine. The fuel pump relay generates a humming sound when the ignition is turned on. When the humming sound is not audible, it is time to have the fuel pump relay replaced by a mechanic. The starter cranks are charged up by a cold start valve, which is operated by the thermo timing switch. This valve injects fuel when heat is at a minimal level. While cold start valve is operating, the thermo timing switch heats up (while cranking). The cold start valve will stop injecting fuel if the engine fails to start after prolonged attempts to start the engine (this is done to avoid flooding the engine with fuel).
Each fuel pump relay has a magnetic coil with an open contact. The relay charges up when the ignition is started. The magnetic coil's open contact clamps shut, closing the circuit between the fuel pump relay and terminal that leads to the electronic control mechanism. A ballast resistor bank receives an electrical charge through the same terminal as the control mechanism. A switch transistor housed in the control mechanism is wired to all injector solenoids. The ballast resistors act to quicken the time it takes for injector solenoids to respond.
In some vehicle designs, there are two fuel pump relays (with one of the fuel pump relay unit's coils running to a diode). The diode is designed to keep electrical current from flowing in the opposite direction (backward). The electrical current runs through the starter solenoid and one of the fuel pump relay terminals. As the engine runs for a while, the air flow sensor executes a pump switch that closes the terminals to the fuel pump relay. The terminal leading to the starter is de-energized. The first fuel relay contains a resistor that maintains enough current to ensure the relay stays electrically charged up. This design ensures that the fuel pump will shut down in the even that the engine should stall.
