Q: How Does the Fuel Pump Know When to Shut Off?

asked by on November 24, 2015

How does the fuel pump know when to shut off?

The are two main components that make most fuel pumps run: the PCM (Powertrain Control Module) and the fuel pump relay. Fuel pumps themselves don’t know when to shut off. They are entirely dependant on the fuel pump relay to provide them with power or remove their power. The fuel pump relay is controlled by the PCM by providing the relay with a ground. The PCM chooses to turn the relay on when it sees the motor is running. If the PCM doesn’t see an indication of the motor running, it will not supply a ground path to the relay and the fuel pump will not receive power.

How does the PCM know the engine is running?

The PCM monitors all signals in the engine compartment. It is the primary component that allows our modern vehicles to run smoothly and efficiently. Each manufacturer will use a different indicator that the motor is running. Some monitor reference pulses from the ignition module. These are used for ignition timing. There won’t be any reference pulses if the motor is not turning. Other options are crank sensors and/or cam sensors. These sensors are used in a variety of ways. Which method utilized is entirely dependant on the manufacturer, and they are constantly coming up with creative ways to revolutionize these systems.

Fuel pump turns on

The PCM will ground the fuel pump relay which supplies power to the fuel pump under some specific circumstances. As a car sits, the fuel pressure gradually decreases and modern cars don’t run without fuel pressure. When you first turn the key on your car, the PCM immediately powers the fuel pump to pressurize the fuel system. If it waited until it recieved a reference signal from one of the many sensors, there would be a delay of several seconds before the car would start. Most people would not appreciate extended cranking. Not to mention it just adds more wear and tear to the starter motor and engine. This usually lasts around two or three seconds, unless the ignition key is turned to crank the position. When the motor is cranking, and the key is in the crank position, the PCM will supply power to the fuel pump until the ignition key is no longer in the cranking position. If everything works perfectly, all of this happens in a split second and you’re off to work another day.

Behavior during an accident

Many cars, but not all, have what is called an inertia switch. Ford in particular is well known for their unique placement of this switch. For many years they placed the inertia switch under the passenger side foot area. Known as the passenger side kick panel, when a rambunctious person would kick their feet in a certain spot, they would kick the inertia switch and that would cut the power to the fuel pump. Of course, the car would instantly die.

The inertia switch is a circuit breaker, so when there is a hard impact from an accident, or someone kicking it, it cuts power to the fuel pump so fuel won’t be sprayed all over the highway, creating a more dangerous situation. This is useful because often times a motor will continue to run after an accident if it doesn’t have an inertia switch.

There are many methods of controlling the power to the fuel pump; in all cases it is the PCM that decides if the fuel pump receives power or not. Some vehicles can vary the voltage to the fuel pump to control fuel pressure. In the case of an accident, there are many crash sensors on modern cars that the PCM will monitor. When an accident has been registered, the PCM has the option to cut power to the fuel pump by turning off the fuel pump relay.

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