A relay is used with a relatively high current draw device that in normal operation requires repeated on and off cycles. Automotive fuel pumps will draw between 3 to 18 amperes in a stock setup. The relay moves the heat and large current draw out the ignition circuit and into its own circuit. Typically, the fuel pump relay is energized by the ECU for several seconds once the ignition key is turned on. Once the engine is running, the ECU, in the presence of a crankshaft signal, maintains an energized fuel pump relay. If the engine quits running with the ignition on, the ECU turns off the fuel pump relay and stops fuel flow. The signs and symptoms associated with a bad fuel pump relay are related to this sequence.
Symptom: Engine Will Not Start
In this case, the engine will turn over but will not start even though there is sufficient fuel in the tank. When the ignition is turned on the, ECU energizes the fuel pump relay for several seconds to pressurize the fuel system. Although other malfunctions will cause this symptom, the first signs to look for are loss of fuel pressure and the loss of the fuel pump whine when the ignition is turned on. In many instances the check engine light will not be on.
Symptom: Engine Suddenly Stops After Startup
In this scenario, the engine starts without difficulty but shortly thereafter stops. The engine acts as if it is running out of fuel. The ECU energizes the fuel pump relay for the first several seconds to pressurize the fuel system. The pressurized system allows the car to start, but the ECU fails to maintain the energized fuel pump relay. With no fuel pressure the car quickly dies. Here, you would look for loss of fuel pressure after startup, and then check the signal from the ECU. The ECU could be the culprit.
Symptom: The Car Slows Down While Driving
This symptom feels like the car is running out of gas. The car may completely stop or may slow down and speed up several times in a row. Again, the ECU is intermittently failing to maintain an energized fuel pump relay. The signs would be a loss of fuel pressure or ECU signal to the fuel pump relay intermittently while driving. The check engine light may or may not be on in this case.
Symptom: Car Will Not Start When Hot
In this scenario, the car starts fine when cold and runs fine otherwise, but it will not start once the engine's hot. After 30 minutes or more of cooling, the car starts as normal. When at normal operating temperature, the initial fuel pump relay sequence fails. The primary sign is the loss of initial start signal from the fuel pump relay to the fuel pump when the car is at normal operating temperature. Other signs include the lack of fuel pump whine when you turn on the ignition, and no relay click. The check engine light may or may not be on in this case also.
- 12 volt relais, relay image by Sascha Zlatkov from Fotolia.com