Circuit Diagnosis - Fuel Pump Relay

Circuit Description
When the Ignition switch is first turned ON, the PCM energizes the fuel pump relay which applies power to the fuel pump through the high speed circuit (controlled by the fuel pump speed control relay). The fuel pump relay will remain ON as long as the engine is running or cranking and the PCM is receiving reference pulses. If no reference pulses are present, the PCM de-energizes the fuel pump relay within 2 seconds after the ignition is turned ON or the engine is stopped. The fuel pump delivers fuel to the fuel rail and injectors, then to the fuel pressure regulator. The fuel pressure regulator controls fuel pressure by allowing excess fuel to be returned to the fuel tank. With the engine stopped, the fuel pump can be turn ON by using the scan tool output controls function.

The PCM alters fuel pump speed by energizing the fuel pump speed control relay Under normal conditions, the fuel pump control speed control relay is energized (normal fuel pump speed). The applied voltage to the fuel pump is controlled by a resistor assembly. When higher fuel volume is required due to increased engine load (MAP sensor value over 90 kPa). the PCM de-energizes the fuel pump speed control relay circuit. The increased voltage to the in-tank fuel pump allows a higher volume of fuel to be delivered to the fuel rail. The PCM also compensates for low system voltage by energizing the fuel pump speed control relay.

Diagnostic Aids
An intermittent may be caused by a poor connection, rubbed through the wire insulation or a wire broken inside of the insulation.

Check for a poor connection or a damaged harness. Inspect the PCM harness and connectors for the following items:
^ Improper mating
^ Broken locks
^ Improperly formed or damaged terminals
^ Poor terminal to wire connections
^ Damaged harnesses

Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table:
3. The fuel pump primes the fuel rail/injectors using the high speed circuit. This check ensures proper fuel pressure for the vehicle to start.
4. Once the engine starts the PCM commands the fuel pump to operate at normal speed (slightly - lower pressure to the fuel rail/injectors). This check ensures proper operation of the fuel pump normal speed circuit.
6. Checks for a short to ground in the feed circuit to the fuel pump relay.
8. Checks for a short to ground in the feed circuit between the fuel pump and fuel pump speed control relay.
10. Checks for a short to ground in the feed circuit or speed control resistor circuit to the fuel pump.
14. Ensures that the fuel pump relay driver circuit is OK and that the PCM is capable of controlling the fuel pump relay. Using the scan tool to command the fuel pump relay allows only a 2 second ON time.
19. Checks the continuity of the speed control and fuel pump feed harness/splices.
33. This vehicle is equipped with a PCM which utilizes an Electrically Erasable Programmable Read Only Memory (EEPROM). When the PCM is being replaced, the new PCM must be programmed.