C 2500 Suburban 2WD V8-350 5.7L (1994)
Fuel Pump Relay: Description and Operation
Fuel Pump Relay Circuit Wiring Diagram
Operation:
When the key is first turned "ON" without the engine running, the control module turns the fuel pump relay "ON" for two seconds. This builds up the
fuel pressure quickly. If the engine is not started within two seconds, the control module shuts the fuel pump "OFF" and waits for ignition reference
pulses. As soon as the engine is cranked, the control module turns the relay "ON" and runs the fuel pump.
As a backup system to the fuel pump relay, the fuel pump is also turned "ON" by an oil pressure switch. When the engine oil pressure reaches about
28 kPa (4 psi), through cranking the oil pressure switch will close to complete the circuit to the fuel pump.
An inoperative fuel pump relay can result in long cranking times, particularly if the engine is cold.
Circuit Description
The status of the fuel pump CKT 120 is monitored by the PCM at terminal "B12" and is used to compensate fuel delivery based on system voltage.
This signal is also used to store a Diagnostic Trouble Code (DTC) if the fuel relay is defective or fuel pump voltage is lost while the engine is
running. There should be at least 12 volts on CKT 120 for at least 2 seconds after the ignition is turned "ON", or any time the reference pulses are
being received by the PCM.
