Fuel Pump Relay Circuit Diagnosis

Fuel Pump Relay Circuit Diagnosis





Circuit Description
When the ignition switch is turned ON, the Powertrain Control Module (PCM) turns ON the in4ank fuel pump. It will remain ON as long as the PCM is receiving ignition reference pulses from the electronic Ignition Control Module (ICM). If there are no reference pulses, the PCM will shut OFF the fuel pump about 2-3 seconds after the ignition is turned ON, about 10 seconds after the reference pulses stop.
The fuel pump delivers fuel to the fuel rail and fuel injectors, then to the fuel pressure regulator, where the system pressure is controlled to 283-324 kPa (41-47) with no manifold vacuum or 214-303 kPa (31-44 psi) at idle. Excess fuel is then returned to the fuel tank.
Improper fuel system pressure will result in one or all of the following symptoms:
- Cranks but will not run
- DTC P0171
- DTC P0172
- DTC P1171
- DTC P0300
- Cuts out, may feel like an ignition problem
- Poor fuel economy, loss of power Hesitation

Diagnostic Aids
An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside the insulation. Check for a poor connection or 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 step number(s) on the

Diagnostic Table.











1. The Powertrain OBD System Check prompts the technician to complete some basic checks and stores the freeze frame and failure records data on the scan tool, if applicable. This creates an electronic copy of the data taken when the malfunction occurred. The information is then stored in the scan tool for later reference.
2. If the no start condition was due to a open fuse, locate and repair any shorts that may have caused the fuse to open before replacement.
3. This step determines if the fuel pump circuit can be controlled by the PCM. The PCM should energize the fuel pump relay and turn the fuel pump ON when commanded by the scan tool. If the engine is not cranking or running, the PCM should de-energize the relay and turn OFF the fuel pump after 2-3 seconds.
4. Jumpering the fuel pump relay injector/fuel pump feed circuit to fuel pump battery feed circuit verifies that the fuel pump relay ignition
feed circuit, the fuel pump feed circuit, the fuel pump and fuel ground are all OK.
7. Check for an open fuel pump relay ground.
16. Replacement PCMs must be reprogrammed and the crankshaft position system variation procedure must be performed. Refer to the latest Isuzu Technical Communication System (ITCS) information for programming procedures and also refer to the CKP System Variation Learn Procedure.
17. This step checks the fuel pump ground circuit on the body side of the body harness only. If the fuel pump ground circuit is OK, then the
fuel pump relay injector/fuel pump feed circuit or the fuel pump ground circuit in the fuel tank harness or the fuel pump itself is malfunctioning.
20. This test will determine if a short to ground on the fuel pump relay injector/fuel pump feed circuit caused the fuse to open. To prevent mis-diagnosis, be sure the fuel pump harness is disconnected before the test.