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Fuel Pump Electrical Circuit Diagnosis - BI-Fuel/Alternative Fuels

FUEL PUMP ELECTRICAL CIRCUIT DIAGNOSIS (ALTERNATIVE FUEL)

CIRCUIT DESCRIPTION
The AF fuel pump relay coil is supplied a ground path all the time. The Alternative Fuels ECM supplies the power to the AF fuel pump relay coil when the engine starts and runs on CNG.

The purpose of the AF fuel pump relay circuitry is to allow the Alternative Fuels ECM the ability to turn OFF the gasoline fuel pump and to supply a voltage source for the lock-off relay.

At initial ignition ON, the AF fuel pump relay is NOT energized by the Alternative Fuels ECM. The gasoline control module energizes the GASOLINE fuel pump relay supplying power to the Fuel Pump Power circuit for a 2 second prime pulse. When the prime pulse is supplied to the Fuel Pump Power circuit, the lock-off relay is energized AND the gasoline fuel pump receives the 2 second prime pulse power. The 2 second prime pulse is allowed to operate the gasoline fuel pump for durability purposes.

When the engine is started, the GASOLINE control module energizes the GASOLINE fuel pump relay supplying power to the Fuel Pump Power circuit. When the engine is operating on CNG, the Alternative Fuels ECM energizes the AF fuel pump relay turning OFF the gasoline fuel pump. The lock-off relay continues to be energized by the GASOLINE fuel pump relay.

TEST DESCRIPTION

Steps 1-9:




Steps 10-18:




Steps 19-22:




The numbers below refer to the step numbers on the diagnostic table.
1. The System Check MUST be performed before proceeding with this diagnostic table. Failure to perform the System Check will result in mis-diagnosis.
2. This step tests if the engine will start and run. A failure that causes the engine to not start on either fuel can be isolated to the CNG harness or a base vehicle problem such as a malfunctioning GASOLINE fuel pump relay.
3. This step tests the gasoline control modules ability to supply power to the gasoline fuel pump circuit.
4. A switch-over to gasoline can be performed by commanding a switch-over using the scan tool alternative fuels special functions. A switch-over to gasoline can also be performed by disconnecting the lock-off solenoid relay and waiting until the CNG fuel pressure drops.
5. This step tests for voltage THROUGH the AF fuel pump relay. With the ignition ON and the engine OFF, the Alternative Fuel ECM does not energize the AF fuel pump relay allowing voltage to flow through the relay and energize the gasoline fuel pump.
6. This step tests the Alternative Fuel ECMs ability to energize the AF fuel pump relay and disable the gasoline fuel pump during CNG operation.
7. This step tests for stuck AF fuel pump relay contacts or a shorted AF Fuel Pump Relay control circuit.
10. If the engine did not operate on gasoline during the System Check but now operates on gasoline, inspect for intermittent terminal contact or a AF fuel pump relay with sticking contacts that may have been disturbed during testing.
11. This step tests if the CNG harness is the cause of the no start on gasoline.
14. If the Fuel Pump Power circuit tests OK, there is a base vehicle problem that is causing the no-start condition.