Circuit Description
The lock-off relay is energized whenever the gasoline control module (VCM/PCM or ECM) supplies power to the gasoline fuel pump. When energized the lock-off relay supplies power to the high and low pressure lock-off solenoids.
At initial ignition ON 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.
When the engine is started the gasoline control module energizes the gasolinefuel pump relay supplying power to the Fuel Pump Power circuit. The gasoline control module will supply power to the Fuel Pump Power circuit regardless of the fuel system that is in operation as long as crankshaft pulses are monitored (the engine is running).
The high and low pressure lock-off solenoids are energized by the AF ECU when the vehicle is operating in the CNG mode. At initial ignition ON, the AF ECU will close the High Pressure Lock-Off Solenoid Control circuit driver (completing the circuit path to ground) for a 2 second CNG prime pulse. The prime pulse occurs at the same time as the gasoline fuel pump prime pulse occurs. This ensures that the lock-off relay will be energized and supply power to the lock-off solenoids at the same time that the AF ECU closes the driver and supplies the ground path.
When the engine is cranking or running, the AF ECU closes both Lock-Off Solenoid Control circuit drivers allowing the engine to operate on CNG.
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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The AF Powertrain On-Board Diagnostic (OBD) System Check must be performed before proceeding with this diagnostic table. Failure to perform the OBD System Check will result in misdiagnosis.
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This step checks the battery positive voltage supply from the lock-off relay to the LPL. The lock-off relay is energized regardless of which fuel system is operating the vehicle.
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This step checks the AF ECU and the Low Pressure Lock-Off Solenoid Control circuit for proper operation. The test lamp should illuminate for a few seconds then turn OFF as the engine cannot start on CNG with the LPL disconnected.
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This step checks the battery positive voltage supply from the lock-off relay to the HPL. The lock-off relay is energized regardless of which fuel system is operating the vehicle.
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This step checks the AF ECU and the High Pressure Lock-Off Solenoid Control circuit for proper operation. The test lamp should illuminate for a few seconds then turn OFF as the AF ECU opens the HPL for system priming prior to cranking the engine.
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This step checks for proper battery positive voltage output from the lock-off relay.
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This step checks for proper relay coil positive voltage and ground supply.
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This step checks for proper battery positive voltage supply from the CNG 20 amp fuse circuit to the lock-off relay.
If the test lamp fails to illuminate, the failure can only exist between the relay and the circuit splice. For example, an open fuse would cause a lack of power to the AF ECU causing no scan tool communication in which the OBD System Check would have directed you to a different diagnostic procedure.
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This step checks for proper positive voltage supply from the Gasoline Fuel Pump circuit to the lock-off relay.
If the test lamp fails to illuminate the failure can only exist between the relay and the circuit splice. For example, an open Fuel Pump circuit would cause a no start condition in which the OBD System Check would have directed you to a different diagnostic procedure.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
Did you perform the AF Powertrain On-Board Diagnostic (OBD) System Check?? | -- | Go to Alternative Fuels (AF) Powertrain On Board (OBD) System Check | ||
Does the test lamp illuminate? | -- | |||
Does the test lamp illuminate as the engine is cranking? | -- | |||
Does the test lamp illuminate? | -- | |||
Does the test lamp flash ON as the ignition is turned ON? | -- | |||
With the engine operating backprobe the lock-off relay terminal 87 using the test lamp connected to a known good ground. Does the test lamp illuminate? | -- | |||
Does the test lamp illuminate? | -- | |||
With the engine operating connect the test lamp between the lock-off relay switch power supply circuit terminal 30 and a known good ground. Does the test lamp illuminate? | -- | |||
With the engine operating connect the test lamp between the lock-off relay coil power supply circuit terminal 85 and a known good ground. Does the test lamp illuminate? | -- | |||
10 |
Was a repair or replacement performed? | -- | ||
11 | Repair the open circuit between the HPL connector power supply terminal A and the circuit splice. Is the repair complete? | -- | -- | |
12 | Repair the open circuit between the LPL connector power supply terminal A and the lock-off relay terminal 87. Is the repair complete? | -- | -- | |
13 | Repair the open circuit between the Lock-Off relay switch power supply terminal 30 and the circuit splice. Is the repair complete? | -- | -- | |
14 | Repair the open ground circuit between the Lock-Off relay coil ground terminal 86 and ground. Is the repair complete? | -- | -- | |
15 | Repair the open circuit between the Lock-Off relay coil power supply terminal 85 and the circuit splice. Is the repair complete? | -- | -- | |
16 | Replace the lock-off relay. Is the replacement complete? | -- | -- | |
17 |
Is the replacement complete? | -- | -- | |
18 |
Does the vehicle operate on CNG with normal driveability, no FIL/MIL illumination, and no stored DTCs? | -- | System OK | Go to Alternative Fuels (AF) Powertrain On Board (OBD) System Check |
