Circuit Description
The lock-off relay coil is supplied power whenever the ignition is in the Start or Run position. When the lock-off relay is energized it supplies power to the high pressure lock-off (HPL) solenoid and the low pressure lock-off (LPL) solenoid.
| Step | Action | Yes | No |
|---|---|---|---|
| Schematic Reference: Engine Controls Schematics Connector End View Reference: Engine Controls Connector End Views or Fuel Injection Control Module Connector End Views | |||
| 1 | Did you perform the Diagnostic System Check Alternative Fuels - Engine Controls? | Go to Step 2 | Go to Diagnostic System Check - Alternative Fuels (Engine Controls) |
| 2 | Disconnect the AF lock-off relay. Probe the relay switch battery circuit terminal (30) with a test lamp connected to ground. Is the test lamp illuminated? | Go to Step 3 | Go to Step 5 |
| 3 | Probe the relay coil ignition circuit terminal (85) with a test lamp connected to ground. Turn ON the ignition with the engine OFF. Is the test lamp illuminated? | Go to Step 4 | Go to Step 7 |
| 4 | Probe the relay coil ground circuit terminal (86) with a test lamp connected to B+. Is the test lamp illuminated? | Go to Step 12 | Go to Step 9 |
| 5 | Inspect the AFS/4WS fuse. Is the fuse open? | Go to Step 6 | Go to Step 10 |
| 6 | Test for one of the following conditions: Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual. A shorted to ground HPL or LPL positive voltage circuit An internally shorted HPL or LPL solenoid A shorted to ground relay switch battery circuit An internally shorted to ground underhood fuse block Repair the condition or replace the component as necessary. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual. Did you complete the repair? | Go to Step 14 | |
| 7 | Inspect the AFS/PCM 1 fuse. Is the fuse open? | Go to Step 8 | Go to Step 11 |
| 8 | Test for one of the following conditions: Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual. A shorted to ground relay coil positive voltage circuit An internally shorted relay An internally shorted to ground underhood fuse block Repair the condition or replace the component as necessary. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual. Did you complete the repair? | Go to Step 14 | |
| 9 | Repair the relay coil ground circuit for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual. Did you complete the repair? | Go to Step 14 | |
| 10 | Repair the relay switch power circuit for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual. Did you complete the repair? | Go to Step 14 | |
| 11 | Repair the relay coil power circuit for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual. Did you complete the repair? | Go to Step 14 | |
| 12 | Test for an intermittent and for a poor connection at the AF lock-off relay. Refer to Testing for Intermittent Conditions and Poor Connections and Repairing Connector Terminals in Wiring Systems in the C/K Pickup Service Manual. Did you find and correct the condition? | Go to Step 14 | Go to Step 13 |
| 13 | Replace the AF lock-off relay. Did you complete the replacement? | Go to Step 14 | |
| 14 | Clear DTC information with a scan tool from the PCM. Turn the ignition OFF and wait 30 seconds. Start and idle the engine. Observe the MIL. Observe vehicle performance and driveability. Does the vehicle operate on CNG with normal driveability and no MIL illumination? | System OK | Go to Diagnostic System Check - Alternative Fuels (Engine Controls) |
Fuel Lock-Off Relay Diagnosis
The powertrain control module (PCM) enables the appropriate CNG fuel injector on the intake stroke for each cylinder. The PCM is not capable of operating the high current CNG fuel injectors. The PCM injector pulse width signals are received by the fuel injector control module (FICM) and duplicate pulse width signals are generated by the FICM in order to operate the CNG injectors. The FICM controls each fuel injector by grounding the control circuit via a solid state device called a driver. A fuel injector coil winding resistance that is too high, or low, will affect engine driveability. A fuel injector control circuit DTC may not set, but a misfire may be apparent. The fuel injector coil windings are affected by temperature. The resistance of the fuel injector coil windings will increase as the temperature of the fuel injector increases.
Diagnostic Aids
- Operating the vehicle over a wide temperature range may help isolate the fuel injector that is causing the condition.
- Perform the fuel injector coil test within the conditions of the customers concern. A fuel injector condition may only be apparent at a certain temperature, or under certain conditions.
- If the fuel injector coil test does not isolate the condition perform the fuel injector balance test. Refer to «Fuel Injector Balance Test with Special Tool»(ref-184503-S21541359122005082300000) or «Fuel Injector Balance Test with Tech 2»(ref-184503-S11192078532005082300000) .
The J 39021 Fuel Injector Tester energizes the CNG fuel injector for a precise amount of time allowing a measured amount of fuel into the manifold. This causes a drop in system fuel pressure that can be recorded and used to compare each fuel injector.
The scan tool first energizes the following components for a precise amount of time allowing a measured amount of fuel into the manifold
- High pressure lock off (HPL)
- Low pressure lock off (LPL)
- CNG fuel injectors
This causes a drop in system fuel pressure that can be recorded and used to compare each fuel injector.