Circuit Description
The lock-off relay coil is supplied power whenever the gasoline control module (VCM/PCM or ECM) energizes the gasoline fuel pump relay. The Alternative Fuels ECM controls the ground circuit of the lock-off relay. When energized the lock-off relay supplies power to the high pressure lock-off solenoids.
At the initial ignition ON, the gasoline control module energizes the gasoline fuel pump relay which supplies power to the Fuel Pump Power circuit for a 2 second prime pulse. At the initial ignition ON, the Alternative Fuels ECM will close the Lock-Off Relay Control circuit driver, completing the circuit path to ground, to allow for a 2 second CNG prime pulse. The prime pulse occurs at the same time as the gasoline fuel pump prime pulse occurs.
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 received.
When the engine is running on CNG, the Alternative Fuels ECM closes the Lock-Off Relay Control circuit driver which opens the high pressure lock-off solenoids.
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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The Diagnostic System Check must be performed before proceeding with this diagnostic table. Failure to perform the System Check will result in misdiagnosis.
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The fuel indicator lamp (FIL), located within the fuel gage select switch button, will illuminate when the engine is operating on gasoline.
In addition, the AFO Signal Command scan tool parameter will indicate the Alternative Fuel ECM's commanded state of the AFO Signal circuit, while the AFO Signal parameter will indicate the gasoline control modules monitored state of the AFO Signal circuit.
When operating on CNG, the displays should indicate that the AF ECM is in command. When operating on gasoline the displays should indicate that the PCM is in command.
Regardless of the fuel system in operation, these 2 data parameters will always indicate the same data unless an AFO Signal circuit fault is present.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
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 | Attempt to start and idle the engine. Does the engine start and idle? | -- | Go to Step 3 | |
3 |
Can you hear high pressure solenoid operation while commanding the high pressure lock-off relay ON and OFF? | -- | System OK | Go to Step 4 |
Observe the AFO Signal and AFO Signal Command scan tool data parameters in order to determine if the engine is operating on Alternative Fuel. Is the engine operating on Alternative Fuel? | -- | Go to Step 18 | Go to Step 5 | |
5 | With the engine idling, backprobe the lock-off relay coil power circuit terminal (85) at the relay connector with the test lamp connected to ground. Is the test lamp illuminated? | -- | Go to Step 6 | Go to Step 16 |
6 | With the engine idling, backprobe the lock-off relay switch power circuit terminal (30) at the relay connector with the test lamp connected to ground. Is the test lamp illuminated? | -- | Go to Step 7 | Go to Step 13 |
7 |
Does the test lamp go ON and OFF as commanded? | -- | Go to Step 8 | Go to Step 11 |
8 |
Did the test lamp turn ON for about 2 seconds at each tank connector? | -- | Go to Step 14 | Go to Step 9 |
9 |
Was a wiring repair performed? | -- | Go to Step 27 | Go to Step 10 |
10 | Repair the open high pressure lock-off solenoid power circuit. Is the repair complete? | -- | Go to Step 27 | -- |
11 |
Does the test lamp go ON and OFF as commanded? | -- | Go to Step 25 | Go to Step 12 |
12 |
Was a wiring or terminal repair performed? | -- | Go to Step 27 | Go to Step 26 |
13 | Observe the fuel gage select switch scan tool parameter as you depress the select switch button. Does the display parameter indicate the switch is applied? | -- | Go to Step 17 | Go to Ignition Relay Diagnosis |
14 |
Is either high pressure lock-off solenoid outside of the specified range ? | 16.3-20.6 ohms | Go to Step 15 | |
15 | Replace the high pressure lock-off solenoid that was not within the specified range. Refer to High Pressure Lock-Off Solenoid Replacement . Is the repair complete? | -- | Go to Step 27 | -- |
16 | Repair the open circuit between the Lock-Off relay coil power supply terminal 85 and the circuit splice. Is the repair complete? | -- | Go to Step 27 | -- |
17 | Repair the open circuit between the Lock-Off relay switch power supply terminal 30 and the ignition relay circuit splice. Is the repair complete? | -- | Go to Step 27 | -- |
18 |
Does the fuel tank pressure decrease while commanding the high pressure lock-off relay OFF? | -- | System OK | Go to Step 19 |
19 |
Does the test lamp go OFF as commanded? | -- | Go to Step 22 | Go to Step 20 |
20 |
Does the test lamp go OFF as commanded? | -- | Go to Step 25 | Go to Step 21 |
21 |
Was a wiring repair performed? | -- | Go to Step 27 | Go to Step 26 |
22 |
Does the fuel tank pressure decrease when the manual tank valve is closed? | -- | Go to Step 23 | Go to Step 24 |
23 | Replace the front tank high pressure lock-off solenoid. Refer to High Pressure Lock-Off Solenoid Replacement . Is the repair complete? | -- | Go to Step 27 | -- |
24 | Replace the rear tank high pressure lock-off solenoid. Refer to High Pressure Lock-Off Solenoid Replacement . Is the repair complete? | -- | Go to Step 27 | -- |
25 | Replace the lock-off relay. Is the repair complete? | -- | Go to Step 27 | -- |
26 |
Is the replacement complete? | -- | Go to Step 27 | -- |
27 |
Does the vehicle operate on CNG with normal driveability, no FIL/MIL illumination, and no stored DTCs? | -- | System OK | Go to Diagnostic System Check - Alternative Fuels (Engine Controls) |
