GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2001 🢒 Cavalier (CNG) 🢒 Engine 🢒 Engine Controls - 2.2L (LN2) 🢒 Fuel Lock-Off Relay Diagnosis

Circuit Description

The lock-off relay is energized whenever the gasoline control module (VCM/PCM or ECM) supplies power to the gasoline fuel pump circuit. The gasoline control module (VCM/PCM or ECM) supplies power to the gasoline fuel pump circuit for about 2 seconds at every ignition ON and whenever 7X reference pulses indicates the engine is cranking or running. 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 two 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 GASOLINE fuel 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 one 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 supplying 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.

  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.

  2. 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.

    3.

  3. 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.

    4.

  4. 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.

    5.

  5. This step check 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.

    6.

  6. This step checks for proper battery positive voltage output from the lock-off relay.

    7.

  7. This step checks for proper relay coil positive voltage and ground supply.

    8.

  8. 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: a blown 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.

    9.

  9. This step checks for proper positive voltage supply from the gasoline fuel pump circuit to the lock-off relay.

Step

Action

Yes

No

Schematic Reference: Engine Controls Schematics

1

Did you perform the Diagnostic System Check?

Go to Step 2

Go to Diagnostic System Check - Alternative Fuels (Engine Controls)

2

  1. Turn the ignition OFF.
  2. Disconnect the low pressure lock-off (LPL) solenoid connector (C014).
  3. Connect the J 35616-200 Unpowered Test Light between the LPL solenoid connector power supply circuit terminal (A) and a known good ground.
  4. Start and idle or crank the engine.

Is the test lamp illuminated while the engine is running or cranking?

Go to Step 3

Go to Step 6

3

  1. Turn the ignition OFF and wait 30 seconds.
  2. Turn the ignition ON leaving the engine OFF.
  3. Clear any stored AF ECU DTC Information using a scan tool.
  4. Connect the test lamp between the LOW pressure lock-off connector terminals (harness side).
  5. Observe the test lamp while cranking the engine.

Did the test lamp illuminate as the engine was cranking?

Go to Step 4

Go to Step 10

4

  1. Turn the ignition OFF.
  2. Remove the fuel tank cover. Refer to Fuel Tank Cover Replacement .
  3. Disconnect the high pressure lock-off (HPL) solenoid connector (C453).
  4. Connect the J 35616-200 Unpowered Test Light between the HPL solenoid connector power supply circuit terminal (A) and a known good ground.
  5. Start and idle or crank the engine.

Is the test lamp illuminated while the engine is cranking or running?

Go to Step 5

Go to Step 11

5

  1. Turn the ignition OFF and wait 30 seconds.
  2. Connect the test lamp between the HPL connector terminals (harness side).
  3. Observe the test lamp while turning the ignition ON.

Did the test lamp flash ON as the ignition was turned ON?

Go to Step 20

Go to Step 10

6

Ensure the engine is idling or cranking.

Backprobe the lock-off relay switch load terminal (87) using the test lamp connected to a known good ground.

Is the test lamp illuminated while the engine is running or cranking?

Go to Step 12

Go to Step 7

7

  1. Disconnect the lock-off relay connector (C011).
  2. Ensure the engine is idling or cranking.

    3. Connect the test lamp between the Lock-Off Relay coil ground circuit terminal (86) and the coil power circuit terminal (85).

Is the test lamp illuminated while the engine is cranking or running?

Go to Step 8

Go to Step 9

8

Ensure the engine is idling or cranking.

Connect the test lamp between the Lock-Off Relay switch power supply circuit terminal (30) and a known good ground.

Is the test lamp illuminated while the engine is cranking or running?

Go to Step 16

Go to Step 13

9

  1. Turn the ignition OFF.
  2. Connect the test lamp between the Lock-Off Relay coil power supply circuit terminal (85) and a known good ground.
  3. Observe and record test lamp operation as you turn the ignition ON while leaving the engine OFF.
  4. Observe and record test lamp operation as you crank or start the engine.

Is the test lamp illuminated while the engine is cranking or running?

Go to Step 14

Go to Step 15

10
  1. Test the lock-off solenoid control circuit for an open or high resistance.
  2. Inspect for poor terminal contact at the lock-off solenoid connector.
  3. Inspect for poor terminal contact at the AF ECU connector.
  4. Repair or replace wiring/terminals as necessary.

Was a repair or replacement performed?

Go to Step 20

Go to Step 19

11

Repair the open circuit between the HPL connector power supply terminal (A) and the circuit splice.

Is the repair complete?

Go to Step 20

--

12

Repair the open circuit between the LPL connector power supply terminal (A) and the Lock-Off Relay terminal (87).

Is the repair complete?

Go to Step 20

--

13

Repair the open circuit between the Lock-Off Relay switch power supply terminal (30) and the circuit splice.

Is the repair complete?

Go to Step 20

--

14

Repair the open ground circuit between the Lock-Off Relay coil ground terminal (86) and ground.

Is the repair complete?

Go to Step 20

--

15

Did the test lamp illuminate for a 2 second bulb check at ignition ON?

Go to Step 16

Go to Step 17

16
  1. Test for an open or shorted 7X reference circuit between the gasoline control module (PCM/VCM or ECM) and the Ignition Control Module (ICM).
  2. Inspect for poor terminal contact at the PCM or ICM connectors.
  3. Repair wiring or terminal contact as necessary.

Was a wiring or terminal repair made?

Go to Step 20

Go to Diagnostic System Check - Engine Controls in Engine Controls in the J platform service manual

17

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 20

--

18

Replace the lock-off relay.

Is the replacement complete?

Go to Step 20

--

19
  1. Replace the AF ECU. Refer to Engine Control Unit Replacement .
  2. Program (flash) the replacement control module with the correct vehicle software and calibration.

Is the replacement complete?

Go to Step 20

--

20
  1. Remove all test equipment except the scan tool.
  2. Connect any dis-connected components, fuses, etc.
  3. Using a scan tool clear DTC Information from the AF ECU and the gasoline control module (VCM/PCM or ECM).
  4. Turn the ignition OFF and wait 30 seconds.
  5. Start the engine and operate the vehicle.
  6. Observe the fuel indicator lamp and the MIL.
  7. Observe vehicle performance and driveability.
  8. After operating the vehicle inspect BOTH control modules for stored DTCs.

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)