GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2002 🢒 GMC C Sierra - 2WD (LPG) 🢒 Engine 🢒 Engine Controls - 6.0L 🢒 Fuel Lock-Off Relay Diagnosis

Circuit Description

The lock-off relay coil is supplied power whenever the powertrain control module (PCM) energizes the gasoline fuel pump relay. 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.

At the initial ignition ON, the PCM energizes the gasoline fuel pump relay control circuit which supplies power to the fuel pump power circuit for a 2 second prime pulse. At the initial ignition ON, the alternative fuels engine control module (ECM) will close the HPL control circuit, 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 PCM will supply power to the fuel pump relay control circuit regardless of the fuel system that is in operation as long as crankshaft pulses are received.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. The diagnostic system check must be performed before proceeding with this diagnostic table. Failure to perform the system check will result in misdiagnosis.

    2.

  2. The PCM energizes the Fuel Pump Relay Control circuit for about 2 seconds everytime the ignition is cycled from OFF to ON.

Step

Action

Value(s)

Yes

No

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
  1. Disconnect the lock-off solenoid relay connector.
  2. Connect a J 35616-200 test lamp between the relay switch power supply circuit terminal (30) and a known good ground.

Does the test lamp illuminate?

--

Go to Step 3

Go to Step 6

3

Connect a J 35616-200 test lamp between the relay coil ground circuit terminal (86) and battery positive.

Does the test lamp illuminate?

--

Go to Step 4

Go to Step 16

4

  1. Connect a J 35616-200 test lamp between the relay coil power circuit terminal (85) and a known good ground.
  2. Observe the test lamp as you turn the ignition ON while leaving the engine OFF.

Does the test lamp briefly illuminate?

--

Go to Step 5

Go to Step 8

5
  1. Turn OFF the ignition.
  2. Connect the lock-off solenoid relay connector.
  3. Backprobe the lock-off solenoid relay switch output circuit terminal (87) with the J 35616-200 test lamp connected to a known good ground.
  4. Observe the test lamp as you turn the ignition ON while leaving the engine OFF.

Does the test lamp briefly illuminate?

--

Go to Intermittent Conditions

Go to Step 18

6

Inspect the AFS 20 amp MaxiFuse® .

Is the fuse open?

--

Go to Step 10

Go to Step 7

7
  1. Test the AFS MaxiFuse® circuit for an open condition between the Fuse Block - Underhood connector terminal and the lock-off solenoid relay connector terminal.
  2. Test the underhood fuse block for proper operation.
  3. Inspect the underhood fuse block connector terminal and the lock-off solenoid relay connector terminal for proper contact.
  4. Inspect the in-line connector terminal (E) for proper terminal contact.
  5. Repair wiring or terminal contact as necessary.

Did you find and correct the condition?

--

Go to Step 21

--

8
  1. Turn OFF the ignition.
  2. Disconnect in-line connector C006.
  3. Connect a J 35616-200 test lamp between the PCM harness fuel pump relay control circuit terminal (E) and a known good ground.
  4. Observe the test lamp as you turn ON the ignition leaving the engine OFF.

Does the test lamp briefly illuminate?

--

Go to Step 15

Go to Step 9

9
  1. Test for an open fuel pump relay control circuit between the gasoline PCM and in-line connector C006. Refer to Circuit Testing in Wiring Systems in the C/K Pickup Service Manual.
  2. Inspect the PCM connector and in-line connector C006 for proper terminal contact. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems in the C/K Pickup Service Manual.
  3. Repair the wiring or terminals as necessary. Refer to Wiring Repairs or Connector Repairs in Wiring Systems in the C/K Pickup Service Manual.

Did you find and correct the condition?

--

Go to Step 21

Go to Step 20

10
  1. Replace the AFS 20 amp MaxiFuse®.
  2. Inspect the AFS 20 amp MaxiFuse®.

Did the replacement fuse open?

--

Go to Step 17

Go to Step 11

11
  1. Turn the ignition OFF.
  2. Disconnect the LPL connector. Refer to Low Pressure Lock-Off Solenoid Replacement .
  3. Remove the fuel tank cover. Refer to Fuel Tank Cover Replacement .
  4. Disconnect the HPL connector. Refer to High Pressure Lock-Off Solenoid Replacement .
  5. Connect theJ 35616-200 test lamp between the lock-off solenoid relay switch output circuit terminal (87) and battery positive.

Is the test lamp illuminated?

--

Go to Step 19

Go to Step 12

12

With the J 39200 DMM measure and record the resistance between the high pressure lock-off solenoid connector terminals.

Is the high pressure lock-off solenoid outside of the specified range ?

16.3-20.6 ohms

Go to High Pressure Lock-Off Solenoid Replacement

Go to Step 13

13

With the J 39200 DMM measure and record the resistance between the low pressure lock-off solenoid connector terminals.

Is the low pressure lock-off solenoid outside of the specified range ?

6.2-10 ohms

Go to Low Pressure Lock-Off Solenoid Replacement

Go to Step 14

14
  1. Replace the AFS 20 amp MaxiFuse®.
  2. Connect the HPL solenoid.
  3. Crank the engine for the specified time.
  4. Inspect the AFS 20 amp MaxiFuse®.

Is the fuse open?

10 seconds

Go to High Pressure Lock-Off Solenoid Replacement

Go to Low Pressure Lock-Off Solenoid Replacement

15
  1. Test for an open circuit between the Lock-Off relay coil power supply terminal (85) and the in-line connector terminal (E). Refer to Circuit Testing in Wiring Systems in the C/K Pickup Service Manual.
  2. Test for a short to ground on the Lock-Off relay coil power supply circuit. Refer to Circuit Testing in Wiring Systems in the C/K Pickup Service Manual.
  3. Repair the wiring as necessary. Refer to Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual.

Did you complete the repair?

--

Go to Step 21

--

16

Repair the open lock-off solenoid relay coil ground circuit. Refer to Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual.

Did you complete the replacement?

--

Go to Step 21

--

17

Repair the shorted lock-off solenoid relay switch power circuit between the relay and the fuse. Refer to Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual.

Did you complete the repair?

--

Go to Step 21

--

18

Replace the lock-off relay.

Did you complete the repair?

--

Go to Step 21

--

19

Repair the short to ground on the lock-off solenoid relay switch output circuit between the relay and the LPL or HPL solenoid. Refer to Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual.

Did you complete the repair?

--

Go to Step 21

--

20

Important: KL6 models require a switchover to gasoline with a scan tool in order to perform the Crankshaft Position Variation Learn Procedure.

KL8 models do not require the Crankshaft Position Variation Learn Procedure.

Replace the PCM. Refer to Powertrain Control Module Replacement .

Did you complete the replacement?

--

Go to Step 21

--

21
  1. Remove all test equipment except the scan tool.
  2. Connect any disconnected components, fuses, etc.
  3. Using a scan tool, clear DTC information from the alternative fuels ECM and the PCM.
  4. Turn OFF the ignition and wait 30 seconds.
  5. Start the engine and operate the vehicle.
  6. Observe the MIL.
  7. Observe the vehicle's 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 MIL illumination, and no stored DTCs?

--

System OK

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