GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2002 🢒 GMC K Sierra - 4WD (LPG) 🢒 Engine 🢒 Engine Controls - 6.0L 🢒 Fuel Pump Electrical Circuit Diagnosis

Circuit Description

The AF fuel pump relay coil is supplied power when the ignition is in RUN and START. The alternative fuels engine control module (ECM) supplies the ground path to the AF fuel pump relay coil when the engine starts and runs on CNG.

The purpose of the AF fuel pump relay circuitry is to allow the alternative fuels ECM the ability to turn OFF the gasoline fuel pump and to supply a voltage source for the lock-off relay.

At initial ignition ON the AF fuel pump relay is not energized by the alternative fuels ECM. The gasoline powertrain control module (PCM) energizes the gasoline fuel pump relay, supplying power to the Fuel Pump Relay Control circuit for a two second prime pulse. When the prime pulse is supplied to the Fuel Pump Relay Control circuit the lock-off relay is energized and the gasoline fuel pump relay receives the two second prime pulse power. The two second prime pulse is for gasoline fuel pump durability purposes.

When the engine is operating on CNG, the alternative fuels ECM energizes the AF fuel pump relay in order to turn OFF the gasoline fuel pump. The lock-off relay continues to be energized by the PCM gasoline Fuel Pump Relay Control circuit.

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 misdiagnosis.

    2.

  2. The scan tool will command the PCM to turn ON the fuel pump and the alternative fuels ECM to turn OFF the AF fuel pump relay.

    3.

  3. The fuel pump prime terminal is located in the underhood fuse block.

    4.

  4. The lock-off solenoid relay is controlled by the PCM Fuel Pump Relay Control circuit.

Step

Action

Yes

No

Schematic Reference: Engine Controls Schematics

1

Did you perform the Diagnostic System Check - Engine Controls (Alternative Fuels)?

Go to Step 2

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

2

Select the scan tool DTC display function for the alternative fuels ECM.

Does the scan tool display DTC P1650?

Go to DTC P1650

Go to Step 3

3

  1. Turn ON the ignition with the engine OFF.
  2. With the scan tool in the Alternative Fuels - Special Function menu command the Fuel Pump ON.
  3. Listen for audible fuel pump operation.

Can you hear the fuel pump operate as commanded?

Go to Step 4

Go to Step 5

4

  1. Start and idle the engine on CNG.
  2. Probe the fuel pump Prime terminal using the J 35616-200 test lamp connected to ground.

Is the test lamp illuminated?

Go to Step 10

Go to Fuel System Diagnosis in the C/K Pickup Service Manual

5

  1. With the scan tool in the Alternative Fuels - Special Function menu command the Fuel Pump ON and OFF.
  2. Listen or feel for operation of the Lock-Off Solenoid Relay.

Does the Lock-Off Solenoid Relay operate when the fuel pump is commanded ON and OFF?

Go to Step 6

Go to Fuel Lock-Off Relay Diagnosis

6
  1. With the scan tool in the Alternative Fuels - Special Function menu command the Fuel Pump ON and OFF.
  2. Listen or feel for operation of the gasoline fuel pump relay.

Does the gasoline fuel pump relay operate when the fuel pump is commanded ON and OFF?

Go to Fuel Pump Electrical Circuit Diagnosis in the C/K Pickup Service Manual

Go to Step 7

7
  1. Disconnect the AF fuel pump relay.
  2. Probe the Fuel Pump Relay Control circuit terminal (30) with a J 35616-200 test lamp connected to ground.
  3. With the scan tool in the Alternative Fuels - Special Function menu command the Fuel Pump ON.

Does the test lamp illuminate when the fuel pump is commanded ON?

Go to Step 8

Go to Step 10

8
  1. Connect the AF fuel pump relay.
  2. Remove the gasoline fuel pump relay.
  3. Probe the gasoline fuel pump relay control terminal (85) using the J 35616-200 test lamp connected to ground.
  4. With the scan tool in the Alternative Fuels - Special Function menu command the Fuel Pump ON.

Does the test lamp illuminate when the fuel pump is commanded ON?

Go to Fuel Pump Electrical Circuit Diagnosis in the C/K Pickup Service Manual

Go to Step 9

9
  1. Test the Fuel Pump Relay circuit between the AF fuel pump relay and the gasoline fuel pump relay for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual.
  2. Repair the circuit as necessary.

Did you find and correct the condition?

Go to Step 12

Go to Step 11

10

Repair the Fuel Pump Relay Control circuit between the AF fuel pump relay terminal (30) and the circuit splice. Refer to Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual.

Did you complete the repair?

Go to Step 12

--

11

Replace the AF Fuel Pump Relay.

Did you complete the replacement?

Go to Step 12

--

12
  1. Remove all test equipment except the scan tool.
  2. Connect any disconnected components, fuses, etc.
  3. With a scan tool, clear DTC information from the alternative fuels ECM and the PCM.
  4. Turn the ignition OFF and wait 30 seconds.
  5. Start the engine and operate the vehicle.
  6. Observe 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 MIL illumination and no stored DTCs?

System OK

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