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Makes 🢒 GMC_Truck 🢒 1998 🢒 GMC C Pickup - 2WD (CNG) 🢒 Engine 🢒 Engine Controls - 5.0L and 5.7L 🢒 Alternative Fuel (AF) Fuel Pump Relay Diagnosis

Alternative Fuel (AF) Fuel Pump Relay Diagnosis 1998


Object Number: 322561 Size: LF
Handling Electrostatic Discharge Sensitive Parts Notice
Engine Controls Components
OBDII Symbol Description Notice
Handling Electrostatic Discharge Sensitive Parts Notice
Engine Controls Schematics (CNG Fuel Controls - 1998)

Step

Action

Value(s)

Yes

No

1

Inspect the 20A CNG fuse.

Is the fuse blown?

--

Go to Step 14

Go to Step 2

2
  1. Backprobe CKT 917 (GRY/BLK) in terminal 87A of fuel pump relay connector C007 using a test light.
  2. Start the vehicle.
  3. Inspect the fuel indicator lamp to verify that the vehicle is operating on CNG.

Does the test light illuminate?

--

Go to Step 5

Go to Step 3

3
  1. Turn the ignition OFF.
  2. Remove the 20A CNG fuse.
  3. Crank or start the engine while observing the test light.

Does the test light illuminate?

--

Go to Step 15

Go to Step 4

4
  1. Disconnect fuel pump relay connector C007.
  2. Probe CKT 120 (GRY) in terminal 30 of connector C007 using a test light.
  3. Crank the engine.

Does the test light illuminate?

--

Go to Step 9

Go to Step 10

5
  1. Disconnect fuel pump relay connector C007.
  2. Turn ON the ignition.
  3. Probe CKT 920 (RED) in terminal 85 of connector C007 using a test light.

Does the test light illuminate?

--

Go to Step 6

Go to Step 11

6

Probe CKT 905 (BRN) in terminal 86 of connector C007 using a test light connected to battery positive.

Does the test light illuminate?

--

Go to Step 9

Go to Step 7

7
  1. Turn the ignition OFF.
  2. Disconnect ECU connector C002 (red).
  3. Inspect for continuity between CKT 905 (BRN) in terminal 86 of connector C007 and C002 terminal 21.

Is there continuity?

--

Go to Step 8

Go to Step 13

8

Inspect for a faulty connection at ECU connector C002.

Is a problem found?

--

Go to Step 13

Go to Step 12

9

Replace the AF fuel pump relay.

Is the action complete?

--

Go to Step 15

--

10

Install a jumper between CKT 120 (GRY) in terminal 30 and CKT 917 (GRY/BLK) in terminal 87A of fuel pump relay connector C007.

Is the action complete?

--

Go to Fuel Pump Circuit Diagnosis in Engine Controls - 5.0, 5.7L the 1998 C/K Truck Service Manual

--

11
  1. Inspect the 40A Heated Seats/CNG fuse.
  2. If the fuse is OK, then locate and repair the open in CKT 920.

Is the action complete?

--

Go to Step 15

--

12

Replace the ECU. Refer to Engine Control Unit Replacement .

Is the action complete?

--

Go to Step 15

--

13

Repair the circuit as necessary. Refer to Repair Procedures in Electrical Diagnosis in the 1998 C/K Truck Service Manual.

Is the action complete?

--

Go to Step 15

--

14
  1. Locate and repair the short to ground. Refer to Repair Procedures in Electrical Diagnosis in the 1998 C/K Truck Service Manual.
  2. Replace fuse(s).

Is the action complete?

--

Go to Step 15

--

15
  1. Reinstall the 20A CNG fuse (if removed).
  2. Start the vehicle.
  3. Inspect the fuel indicator lamp to verify that the vehicle is operating on CNG.
  4. Listen for gasoline fuel pump operation.

Is the fuel pump operating?

--

Go to Step 1

Relay OK