GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Isuzu 🢒 2000 🢒 S/T Isuzu - 2WD 🢒 Engine 🢒 Engine Controls - 4.3L 🢒 Fuel Pump Electrical Circuit Diagnosis

Refer to

Cell 21: Fuel Pump and Sender


Object Number: 385578 Size: FS
VCM Connector End Views
Cell 10: ECM B, HORN Fuses and Horn, Fuel Pump Relays
Power and Grounding Connector End Views
Handling ESD Sensitive Parts Notice
Engine Controls Components
Vehicle Control Module Description
Cell 21: Fuel Injection Controls
Cell 21: Ignition Controls
OBD II Symbol Description Notice
Handling ESD Sensitive Parts Notice
Cell 21: Vehicle Speed Sensor
Cell 21: MAP, IAT, ECT, TP and Fuel Tank Pressure Sensors
Cell 21: Heated Oxygen Sensors
Cell 14: Underhood Grounds
 .

Circuit Description

When the ignition switch is turned ON, the vehicle control module (VCM) turns the in-tank fuel pump ON. The in-tank fuel pump remains ON as long as the engine is cranking or running and the VCM is receiving the distributor reference pulses. If there are no reference pulses, the VCM shuts the fuel pump OFF within 2 seconds after the ignition was turned ON or if the engine stops. The VCM will also turn ON the fuel pump when the ignition is turned to the OFF position.

Diagnostic Aids

If the fuel pump relay circuit is OK, refer to Fuel System Diagnosis .

Excessive fuel may also cause long cranking times, which are accompanied by the following conditions:

    • A start that is not as fast as normal
    • A puff of black smoke at the tailpipe

An improperly-connected or faulty evaporative emissions system (EVAP) component can cause this problem. Refer to Evaporative Emission Control System Diagnosis .

One or more leaking poppet nozzles (CSFI injectors) may also extend the cranking time. Perform the Injector Balance Test. Refer to Fuel Injector Balance Test with Tech 2 .

Test Description

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

  1. Command both the ON and the OFF states. Repeat the commands as necessary.

    2.

  2. This step determines if the condition is located on the coil side or on the switch side of the circuit.

    3.

  3. This step verifies that the VCM is providing voltage to the fuel pump relay.

    4.

  4. This step tests for an open in the ground circuit to the fuel pump relay.

    5.

  5. This step determines if a voltage is constantly being applied to the fuel pump relay.

    6.

  6. In order to gain access to the fuel pump connector, the fuel tank may need to be removed. Refer to Fuel Tank Replacement .

  7. This step determines if the condition with the circuit is intermittent. If the fuse does not open, inspect the supply voltage circuit between the fuse and the fuel pump for an intermittent condition.

    8.

  8. Use the same amperage fuse in the jumper as is used in order to protect the fuel pump circuit.

    9.

  9. In order to gain access to the fuel pump connector, the fuel tank may need to be removed. Refer to Fuel Tank Replacement .

  10. Inspect the splice pack which contains the fuel pump ground and other ground circuits. Be certain that all connections within the splice pack are clean and tight.

Fuel Pump Electrical Diagnosis (wo/Jumper Harness)

Step

Action

Yes

No

1

Did you perform the Diagnostic System Check-Engine Controls?

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

2

  1. Install a scan tool.
  2. Turn ON the ignition, with the engine OFF.
  3. Use a scan tool in order to command ON and OFF the fuel pump relay.

Does the fuel pump turn ON and OFF?

Go to Intermittent Conditions

Go to Step 3

3

Use a scan tool in order to command ON and OFF the fuel pump relay.

Do you hear a click when you command the fuel pump relay ON and OFF?

Go to Step 9

Go to Step 4

4

  1. Turn OFF the ignition.
  2. Disconnect the fuel pump relay.
  3. Turn ON the ignition, with the engine OFF.
  4. Probe the control circuit of the fuel pump relay with a test lamp that is connected to a good ground.
  5. Use a scan tool in order to command ON and OFF the fuel pump relay.

Does the test lamp turn ON and OFF?

Go to Step 5

Go to Step 6

5

  1. Connect a test lamp between the control circuit of the fuel pump relay and the ground circuit of the fuel pump relay.
  2. Use a scan tool in order to command ON and OFF the fuel pump relay.

Does the test lamp turn ON and OFF?

Go to Step 19

Go to Step 22

6

Does the test lamp remain illuminated with each command?

Go to Step 7

Go to Step 8

7
  1. Disconnect the VCM. Refer to VCM Replacement/Programming .
  2. Test the control circuit of the fuel pump relay for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 27

Go to Step 26

8
  1. Disconnect the VCM. Refer to VCM Replacement/Programming .
  2. Test the control circuit of the fuel pump relay for a short to ground or for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 27

Go to Step 20

9

Turn ON the ignition, with the engine OFF.

Does the fuel pump operate continuously?

Go to Step 10

Go to Step 11

10
  1. Turn OFF the ignition.
  2. Disconnect the fuel pump relay.
  3. Turn ON the ignition, with the engine OFF.

Does the fuel pump operate continuously?

Go to Step 21

Go to Step 25

11

Is the fuel pump fuse open?

Go to Step 12

Go to Step 14

12

  1. Test the supply voltage circuit of the fuel pump for a grounded circuit between the fuel pump fuse and the fuel pump. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
  2. Replace the fuel pump fuse if necessary.

Did you find and correct the condition?

Go to Step 27

Go to Step 13

13

  1. Install all of the disconnected electrical components.
  2. Install a new fuel pump fuse.
  3. Use a scan tool in order to turn ON the fuel pump.

Is the fuel pump fuse open?

Go to Step 24

Go to Testing for Electrical Intermittents in Wiring Systems

14
  1. Turn OFF the ignition.
  2. Disconnect the fuel pump relay.
  3. Turn ON the ignition, with the engine OFF.
  4. Probe the battery voltage circuit of the fuel pump relay switch with a test lamp that is connected to a good ground.

Does the test lamp illuminate?

Go to Step 15

Go to Step 23

15

Connect a 20-amp fused jumper wire between the battery voltage circuit of the fuel pump relay switch and the supply voltage circuit of the fuel pump.

Does the fuel pump operate?

Go to Step 19

Go to Step 16

16

Test the supply voltage circuit of the fuel pump for an open or a high resistance between the fuel pump relay and the fuel pump. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 27

Go to Step 17

17

Important: Inspect the ground circuit for:

   • Tightness
   • Corrosion on the terminals
   • Damage to the wiring harness

Test the ground circuit of the fuel pump for an open or for high resistance. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 27

Go to Step 18

18

Inspect for faulty connections at the fuel pump. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 27

Go to Step 24

19

Inspect for faulty connections at the fuel pump relay. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 27

Go to Step 25

20

Inspect for faulty connections at the harness connector of the VCM. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 27

Go to Step 26

21

Repair the supply voltage circuit of the fuel pump for a short to voltage. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

Go to Step 27

--

22

Repair the open ground circuit of the fuel pump relay. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

Go to Step 27

--

23

Repair the battery voltage circuit of the fuel pump relay switch. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

Go to Step 27

--

24

Important: Inspect for faulty connections at the fuel pump and within the fuel tank before you replace the fuel pump.

  1. Replace the fuel pump. Refer to Fuel Sender Assembly Replacement .
  2. Replace the fuel pump fuse if necessary.

Did you complete the replacement?

Go to Step 27

--

25

Replace the fuel pump relay. Refer to Fuel Pump Relay Replacement .

Did you complete the replacement?

Go to Step 27

--

26

Replace the VCM. Refer to VCM Replacement/Programming .

Did you complete the replacement?

Go to Step 27

--

27

Operate the system in order to verify the repair.

Did you correct the condition?

System OK

Go to Step 2