Fuel Pump Electrical Circuit Diagnosis. GMC Truck Savana (CNG) 2001

For 1990-2009 cars only

Makes 🢒 GMC_Truck 🢒 2001 🢒 Savana (CNG) 🢒 Engine 🢒 Engine Controls - 5.0L and 5.7L 🢒 Fuel Pump Electrical Circuit Diagnosis

Fuel Pump Electrical Circuit Diagnosis Alternative Fuel

Circuit Description

The AF fuel pump relay coil is supplied a ground path all the time. The Alternative Fuels ECM supplies the power 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 control module energizes the GASOLINE fuel pump relay supplying power to the Fuel Pump Power circuit for a 2 second prime pulse. When the prime pulse is supplied to the Fuel Pump Power circuit, the lock-off relay is energized AND the gasoline fuel pump receives the 2 second prime pulse power. The 2 second prime pulse is allowed to operate the gasoline fuel pump for durability purposes.

When the engine is started, the GASOLINE control module energizes the GASOLINE fuel pump relay supplying power to the Fuel Pump Power circuit. When the engine is operating on CNG, the Alternative Fuels ECM energizes the AF fuel pump relay turning OFF the gasoline fuel pump. The lock-off relay continues to be energized by the GASOLINE fuel pump relay.

Test Description

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

The System Check MUST be performed before proceeding with this diagnostic table. Failure to perform the System Check will result in mis-diagnosis.

This step tests if the engine will start and run. A failure that causes the engine to not start on either fuel can be isolated to the CNG harness or a base vehicle problem such as a malfunctioning GASOLINE fuel pump relay.

This step tests the gasoline control modules ability to supply power to the gasoline fuel pump circuit.

A switch-over to gasoline can be performed by commanding a switch-over using the scan tool alternative fuels special functions. A switch-over to gasoline can also be performed by disconnecting the lock-off solenoid relay and waiting until the CNG fuel pressure drops.

This step tests for voltage THROUGH the AF fuel pump relay. With the ignition ON and the engine OFF, the Alternative Fuel ECM does not energize the AF fuel pump relay allowing voltage to flow through the relay and energize the gasoline fuel pump.

This step tests the Alternative Fuel ECMs ability to energize the AF fuel pump relay and disable the gasoline fuel pump during CNG operation.

This step tests for stuck AF fuel pump relay contacts or a shorted AF Fuel Pump Relay control circuit.

If the engine did not operate on gasoline during the System Check but now operates on gasoline, inspect for intermittent terminal contact or a AF fuel pump relay with sticking contacts that may have been disturbed during testing.

This step tests if the CNG harness is the cause of the no start on gasoline.

If the Fuel Pump Power circuit tests OK, there is a base vehicle problem that is causing the no-start condition.

Step

Action

Yes

Schematic Reference: Engine Controls Schematics

Did you perform the Alternative Fuels Engine Controls System Check?

Go to Step 2

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

Physically ensure BOTH fuel systems each have at least ¼ tank of fuel.
Install the CNG 20-amp fuse if previously removed.
Crank and attempt to start the engine.

Does the engine start and run at all?

Go to Step 4

Go to Step 3

Turn the ignition OFF and wait 30 seconds.
Disconnect the alternative fuels engine harness connector that contains the gasoline fuel pump circuits (C007).
Connect the J 35616-200 Unpowered Test Lamp between the Fuel Pump Power circuit terminal (A) and a known good ground. Connect to the engine harness side of the connector in order to monitor for power from the GASOLINE fuel pump relay.
Observe the test lamp as you turn the ignition ON leaving the engine OFF.

Did the test lamp illuminate for about 2 seconds?

Go to Step 14

Go to Step 13

Does the engine operate normally while on CNG but stall and not start during a switch-over to gasoline?

Go to Step 5

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

Turn the ignition OFF and wait 30 seconds.
Backprobe the AF Fuel Pump Relay switch power supply terminal (30) with the J 35616-200 connected to a known good ground.
Observe the test lamp as you turn the ignition ON, with the engine OFF.

Did the test lamp illuminate for about 2 seconds?

Go to Step 6

Go to Step 15

Start and idle the engine. Ensure the engine is operating on CNG.
Backprobe the AF Fuel Pump Relay switch load terminal (87A) with the J 35616-200 connected to a known good ground.

Is the test lamp illuminated?

Go to Step 8

Go to Step 7

Using the scan tool, establish communications with the Alternative Fuels ECM.
Ensure that the engine is idling.

Command the Fuel Pump Relay (Alternative Fuels) OFF.

Does the test lamp illuminate when the AF Fuel Pump Relay is commanded OFF?

Go to Step 10

Go to Step 16

Ensure that the engine is idling.
Backprobe the AF Fuel Pump Relay coil ground supply terminal (86) with the J 35616-200 connected to BATTERY POSITIVE.

Is the test lamp illuminated?

Go to Step 9

Go to Step 17

Ensure that the engine is idling.
Backprobe the AF Fuel Pump Relay coil power supply terminal (85) with the J 35616-200 connected to a known good ground.

Is the test lamp illuminated?

Go to Step 20

Go to Step 18

With the ignition OFF, remove the CNG 20-amp fuse in order to force a switch-over to gasoline.
Crank the engine.

Does the engine start and run on gasoline?

Go to Step 22

Go to Step 11

Turn the ignition OFF and wait 30 seconds.
Disconnect the alternative fuels engine harness connector that contains the gasoline fuel pump circuits (C007).
Jumper the Fuel Pump Power circuit terminal (A) and the Fuel Pump circuit terminal (B) together. Jumper at the engine harness side of the connector in order to bypass the alternative fuels harness.
Crank the engine.

Does the engine start and run on gasoline?

Go to Step 19

Go to Step 12

A failure exists in the one of the following:

•	The gasoline fuel pump

•	The fuel pump power circuit between the alternative fuels engine harness connector (C007) and the fuel pump connector

•	The fuel pump ground circuit

Refer to the gasoline service information in order to diagnose and repair the concern.

Are repairs complete?

Go to Step 22

A failure exists in the GASOLINE fuel pump relay circuitry.
Refer to the gasoline service information in order to diagnose and repair the concern.
If a blown fuel pump fuse is indicated by the gasoline service information ensure that the related CNG harness circuits are inspected for shorts.

Are repairs complete?

Go to Step 22

Test the Fuel Pump Power circuit between the alternative fuels engine harness connector (C007) terminal (A) and the splice on the CNG harness side for an open or poor terminal contact.
Repair wiring or terminal contact as necessary.

Was a wiring or terminal repair made?

Go to Step 22

Go to Engine Cranks but Does Not Run in Engine Controls in the G Van Service Manual

Repair the open Fuel Pump Power circuit between the AF fuel pump relay terminal (30) and the splice.

Is the repair complete?

Go to Step 22

Inspect for proper AF fuel pump relay terminal contact.
Test the AF fuel pump relay control circuit for a short to battery positive.
Repair terminals or wiring as necessary.

Was a terminal or wiring repair made?

Go to Step 22

Go to Step 20

Repair the open AF fuel pump relay coil ground circuit between the AF fuel pump relay terminal (86) and the splice.

Is the repair complete?

Go to Step 22

Inspect the Alternative Fuels ECM connectors for proper terminal contact .
Test for AF Fuel Pump Relay Control circuit continuity.
Test the AF Fuel Pump Relay Control circuit for a short to ground.
Repair terminals or wiring as necessary.

Was a terminal or wiring repair made?

Go to Step 22

Go to Step 21

Repair the open Fuel Pump circuit between the alternative fuels engine harness connector (C007) terminal (B) and the AF fuel pump relay terminal (87A).

Is the repair complete?

Go to Step 22

Replace the AF fuel pump relay.

Is the replacement complete?

Go to Step 22

Replace the Alternative Fuels ECM. Refer to Alternative Fuels Engine Control Module Replacement .
Program the replacement control module with the correct vehicle software and calibration.

Is the replacement complete?

Go to Step 22

Remove all test equipment except the scan tool.
Connect any disconnected components, fuses, etc.
Using a scan tool, clear DTC Information from the Alternative Fuels ECM and the gasoline control module (VCM/PCM or ECM).
Turn the ignition OFF and wait 30 seconds.
Start the engine and operate the vehicle.
Observe the fuel indicator lamp and the MIL.
Observe vehicle performance and driveability.
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)