Fuel Gauge Relay Diagnosis. GMC Truck GMC K Pickup

For 1990-2009 cars only

Makes 🢒 GMC_Truck 🢒 1999 🢒 GMC K Pickup - 4WD (CNG) 🢒 Engine 🢒 Engine Controls - 5.0L and 5.7L 🢒 Fuel Gauge Relay Diagnosis

Engine Controls Components
Engine Controls Schematics
OBD II Symbol Description Notice
Handling ESD Sensitive Parts Notice
Handling ESD Sensitive Parts Notice
Handling ESD Sensitive Parts Notice

Circuit Description

The fuel gauge relay coil is supplied a ground path all the time. The coil power is supplied by the AF ECU. When the vehicle is operating in the CNG mode the AF ECU energizes the fuel gauge relay allowing the CNG Fuel Level Output to be displayed by the fuel gauge. When the vehicle is operating in the gasoline mode the AF ECU does NOT energize the fuel gauge relay allowing the gasoline Fuel Level Output to be displayed by the fuel gauge.

Regardless of the fuel system in operation the AF ECU monitors the Fuel Gauge Select Switch Input circuit. When the vehicle operator depresses the switch the AF ECU will display the DORMANT (non-operating) fuel system level by energizing (when operating on gasoline) or de-energizing (when operating on CNG) the fuel gauge relay. The dormant fuel level will be displayed for about 10 seconds.

Test Description

The number(s) below refer to the step number(s) on the diagnostic table.

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

This step checks the AF ECUs ability to operate the fuel gauge.

This step checks the ability of the fuel gauge relay to switch to the gasoline fuel level.

With CNG enabled the AF ECU powers the Fuel Gauge Relay Control circuit energizing the fuel gauge relay.

This step checks for a shorted Fuel Gauge circuit or an instrument panel failure. With the Fuel Gauge circuit disconnected the fuel gauge should show a defaulted reading of PAST full.

Jumpering the Fuel Gauge circuit to ground should cause the fuel gauge to indicate Empty.

This step determines if the fuel gauge relay is the cause of the CNG fuel gauge concern.

This step checks for proper operation of the fuel gauge select switch.

This step determines if the fuel gauge relay is the cause of the GASOLINE fuel gauge concern.

Step

Action

Value(s)

Yes

Important: The vehicle MUST operate on CNG in order to use this diagnostic procedure.

Were you sent here from other diagnostic service information?

Go to Step 2

Go to Alternative Fuels (AF) Powertrain On Board (OBD) System Check

Using the scan tool communicate with the AF ECU and command the Fuel Gauge Output from Full to Empty.

Does the fuel gauge respond correctly?

Go to Step 3

Go to Step 6

Using the scan tool communicate with the GASOLINE control module (VCM/ECM or PCM) and observe the gasoline Fuel Level scan tool parameter.
Depress and Release the fuel gauge select switch button.
Compare the gasoline Fuel Level scan tool parameter to the fuel gauge level indicated by the instrument panel.

Does the fuel gauge indicate the same level as the gasoline Fuel Level scan tool parameter for about 10 seconds and then return to displaying the CNG fuel tank level?

Go to Step 4

Go to Step 10

Is there a CNG fuel gauge accuracy concern?

Go to CNG Fuel Gauge Reading Inaccurate

Go to Step 5

Is there a gasoline fuel gauge accuracy concern?

Go to the fuel level/gauge diagnosis in the C/K Truck Service Manual.

System OK

Disconnect the fuel gauge relay connector (C012).
Ignition ON. Connect the J 35616-200 Un-powered Test Lamp between the Fuel Gauge Relay Control circuit terminal (86) and the Fuel Gauge Relay coil ground circuit terminal (85).

Is the test lamp illuminated?

Go to Step 7

Go to Step 12

Remove the test lamp.
With the fuel gauge relay disconnected observe the fuel gauge on the instrument panel.

Does the fuel gauge indicate PAST Full?

Go to Step 8

Go to Step 13

Jumper the Fuel Gauge circuit terminal (30) to a known good ground using a fused jumper.

Does the fuel gauge indicate Empty after being jumpered for several seconds?

Go to Step 9

Go to Step 14

Remove the jumper from ground.
Connect the fused jumper between the Fuel Gauge circuit terminal (30) and the CNG Fuel Level circuit terminal (87).

Using the scan tool communicate with the AF ECU and command the Fuel Gauge Output from Full to Empty.

Does the fuel gauge respond correctly?

Go to Step 20

Go to Step 15

Disconnect the fuel gauge relay connector (C012).
With the Ignition ON, connect the J 35616-200 Un-powered Test Lamp between the Fuel Gauge Relay Control circuit terminal (86) and the Fuel Gauge Relay coil ground circuit terminal (85).
Depress and Release the fuel gauge select switch button.

Did the test lamp illuminate, turn OFF for about 10 seconds and then illuminate again?

Go to Step 11

Go to Step 16

Remove the test lamp.
Connect a fused jumper between the Fuel Gauge circuit terminal (30) and the GASOLINE Fuel Level circuit terminal (87A).
Using the scan tool communicate with the GASOLINE control module (VCM/ECM or PCM) and observe the gasoline Fuel Level scan tool parameter.
Compare the gasoline Fuel Level scan tool parameter to the fuel gauge level indicated by the instrument panel.

Does the fuel gauge indicate the same level as the scan tool Fuel Level parameter?

Go to Step 20

Go to Step 17

Connect the test lamp between the Fuel Gauge Relay coil ground circuit terminal (85) and BATTERY POSITIVE.

Is the test lamp illuminated?

Go to Step 18

Go to Step 19

Check the Fuel Gauge circuit for a short to ground or a short to voltage (jumper fuse blown).
If the circuit is OK repair the instrument panel. Refer to instrument panel diagnosis in the Body and Accessories section of the C/K Truck Service Manual.

Is the repair complete?

Go to Step 22

Check the Fuel Gauge circuit for an open or poor terminal contact.
If the circuit is OK repair the instrument panel. Refer to instrument panel diagnosis in the Body and Accessories section of the C/K Truck Service Manual.

Is the repair complete?

Go to Step 22

Check the CNG Fuel Level Output circuit for the following:

•

an open

•	poor terminal contact

•	a short to ground

•	a short to voltage

Repair circuit as necessary.

Was a circuit repair necessary?

Go to Step 22

Go to Step 21

Check the Fuel Gauge Relay Control circuit for a short to voltage.
Repair circuit as necessary.

Was a circuit repair necessary?

Go to Step 22

Go to Step 21

Check the GASOLINE Fuel Level Output circuit for the following:

•

an open

•	poor terminal contact

•	a short to ground

•	a short to voltage

•	a gasoline control module (VCM/PCM or ECM) fault.

Repair as necessary.

Is the repair complete?

Go to Step 22

Check the Fuel Gauge Relay Control circuit for an open or a short to ground.
Check for proper terminal contact at the AF ECU connector.
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 Gauge Relay coil ground circuit between the relay terminal (85) and the circuit splice.

Is the repair complete?

Go to Step 22

Replace the Fuel Gauge Relay.

Is the replacement complete?

Go to Step 22

Replace the AF ECU. Refer to Engine Control Unit Replacement .
Program (flash) 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 dis-connected components, fuses, etc.
Using a scan tool clear DTC information from the AF ECU 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 Alternative Fuels (AF) Powertrain On Board (OBD) System Check