GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2001 🢒 Express (CNG) 🢒 Engine 🢒 Engine Controls - 5.0L and 5.7L 🢒 Fuel Gauge Relay Diagnosis - Alternative Fuels

Circuit Description

The fuel gage relay coil is supplied a ground path all the time. The coil power is supplied by the Alternative Fuels ECM. When the vehicle is operating in the CNG mode the Alternative Fuels ECM energizes the fuel gage relay allowing the CNG Fuel Level Output to be monitored by the Instrument Panel (IP). When the vehicle is operating in the gasoline mode the Alternative Fuels ECM does NOT energize the fuel gage relay allowing the gasoline Fuel Level Output to be monitored by the IP.

Regardless of the fuel system in operation the Alternative Fuels ECM monitors the Fuel Gage Select Switch Input circuit. When the vehicle operator depresses the switch, the Alternative Fuels ECM will energize or de-energize the fuel gage relay causing the IP to display the DORMANT-non-operating- fuel system level. The dormant fuel level will be displayed for about 10 seconds.

Test Description

The number below refers to the step number on the diagnostic table.

  1. If the CNG fuse opens when the switch is depressed, test for a shorted to ground fuel gage select switch input circuit or an internally shorted Alternative Fuels ECM. In addition a short may have damaged the fuel gage select switch.

    This step tests the Alternative Fuels ECM's ability to monitor the fuel gage select switch input circuit.

Step

Action

Yes

No

1

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 Diagnostic System Check - Alternative Fuels (Engine Controls)

2

  1. Using the scan tool, observe the Fuel Gage Select Switch input parameter.
  2. Depress and release the fuel gage select switch button.

Did the scan tool parameter indicate a change in the fuel gage type was requested?

Go to Step 3

Go to Step 6

3

    Important: Before commanding the relay wait at least 10 seconds from the last time you depressed the fuel gage select switch.

  1. Command the Fuel Gage Relay ON and OFF using the scan tool Alt. Fuel Special Functions.
  2. Listen for audible fuel gage relay operation.

Does the relay turn ON and OFF as commanded?

Go to Step 4

Go to Step 20

4

    Important: Ensure the engine is operating on CNG.

    Before commanding the gage wait at least 10 seconds from the last time you depressed the fuel gage select switch.

  1. Command the Fuel Gage Output from 100% to 0% using the scan tool Alt. Fuel Special Functions.
  2. Observe the instrument panel fuel gage while commanding the Fuel Gage Output.

Does the instrument panel fuel gage accurately respond to the fuel gage commands?

Go to Step 5

Go to Step 18

5
  1. Establish communication with the gasoline control module (VCM/PCM or ECM).
  2. Observe and record the EVAP Data gasoline fuel level scan tool data parameter.
  3. Command the Fuel Gage Relay OFF using the scan tool ALT. FUEL Special Functions.
  4. Compare the instrument panel fuel gage to the scan tool gasoline fuel level.

Does the instrument panel fuel gage indicate the same as the gasoline fuel level data parameter?

Go to Step 17

Go to Step 16

6

Does the scan tool parameter always indicate the switch is applied?

Go to Step 7

Go to Step 8

7
  1. Remove the fuel gage select switch from the instrument panel. Refer to Alternative Fuel Gage Selector Switch/Fuel Indicator Lamp Replacement .
  2. Disconnect the fuel gage select switch connector.
  3. Using the scan tool observe the Fuel Gage Select Switch input parameter.

Does the scan tool parameter indicate the switch is applied?

Go to Step 11

Go to Step 28

8
  1. Remove the fuel gage select switch from the instrument panel. Refer to Alternative Fuel Gage Selector Switch/Fuel Indicator Lamp Replacement .
  2. Disconnect the fuel gage select switch connector.
  3. Connect the J 35616-200 Un-powered Test Lamp between GROUND and the switch power circuit terminal .
  4. Observe the test lamp with the ignition ON.

Is the test lamp illuminated?

Go to Step 9

Go to Step 12

9
  1. Connect a fused jumper between the switch supply circuit terminal and the fuel gage select switch input circuit terminal.

    2. If the jumper fuse opens, repair the short to ground on the fuel gage select switch input circuit.

  2. Using the scan tool observe the fuel gage select switch input parameter.

Did the scan tool parameter indicate a change in the fuel gage type was requested when the circuit was jumpered?

Go to Step 28

Go to Step 10

10
  1. Turn the ignition OFF and wait 30 seconds.
  2. Disconnect the Alternative Fuels ECM connectors. Refer to Alternative Fuels Engine Control Module Replacement in Engine Controls .
  3. Test the fuel gage select switch input circuit for the following:
    4. • An open circuit
    • Poor terminal contact at the Alternative Fuels ECM connector or at the switch connector
  4. Repair wiring or terminals as necessary.

Was a wiring or terminal repair necessary?

Go to Step 31

Go to Step 30

11
  1. Turn the ignition OFF and wait 30 seconds.
  2. Disconnect the Alternative Fuels ECM connectors. Refer to Alternative Fuels Engine Control Module Replacement in Engine Controls .
  3. Test the fuel gage select switch Input circuit for a short to voltage.
  4. Repair wiring or terminals as necessary.

Was a wiring repair necessary?

Go to Step 31

Go to Step 30

12
  1. Disconnect the ignition relay connector.
  2. Connect the J 35616-200 between a known good ground and the relay switch and coil supply circuit terminals.

Does the test lamp illuminate on both circuits?

Go to Step 13

Go to Step 15

13

Connect the J 35616-200 between the relay switch supply circuit terminal (30) and the relay ground circuit terminal.

Does the test lamp illuminate?

Go to Step 14

Go to Step 25

14
  1. Connect a fused jumper between the CNG Fuse circuit terminal (30) and the fuel gage select switch output circuit terminal (87) at the ignition relay connector.
  2. Connect the J 35616-200 between GROUND and the switch power circuit terminal at the fuel gage select switch connector.
  3. Observe the test lamp.

Is the test lamp illuminated?

Go to Step 29

Go to Step 26

15

Repair the open in the circuit that did not illuminate the test lamp.

Is the repair complete?

Go to Step 31

--

16
  1. Test the fuel level input circuit between the fuel gage relay and the PCM for an open, short to ground or short to battery positive.
  2. Inspect for proper terminal contact at the fuel gage relay, PCM and any in-line connectors.
  3. Repair wiring or terminal contact as necessary.

Was a terminal or wiring repair made?

Go to Step 31

Go to Diagnostic System Check - Instrument Cluster in Instrument Panel, Gages, and Console in the G-Van Service Manual

17

Is there a CNG fuel gage accuracy concern?

Go to Fuel Gauge Inaccurate or Inoperative - Alternative Fuels

System OK

18
  1. Connect the unpowered test lamp between the Alternative Fuels ECM CNG fuel level output circuit at the fuel gage relay terminal (87) and battery positive.
  2. Command the Fuel Gage Output from full to empty using the scan tool Alt. Fuel Special Functions.
  3. Observe the test lamp while commanding the Fuel Gage Output from full to empty.

Does the test lamp blink and/or increase and decrease in brightness as the Fuel Gage Output is commanded?

Go to Step 19

Go to Step 22

19
  1. Connect a fused jumper between the instrument panel fuel gage circuit terminal (30) and the CNG Fuel level Output circuit terminal (87).
  2. Command the Fuel Gage Output from full to empty using the scan tool Alt. Fuel Special Functions.
  3. Observe the instrument panel fuel gage while commanding the Fuel Gage Output from full to empty.

Does the instrument panel fuel gage accurately respond to the fuel gage commands?

Go to Step 27

Go to Step 23

20
  1. Disconnect the fuel gage relay.
  2. Connect the unpowered test lamp between the relay coil ground circuit at the fuel gage connector terminal (85) and battery positive.

Is the test lamp illuminated?

Go to Step 21

Go to Step 25

21
  1. Connect the unpowered test lamp between the relay coil control circuit at the fuel gage connector terminal (86) and ground.
  2. Turn the ignition ON leaving the engine OFF.
  3. Command the Fuel Gage Relay ON and OFF using the scan tool Alt. Fuel Special Functions.

Does the test lamp turn ON and OFF when the fuel gage relay is commanded ON and OFF?

Go to Step 27

Go to Step 24

22
  1. Test the CNG fuel level output circuit between the fuel gage relay and the Alternative Fuels ECM for an open, short to ground or short to battery positive.
  2. Inspect for proper terminal contact at the fuel gage relay and Alternative Fuels ECM connectors.
  3. Repair wiring or terminal contact as necessary.

Was a terminal or wiring repair made?

Go to Step 31

Go to Step 30

23
  1. Test the relay fuel level output circuit between the fuel gage relay and the instrument panel for an open, short to ground or short to battery positive.
  2. Inspect for proper terminal contact at the fuel gage relay and instrument panel connectors.
  3. Repair wiring or terminal contact as necessary.

Was a terminal or wiring repair made?

Go to Step 31

Go to Diagnostic System Check - Instrument Cluster in Instrument Panel, gages, and Console in the G-Van Service Manual

24
  1. Test the fuel gage relay control circuit between the fuel gage relay and the Alternative Fuels ECM for an open, short to ground or short to battery positive.
  2. Inspect for proper terminal contact at the fuel gage relay and Alternative Fuels ECM connectors.
  3. Repair wiring or terminal contact as necessary.

Was a terminal or wiring repair made?

Go to Step 31

Go to Step 30

25

Repair the open fuel gage relay ground circuit.

Is the repair complete?

Go to Step 31

--

26

Repair the open fuel gage select switch power circuit between the ignition relay terminal (87) and the fuel gage select switch terminal (A).

Is the repair complete?

Go to Step 31

--

27

Replace the fuel gage relay.

Is the replacement complete?

Go to Step 31

--

28

Replace the fuel gage selector switch. Refer to Alternative Fuel Gage Selector Switch/Fuel Indicator Lamp Replacement .

Is the replacement complete?

Go to Step 31

--

29

Replace the ignition relay.

Is the replacement complete?

Go to Step 31

--

30
  1. Replace the Alternative Fuels ECM. Refer to Alternative Fuels Engine Control Module Replacement in Engine Controls.
  2. Program the replacement control module with the correct vehicle software and calibration.

Is the replacement complete?

Go to Step 31

--

31
  1. Remove all test equipment except the scan tool.
  2. Connect any disconnected components, fuses, etc.
  3. Using a scan tool clear DTC Information from the Alternative Fuels ECM and the gasoline control module (VCM/PCM or ECM).
  4. Turn the ignition OFF and wait 30 seconds.
  5. Start the engine and operate the vehicle.
  6. Observe the fuel indicator lamp and 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 FIL/MIL illumination and no stored DTCs?

System OK

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