Fuel Gauge Relay Diagnosis - Alternative Fuels. Chevrolet Cavalier (CNG) 2001

For 1990-2009 cars only

Makes 🢒 Chevrolet 🢒 2001 🢒 Cavalier (CNG) 🢒 Engine 🢒 Engine Controls - 2.2L (LN2) 🢒 Fuel Gauge Relay Diagnosis - Alternative Fuels

Engine Controls Component Views
Engine Controls Schematics
OBD II Symbol Description 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 Alternative Fuels Engine Control Unit (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 monitored by the PCM. 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 monitored by the PCM. The PCM sends the fuel level information to the instrument panel using the Class 2 serial data circuit.

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 energize (when operating on gasoline) or de-energize (when operating on CNG) the fuel gauge relay causing the IP to display the DORMANT (non-operating) fuel system level. The dormant fuel level will be displayed for about 10 seconds.

Passenger vehicles have a filtered fuel level display, depress the fuel gauge select switch 30 seconds after the vehicle speed sensor indicates the vehicle is at rest in order to avoid inaccurate gauge displays.

Test Description

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

If the CNG fuse opens when the switch is depressed, test for a shorted to ground fuel gauge select switch input circuit or an internally shorted AF ECU. In addition a short may have damaged the fuel gauge select switch.

This step tests the AF ECU's ability to monitor the fuel gauge select switch input circuit.

This step tests for proper communication between the PCM and the instrument panel. Without communication the instrument panel may default the fuel gauge to empty.

With the vehicle operating on CNG this step inspects for PCM DTCs which may have detected a circuit fault on the CNG fuel level output or the PCM fuel level input circuits.

With the vehicle operating on gasoline this step inspects for PCM DTCs which may have detected a circuit fault on the fuel level sending unit (gasoline) circuits.

Step

Action

Value(s)

Yes

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

Did you perform the Diagnostic System Check?

Go to Step 2

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

Using the scan tool, observe the Fuel Gauge Select Switch input parameter.
Apply and release the fuel gauge select switch button.

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

Go to Step 3

Go to Step 5

Is there a fuel gauge operation or accuracy concern?

Go to Step 12

Go to Step 4

Is there a fuel indicator lamp operation concern?

Go to Fuel Indicator Lamp Always On - Alternative Fuels or Fuel Indicator Lamp Inoperative - Alternative Fuels

System OK

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

Go to Step 6

Go to Step 7

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

Does the scan tool parameter indicate the switch is applied?

Go to Step 10

Go to Step 29

Remove the fuel gauge select switch from the instrument panel. Refer to Alternative Fuel Gage Selector Switch/Fuel Indicator Lamp Replacement .
Disconnect the fuel gauge select switch connector.
Connect the J 35616-200 12-V Unpowered Test Lamp between GROUND and the switch power circuit terminal .
Observe the test lamp with the ignition ON.

Is the test lamp illuminated?

Go to Step 8

Go to Step 11

Connect a fused jumper between the switch supply circuit terminal and the fuel gauge select switch Input circuit terminal.

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

Using the scan tool observe the fuel gauge select switch input parameter.

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

Go to Step 29

Go to Step 9

Disconnect the AF ECU connectors. Refer to Engine Control Unit Replacement in Engine Controls.
Test the fuel gauge select switch Input circuit for the following:

•	an open circuit

•	poor terminal contact at the AF ECU connector or the switch connector

Repair wiring or terminals as necessary.

Was a wiring or terminal repair necessary?

Go to Step 32

Go to Step 30

Disconnect the AF ECU connectors. Refer to Engine Control Unit Replacement in Engine Controls.
Test the fuel gauge select switch Input circuit for a short to voltage.
Repair wiring or terminals as necessary.

Was a wiring repair necessary?

Go to Step 32

Go to Step 30

Repair the open fuel gauge select switch power supply circuit between the switch terminal and the ignition relay.

Is the repair complete?

Go to Step 32

Turn the ignition ON leaving the engine OFF.
Command the Fuel Gauge Relay ON and OFF using the scan tool Alt. Fuel Special Functions.
Listen for audible fuel gauge relay operation. The relay is located behind the left side headlamp assembly.

Does the relay turn ON and OFF as commanded?

Go to Step 13

Go to Step 21

Inspect the PCM for stored DTCs.

Is a PCM to IPC serial data DTC present?

Go to Diagnostic System Check - Engine Controls

Go to Step 14

Ensure the ignition is ON for at least 30 seconds.
Inspect the PCM for Fuel Level Sensor DTCs.

Is a PCM Fuel Level Sensor DTC present?

Go to Step 18

Go to Step 15

In order to test the gasoline fuel sending unit circuitry command the Fuel Gauge Relay OFF using the scan tool Alt. Fuel Special Functions for at least 30 seconds. Do not turn the ignition OFF.
Inspect the PCM for Fuel Level Sensor DTCs.

Is a PCM Fuel Level Sensor DTC present?

Go to Step 20

Go to Step 16

Is there a gasoline fuel gauge accuracy concern?

Go to Fuel Gage Inaccurate or Inoperative in Instrument Panel, Gauges, and Console in the J Platform Service Manual

Go to Step 17

Is there a CNG fuel gauge accuracy concern?

Go to Fuel Gauge Inaccurate or Inoperative - Alternative Fuels

System OK

Disconnect the fuel gauge relay.
Connect the J 39200 DMM between the PCM fuel level input circuit at the fuel gauge relay terminal (30) and a known good ground.
Measure the voltage of the PCM fuel level input circuit.

Is the voltage within the value range specified?

4.8-5.2 V

Go to Step 19

Go to Step 23

Connect the unpowered test lamp between the AF ECU CNG fuel level output circuit at the fuel gauge relay terminal (87) and battery positive.
Command the Fuel Gauge Output from full to empty using the scan tool Alt. Fuel Special Functions.
Observe the test lamp while commanding the Fuel Gauge Output from full to empty.

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

Go to Step 28

Go to Step 24

Turn the ignition OFF.
Disconnect the PCM.
Using the J 39200 DMM measure the resistance between the gasoline fuel sending unit circuit at the fuel gauge relay connector terminal (87A) and the sensor ground circuit terminal at the PCM connector.

Is the resistance within the value range specified?

40-250 ohms

Go to Step 28

Go to Step 25

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

Is the test lamp illuminated?

Go to Step 22

Go to Step 27

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

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

Go to Step 28

Go to Step 26

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

Was a terminal or wiring repair made?

Go to Step 32

Go to Step 31

Test the CNG fuel level output circuit between the fuel gauge relay and the AF ECU for an open, short to ground or short to battery positive.
Inspect for proper terminal contact at the fuel gauge relay and AF ECU connectors.
Repair wiring or terminal contact as necessary.

Was a terminal or wiring repair made?

Go to Step 32

Go to Step 30

The fault exists in the gasoline fuel sending unit circuitry.

Test the fuel sending unit output circuit between the fuel gauge relay and the fuel tank module (gasoline) for an open, short to ground or short to battery positive.

Test the fuel sending unit sensor ground circuit between the fuel tank module (gasoline) and the PCM for an open, short to ground or short to battery positive.
Test the fuel sending unit for an open or short.
Inspect for proper terminal contact at the fuel gauge relay, PCM and any in-line connectors.
Repair wiring, terminal contact or replace the fuel sending unit as necessary.

Was a terminal, wiring repair or fuel sending unit replacement made?

Go to Step 32

Go to Step 31

Test the fuel gauge relay control circuit between the fuel gauge relay and the AF ECU for an open, short to ground or short to battery positive.
Inspect for proper terminal contact at the fuel gauge relay and AF ECU connectors.
Repair wiring or terminal contact as necessary.

Was a terminal or wiring repair made?

Go to Step 32

Go to Step 30

Repair the open fuel gauge relay ground circuit.

Is the repair complete?

Go to Step 32

Replace the fuel gauge relay.

Is the replacement complete?

Go to Step 32

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

Is the replacement complete?

Go to Step 32

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 32

Important: The following control module replacement procedures must all be completed before attempting to operate the vehicle.

Replace the gasoline control module (VCM/PCM or ECM). Refer to Powertrain Control Module Replacement in the Engine Controls section.
Program (flash) the replacement control module with the correct vehicle software and calibration. Refer to Powertrain Control Module Programming .

Important: In order to perform the variation learn procedure a switch-over to gasoline operation is necessary. Remove the CNG 20 amp fuse in order to perform the switch-over.

Perform the Crankshaft Position System Variation Learn in the Engine Controls section.

All of the above procedures are located in the J Platform Service Manual.

Is the replacement complete?

Go to Step 32

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 Diagnostic System Check - Alternative Fuels (Engine Controls)