GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 1997 🢒 GMC C Pickup - 2WD (CNG) 🢒 Engine 🢒 Engine Controls - 5.0L and 5.7L 🢒 Fuel Gauge Indicates Empty with CNG in Tank

Object Number: 322565 Size: LF
Handling Electrostatic Discharge Sensitive Parts Notice
Engine Controls Components
OBDII Symbol Description Notice
Engine Controls Schematics (PWR, GND and DLC - 1997)

Diagnostic Aids

Important:  The gauge will display the dormant fuel level for 10 seconds when the fuel gauge selector switch is pressed. It will then return to normal operation.

Check for the following conditions:

    • Verify that the CNG fuel tank is at least one-third full (1,200 psi). Add fuel if needed.
    • Verify that the High Pressure Lock-off (HPL) connector is connected.
    • Verify that the manual lock-down screw in the HPL is open.

Step

Action

Value(s)

Yes

No

1
  1. Turn ON the ignition.
  2. Inspect the fuel indicator light. Depress the fuel gauge selector switch if the light is illuminated.
  3. Read the gauge.

Does the fuel gauge read above empty?

--

Go to Step 23

Go to Step 2

2
  1. Start the engine.
  2. Close the manual shut-off valve (MSV).
  3. Turn OFF the ignition after the vehicle switches to gasoline or stalls.

    4. Important:: This procedure may set a DTC 51 (GMS Frequency Low) in the AF fuel system.

  4. Remove the Pressure Relief Device (PRD) from the High Pressure Regulator (HPR).
  5. Connect the J 39970 CNG Electronic Pressure Gauge using the J 39970-UPD Electronic Pressure Gauge Adapter in the PRD port of the HPR.
  6. Open the CNG MSV.
  7. Crank or start the engine.

Was the HPR pressure within the specified range?

130-220 psi

Go to Step 3

Go to Fuel System Diagnosis

3
  1. Disconnect the CNG Fuel Sending Unit (FSU) connector C315.
  2. Turn ON the ignition.

Does the fuel gauge indicate FULL?

--

Go to Step 4

Go to Step 5

4
  1. Connect a DVOM between CKT 920 (RED) at terminal B and CKT 450 (BLK) at terminal A of FSU connector C315.
  2. Turn ON the ignition.

Does the DVOM display a voltage within the specified range?

11-13 V

Go to Step 11

Go to Step 17

5

Inspect for voltage at CKT 909 (TAN) at terminal C of C315 using a DVOM.

Is the voltage within the specified range?

11-13 V

Go to Step 6

Go to Step 9

6
  1. Disconnect connector C012 at the fuel gauge relay.
  2. Inspect the voltage at CKT 928 (TAN/WHT) at terminal 87 using a DVOM.

Is the voltage within the specified range?

6-7 V

Go to Step 7

Go to Step 10

7

Ground CKT 30 (PPL) at terminal 30 of C012 using a jumper wire.

Does the gauge read empty?

--

Go to Step 23

Go to Step 8

8

Install a jumper wire between CKT 30 (PPL) at terminal 30 and CKT 918 (PPL) at terminal 87A of C012.

Is the action complete?

--

Go to Instrument Cluster Diagnosis in Instrument Panel, Gauges and Console in the 1997 C/K Truck Service Manual

--

9
  1. Disconnect ECU connector C001 (blue).
  2. Inspect for continuity of CKT 909 (TAN) at terminal C of C315, pin 17, using a DVOM.

Is there continuity?

--

Go to Step 13

Go to Step 21

10
  1. Disconnect ECU connector C001.
  2. Inspect for continuity of CKT 928 (TAN/WHT) at terminal 87 of C012, pin 32, using a DVOM.

Is there continuity?

--

Go to Step 13

Go to Step 21

11

Inspect for a faulty connection at ECU connector C001.

Was a problem found?

--

Go to Step 21

Go to Step 12

12

Inspect for continuity of CKT 909 (TAN) at terminal C of C315 to C001, pin 17, using a DVOM.

Is there continuity?

--

Go to Step 14

Go to Step 21

13

Inspect for a faulty connection at ECU connector C001.

Was a problem found?

--

Go to Step 21

Go to Step 20

14

Inspect for a faulty connection at FSU connector C315.

Was a problem found?

--

Go to Step 21

Go to Step 15

15

Inspect for a short to ground on CKT 909 (TAN) at C001, pin 17, using a DVOM.

Was a problem found?

--

Go to Step 21

Go to Step 16

16

Replace the Fuel Sending Unit (FSU). Refer to Fuel Sender Replacement .

Is the repair complete?

--

Go to Step 23

--

17
  1. Connect the DVOM to chassis ground.
  2. Turn ON the ignition.
  3. Probe CKT 920 (RED) at terminal B.

Does the DVOM display a voltage within the specified range?

11-13 V

Go to Step 18

Go to Step 19

18

Repair the open in ground circuit 450 between C315 and G013. Refer to Repair Procedures in Electrical Diagnosis in the 1997 C/K Truck Service Manual.

Is the repair complete?

--

Go to Step 23

--

19
  1. Inspect AF fuse 2.
  2. If the fuse is OK, repair CKT 920. Refer to Repair Procedures in Electrical Diagnosis in the 1997 C/K Truck Service Manual.

Is the action complete?

--

Go to Step 23

--

20
  1. Inspect for a short to ground of CKT 909 (TAN) at C001, pin 17, using a DVOM.
  2. Inspect for a short to ground of CKT 928 (TAN/WHT) wire at C001, pin 32.

Was a problem found?

--

Go to Step 21

Go to Step 22

21

Repair the circuit as necessary. Refer to Repair Procedures in Electrical Diagnosis in the 1997 C/K Truck Service Manual.

Is the repair complete?

--

Go to Step 23

--

22

Replace the ECU. Refer to Engine Control Unit Replacement .

Is the repair complete?

--

Go to Step 23

--

23
  1. Turn ON the ignition.
  2. Inspect the fuel gauge indicator light.
  3. Press the fuel gauge selector switch if the light is illuminated and read the gauge.

Does the fuel gauge read above empty?

--

Fuel Gauge OK

Go to Step 4