Circuit Description
The CNG fuel gage system consists of the following components:
| • | The fuel tank pressure (FTP) sensor |
| • | The fuel tank temperature (FTT) sensor |
| • | The pulse width modulated (PWM) output circuit |
| • | The dash mounted fuel gage |
| • | The dash mounted fuel gage select switch - KL6 only |
The alternative fuels Engine Control Module (ECM) monitors the fuel pressure and fuel temperature in the CNG fuel tank. The volume of CNG varies with pressure and temperature. In order to display an accurate fuel gage reading, the alternative fuels ECM will perform a calculation based upon fuel pressure and temperature and determine the correct amount of fuel remaining in the fuel tank. The alternative fuels ECM converts the calculated fuel level into a PWM signal that is monitored by the PCM. The PCM sends the fuel level information to the instrument panel fuel gage via the Class 2 serial data circuit. Vehicles that are equipped with Bi-fuel option KL6 also utilize a fuel gage select switch. The fuel gage select switch is monitored by the PCM. When the switch is pressed, the PCM will display the fuel level for the non-operating fuel system, for about 10 seconds.
Diagnostic Aids
If the customers concern is related to fast filling the fuel tank refer to the following:
| • | Fast-filling the CNG fuel tank causes the fuel to heat-up and expand. |
| • | The fuel temperature may remain hotter than the ambient (IAT) temperature for up to 3 hours after filling. |
| • | The fuel tank pressure may decrease by up to 800 psi within 3 hours of fast filling. |
The fuel tank will not completely fill during a fast fill, due to the expansion of the fuel. The fuel gage will display full as long as the fuel is hot. The fuel gage display reading will decrease as the fuel cools.
Step | Action | Values | Yes | No |
|---|---|---|---|---|
1 | Did you perform the Diagnostic System Check Alternative Fuels - Engine Controls? | -- | Go to Step 2 | Go to Diagnostic System Check - Alternative Fuels (Engine Controls) |
2 |
Important: Verify that the customers concern is not a normal operating condition. Refer to Diagnostic Aids. Is the customers concern a normal operating condition? | -- | Go to Intermittent Conditions in Engine Controls | Go to Step 3 |
3 |
Does the IP fuel gage accurately display the Alternative Fuel Level? | -- | Go to Step 10 | Go to Step 4 |
4 |
Do both data parameters indicate that the AF ECM is in control of engine fueling? | -- | Go to Step 5 | Go to Fuel System Unwanted Switch to Gasoline in Engine Controls |
5 |
Does the fuel gage operate as commanded? | -- | Go to Step 6 | Go to Diagnostic System Check - Instrument Cluster in Instrument Panel, Gages and Console in the C/K Pickup Service Manual |
6 | Is this vehicle equipped with the Bi-fuel KL6 option? | -- | Go to Step 7 | Go to Step 15 |
7 |
Does the Fuel Gage Selected data display change when the switch is Pressed? | -- | Go to Step 15 | Go to Step 8 |
8 |
Is the test lamp illuminated? | -- | Go to Step 17 | Go to Step 9 |
9 | Press the fuel gage select switch while observing the test lamp. Does the test lamp illuminate when the switch is pressed? | -- | Go to Step 23 | Go to Step 18 |
10 | Observe the Fuel Temperature Sensor voltage display. Does the display indicate a voltage greater than the specified value? | 4.8 V | Go to Step 20 | Go to Step 11 |
11 | Observe the Fuel Tank Pressure Sensor voltage display. Does the display indicate a voltage greater than the specified value? | 4.8 V | Go to Step 21 | Go to Step 12 |
12 |
Important: Fast-filling the CNG fuel tank causes the fuel to heat-up and expand. The fuel temperature may remain hotter than the ambient (IAT) temperature for up to 3 hours after filling.
Does each sensor indicate the same temperature plus or minus the specified value? | 20°C (35°F) | Go to Step 13 | Go to Step 25 |
13 |
Important: Fast-filling the CNG fuel tank causes the fuel to heat-up and expand. The fuel tank pressure may DECREASE by up to 800 psi within three hours of fast filling.
Does the FTP sensor indicate fuel pressure near the value specified? | 24,820 kPa (3600 psi) @ 21°C (70°F) | Go to Step 14 | Go to Step 26 |
14 |
Does the Fuel Tank Pressure Sensor data display smoothly indicate the fuel pressure decrease and also indicate fuel pressure less than the value specified? | 1379 kPa (200 psi) | Go to Intermittent Conditions in Engine Controls | Go to Step 26 |
15 |
Is the voltage between the value specified? | 4.8-5.2V | Go to Step 16 | Go to Step 19 |
16 |
Does the fuel gage display near empty? | -- | Go to Step 24 | Go to Step 27 |
17 |
Is the test lamp illuminated? | -- | Go to Step 22 | Go to Step 29 |
18 | Test the Fuel Gage Select Switch Signal circuit between the PCM connector terminal and the switch for an open or short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual. Did you find and correct the condition? | -- | Go to Step 30 | Go to Step 29 |
19 | Test the Alternative Fuel Level Signal circuit between the PCM connector terminal and the alternative fuels ECM for an open, short to ground or short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual. Did you find and correct the condition? | -- | Go to Step 30 | Go to Step 23 |
20 | Test the Fuel Tank Temperature Sensor Siganl circuit for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual. Did you find and correct the condition? | -- | Go to Step 30 | Go to Step 25 |
21 | Test the Fuel Tank Pressure Sensor Signal circuit for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual. Did you find and correct the condition? | -- | Go to Step 30 | Go to Step 26 |
22 | Repair the Fuel Gage Select Switch Signal circuit for a short to ground condition . Refer to Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual. Did you correct the condition? | -- | Go to Step 30 | -- |
23 | Inspect for proper terminal contact at the PCM connector. Refer to Testing for Intermittent Conditions and Poor Connections and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual. Did you find and correct the condition? | -- | Go to Step 30 | Go to Step 27 |
24 | Inspect for proper terminal contact at the alternative fuels ECM connector. Refer to Testing for Intermittent Conditions and Poor Connections and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual. Did you find and correct the condition? | -- | Go to Step 30 | Go to Step 28 |
25 | Replace the fuel tank temperature sensor. The temperature sensor is not Serviceable separately from the HPL. Refer to High Pressure Lock-Off Solenoid Replacement in Engine Controls. Did you complete the replacement? | -- | Go to Step 30 | -- |
26 | Replace the fuel tank pressure sensor. Refer to Fuel Tank Pressure Sensor Replacement in Engine Controls. Did you complete the replacement? | -- | Go to Step 30 | -- |
27 |
Important: KL6 models require a switchover to gasoline with a scan tool in order to perform the Crankshaft Position Variation Learn Procedure. KL8 models do not require the Crankshaft Position Variation Learn Procedure.Replace the PCM. Refer to Powertrain Control Module Replacement in Engine Controls in the C/K Pickup Service Manual. Did you complete the replacement? | -- | Go to Step 30 | -- |
28 | Replace the alternative fuels ECM. Refer to Alternative Fuels Engine Control Module Replacement in Engine Controls. Did you complete the replacement? | -- | Go to Step 30 | -- |
29 | Replace the fuel gage select switch. Refer to Fuel Gage Selector Switch Replacement - Alternative Fuels . Did you complete the replacement? | -- | Go to Step 30 | -- |
30 |
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) |
