Circuit Description
The Gas Mass Sensor (GMS) provides a frequency signal proportional to the mass-flow of gaseous fuel entering the engine. The GMS actual gas signal varies from about 700 Hz at idle to about 2700 Hz at Wide Open Throttle (WOT).
The GMS desired gas signal from the ECU is an input to the GMS which is a frequency ranging from about 700 Hz to 5000 Hz. This signal, when present, activates the GMS and commands a desired Mixture Control Valve (MCV) position.
The GMS actual gas signal is one of the most important inputs used by the ECU for fuel control while operating on CNG. The GMS provides fuel mass-flow feedback to the ECU which is used to precisely trim the position of the MCV for the required fuel flow rate.
This is a type A DTC.
Conditions for Setting the DTC
The following conditions will set the DTC:
| • | If the engine speed is between 25 and 500 RPM: |
| - | The battery voltage is greater than 8 volts. |
| - | The GMS frequency output is less than 250 Hz for 0.5 second. |
| • | If the engine speed is greater than 500 RPM: |
| - | The battery voltage is greater than 10 volts. |
| - | The GMS frequency output is less than 500 Hz for 0.5 second. |
Action Taken When the DTC Sets
The ECU will set DTC 51 and illuminate the MIL lamp after one fault has occurred if a low frequency failure is detected. The ECU will simultaneously initiate a fuel mode switch-over to gasoline. The vehicle will not attempt to operate on CNG again until the next key cycle.
Conditions for Clearing the MIL/DTC
The ECU turns the MIL OFF after 3 consecutive driving trips without a fault condition present. A DTC will clear if no fault conditions have been detected for 40 warm-up cycles (coolant temperature has risen 40°C (72°F) from the start-up coolant temperature and the engine coolant temperature exceeds 70°C (160°F) during that same ignition cycle) or the code clearing procedure has been used.
Diagnostic Aids
Always diagnose ANY VCM codes that are present before beginning this diagnostic procedure.
Important: The GMS mass-flow and commanded flow frequencies cannot be monitored in the gasoline mode at any time. These frequencies cannot be monitored in the CNG mode with the Key On/Engine Off. The signals will only be present while operating on CNG and only with the engine in the crank or run modes.
Any condition that results in no fuel flow being measured by the GMS will cause a DTC 51 to set. Check for the following conditions:
| • | A closed CNG manual fuel shut-off valve. |
| • | An empty CNG fuel tank. |
| • | Malfunctioning or disconnected low- or high-pressure lock-offs. |
| • | Malfunctioning or disconnected fuel sending unit (FSU). |
| • | Inspect the harness for the following conditions: |
| - | Backed out terminals |
| - | Improper mating |
| - | Broken locks |
| - | Improperly formed or damaged terminals |
| - | Poor terminal to wire connection |
| - | A damaged harness |
| • | A DTC 51 will be set if the GMS output signal circuit is open or shorted. |
| • | A DTC 51 will be set if the commanded flow signal to the GMS is open or shorted. |
Test Description
The number(s) below refer to the step number(s) on the diagnostic table.
-
This step verifies that the manual shut-off valve is open.
-
This step verifies that the fuel tank contains fuel. It also verifies that the lock-offs are connected and functioning and that system pressures are within specifications.
-
This step verifies continuity of the GMS desired gas circuit.
-
This step verifies continuity of the GMS actual gas circuit.
Step | Action | Value(s) | Yes | No | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
1 | Was the Alternative Fuels (AF) Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | Go to Step 2 | Go to Alternative Fuels (AF) Powertrain On Board (OBD) System Check | ||||||
2 |
Does the DTC set and the MIL illuminate? | -- | Go to Step 3 | Go to Step 14 | ||||||
Inspect the CNG manual shut-off valve (MSV). Is the MSV in the ON position? | -- | Go to Step 5 | Go to Step 4 | |||||||
4 | Place the manual shut-off valve (MSV) in the ON position. Is the action complete? | -- | Go to Step 22 | -- | ||||||
Perform the AF Fuel System Diagnosis. Refer to Fuel System Diagnosis . Was a problem found and repaired? | -- | Go to Step 22 | Go to Step 6 | |||||||
6 |
Is the voltage within the specified range? | 4.0-6.0 V | Go to Step 10 | Go to Step 7 | ||||||
7 |
Was a problem found? | -- | Go to Step 19 | Go to Step 8 | ||||||
Is the voltage within the specified range? | 11-13 V | Go to Step 9 | Go to Step 19 | |||||||
9 |
Is there continuity? | -- | Go to Step 16 | Go to Step 19 | ||||||
Is the voltage greater than the specified value? | 7.0 V | Go to Step 11 | Go to Step 13 | |||||||
11 |
Is the maximum recorded frequency greater than the specified value? | 750 Hz | Go to Step 12 | Go to Step 18 | ||||||
12 |
Is the maximum recorded frequency greater than the specified value? | 250 Hz | Go to Step 14 | Go to Step 15 | ||||||
13 |
Was a problem found? | -- | Go to Step 19 | Go to Step 18 | ||||||
14 | The DTC is intermittent. If no additional DTCs are stored, refer to Diagnostic Aids. If additional DTCs are stored, refer to those tables first. Are any additional DTCs stored? | -- | Go to the applicable DTC table | -- | ||||||
15 |
Is there continuity? | -- | Go to Step 16 | Go to Step 19 | ||||||
16 | Inspect for continuity of the engine sync. baro. circuit between C010 and C002 using a DVOM. Is there continuity? | -- | Go to Step 17 | Go to Step 19 | ||||||
17 | Inspect the GMS connector and pins for loose or corroded connections. Was a problem found? | -- | Go to Step 19 | Go to Step 20 | ||||||
18 | Inspect the ECU connector and pins for loose or corroded connections. Was a problem found? | -- | Go to Step 19 | Go to Step 21 | ||||||
19 | Repair circuit as necessary. Refer to Wiring Repairs in Engine Electrical in the C/K Truck Service Manual. Is the action complete? | -- | Go to Step 22 | -- | ||||||
20 | Replace the GMS/MCV assembly. Refer to Gas Mass Sensor/Mixture Control Valve and Mounting Bracket Replacement . Is the action complete? | -- | Go to Step 22 | -- | ||||||
21 | Replace the ECU. Refer to Engine Control Unit Replacement . Is the action complete? | -- | Go to Step 22 | -- | ||||||
22 |
Does the MIL illuminate? | -- | Go to Step 3 | Go to Step 23 | ||||||
23 | Are any ECU or VCM DTCs displayed that have not been diagnosed? | -- | Go to the applicable DTC table | System OK |
