Circuit Description
The Gas Mass Sensor (GMS) and the Mixture Control Valve (MCV) are contained in one non-serviceable assembly. The GMS/MCV is supplied ignition voltage and a ground. The GMS monitors the mass and flow of the gaseous fuel entering the engine and converts this information into a GMS ACTUAL Gas Flow signal circuit frequency . The AF ECU commands fuel flow by supplying the MCV with a frequency signal. The DESIRED Gas Flow signal circuit frequency varies from around 1050 Hz (0.40 gms) at idle, to around 5000 Hz (18 gms) at wide open throttle.
The AF ECU converts the ACTUAL Gas Flow signal circuit frequency into a grams per second value. During low fuel flow rates (such as at engine idle) the GMS sensor Actual Gas circuit will produce a low frequency signal of around 700 Hz (0.21 gms). During high fuel flow rates (such as at wide open throttle-road load) the GMS sensor will produce a high frequency signal of around 2700 Hz (16.65 gms).
This DTC is used to indicate a GMS ACTUAL Gas Flow signal circuit frequency that is above the normal operating range of the sensor.
Conditions for Running the DTC
Cranking Test
| • | The engine is cranking on alternative fuel. |
| The engine Speed is between 25-500 RPM. |
| • | The battery voltage is greater than 8 volts. |
Running Test
| • | The engine is operating on alternative fuel. |
| • | The engine speed is greater than 500 RPM. |
| • | The engine has been operating for greater than 10 seconds. |
| • | The battery voltage is greater than 10 volts. |
Conditions for Setting the DTC
Cranking and Running Test
The AF ECU monitors a GMS Actual Gas Flow signal circuit frequency of greater than 2900 Hz for at least 0.5 seconds continuously.
Action Taken When the DTC Sets
The AF ECU will switch-over to gasoline fuel operation. The AF ECU will not attempt alternative fuel operation until the next ignition key cycle.
| • | The AF ECU will illuminate the MIL and store the DTC as History during ONE drive trip in which the diagnostic runs and fails. |
| • | The AF ECU will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data. |
Conditions for Clearing the MIL/DTC
| • | The Last Test Failed status will clear when the DTC runs and passes. |
| • | The AF ECU will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes. |
| • | The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC. |
| • | All DTC Information can be cleared using a scan tool. |
| • | Interruption of Control Module (AF ECU / VCM / PCM or ECM) power or ground MAY clear DTC Information. Clearing DTC Information with this method is inconsistent and should not be performed. |
Diagnostic Aids
Important: The Desired Gas and Actual Gas circuits only output a frequency signal when the engine is cranking or running on CNG. No frequency SIGNAL is present with the ignition ON and the engine OFF or during any time the vehicle is operating in the gasoline mode.
Intermittent Condition
A fault not present indicates the condition that caused the DTC to set is intermittent and not currently present. Refer to Intermittent Conditions diagnostic information located in Symptoms-Engine Controls. If any additional DTCs were stored, proceed to the applicable DTC table in the order stated in the Diagnostic System Check - Alternative Fuels (Engine Controls) .
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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This step determines if the high frequency condition is present.
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This step determines if the proper voltage is available to the GMS/MCV.
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This step determines if the proper ground is available to the GMS/MCV.
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This step determines if the high frequency is being induced by a source other than the GMS/MCV.
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This step determines if excessively high CNG fuel pressure caused the GMS/MCV to produce a higher than normal frequency.
Step | Action | Value(s) | Yes | No | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Alternative Fuels Engine Controls Diagnostic System Check? | -- | Go to Step 2 | Go to Diagnostic System Check - Alternative Fuels (Engine Controls) | ||||||
Observe the data parameters that give an indication of how the vehicle was being operated when this DTC set. Does the scan tool indicate the diagnostic Passed? | -- | Go to Step 3 | Go to Step 4 | |||||||
3 | The fault is not present. Refer to Diagnostic Aids. Are any DTCs stored that have not been diagnosed? | -- | Go to Intermittent Conditions | |||||||
Is the voltage WITHIN the value range specified? | 11-13 V | Go to Step 5 | Go to Step 9 | |||||||
Connect the J 35616-200 Unpowered Test Light between BATTERY POSITIVE and the two GMS/MCV ground circuit terminals one at a time. Does the test lamp illuminate brightly when connected to each ground circuit terminal? | -- | Go to Step 6 | Go to Step 10 | |||||||
Inspect the vehicle for the following conditions:
Was an induced high frequency source found and corrected? | -- | Go to Step 12 | Go to Step 7 | |||||||
Perform the AF Fuel System diagnosis and return to this step. Refer to Fuel System Diagnosis . Was a CNG fuel system problem found and repaired? | -- | Go to Step 12 | Go to Step 8 | |||||||
8 | Replace the GMS/MCV. Refer to Gas Mass Sensor/Mixture Control Valve Replacement . Is the replacement complete? | -- | Go to Step 12 | -- | ||||||
9 |
Did the FIL illuminate for a 2 second bulb check or stay ON? | -- | Go to Step 11 | Go to Ignition Relay Diagnosis | ||||||
10 | Repair the GMS/MCV ground circuit with high or infinite (open) resistance. Is the repair complete? | -- | Go to Step 12 | -- | ||||||
11 | Repair the open ignition circuit between the GMS/MCV connector and the ignition circuit splice. Is the repair complete? | -- | Go to Step 12 | -- | ||||||
12 |
Observe the data parameters that give an indication of how the vehicle was being operated when this DTC set. Does the scan tool indicate the diagnostic Passed? | -- | System OK | Go to Step 4 |
