The Diagnostic System Check-Engine Controls is an organized approach to identifying a condition that is created by a malfunction in the engine control system. The Diagnostic System Check must be the starting point for any driveability concern. The Diagnostic System Check directs the service technician to the next logical step in order to diagnose the concern. Understanding and correctly using the diagnostic table reduces diagnostic time and prevents the replacement of good parts.
Important: System voltage must remain between 9-16 volts for serial data communication.
Always store control module - Alternative Fuels ECM, VCM/PCM or ECM - DTC information into a scan tool before proceeding with diagnosis. The stored control module DTC information may contain data - Freeze Frame or Failure Records - useful for intermittent diagnosis. Clearing DTC information with a scan tool or the loss of control module power or ground will erase the control module DTC information. Some diagnostic procedures call for disconnection of control module connectors and loss of control module stored DTC information may occur.
The numbers below refer to the step numbers on the diagnostic table.
This step inspects for proper initial MIL operation.
If the scan tool fails to turn ON, check scan tool operation on a known good vehicle. The data link connector (DLC) supplies the scan tool with a power circuit - terminal 16 and 2 ground circuits - terminals 4 and 5. All 3 circuits are required for proper scan tool operation.
Both control modules simultaneously test certain DTCs regardless of the fuel system that is in operation.
Checking for a shorted MIL circuit and proper control module operation of the MIL circuit.
The fuel indicator lamp (FIL) located within the fuel gage select switch button will illuminate when the engine is operating on gasoline.
The AFO Signal Command scan tool parameter will indicate the Alternative Fuels ECM's commanded state of the Alternative Fuel Operation circuit. The AFO Signal parameter will indicate the gasoline control modules monitored state of the Alternative Fuel Operation circuit. Regardless of the fuel system in operation these 2 data parameters will always indicate the same data unless an Alternative Fuel Operation circuit fault is present or the gasoline control module is programmed with standard gasoline software.
If actual and typical data values are different and a fault is suspected check the DTC information with a scan tool and verify that the suspected component or system DTCs have ran and passed. If they have passed use the sensor information in Description and Operation to help diagnose suspected sensor faults. If a system fault is suspected - EGR operation, EVAP operation, etc. - refer to the DTC that applies to the system and if possible use the diagnostic table to aid in diagnosis.
DTC P1665 may set if there is an AFO Signal circuit fault or if the PCM is programmed with standard gasoline software.
Check for applicable bulletins before proceeding with DTC diagnosis.
After determining which control module has stored DTCs, diagnose DTCs in the following order:
1. | Control Module MEMORY or INTERNAL FAULT DTCs - PCM Memory, PCM Not Programmed, etc. |
2. | SYSTEM and IGNITION VOLTAGE DTCs - System Voltage High or Low, etc. |
3. | COMPONENT / CIRCUIT DTCs - TP Sensor Circuit Voltage High or Low, Oxygen Sensor Circuit Insufficient Activity, etc. |
4. | SYSTEM DTCs (MAP System Performance, TP Sensor Performance, etc.) |
Step | Action | Yes | No | ||||||
---|---|---|---|---|---|---|---|---|---|
Important: Inspect for applicable service bulletins before proceeding with this test. Ensure BOTH fuel tanks are at least ¼ full.Do not clear DTCs unless instructed by a diagnostic procedure.
Is the MIL on continuously? | Go to Step 2 | ||||||||
Does the scan tool display gasoline control module data? | Go to Step 3 | Go to Scan Tool Does Not Communicate with Class 2 Device in Data Link Communications in the G Van Service Manual | |||||||
3 | Using a scan tool attempt to communicate and retrieve data from the Alternative Fuels ECM. Does the scan tool display alternative fuels data? | Go to Step 4 | Go to Ignition Relay Diagnosis | ||||||
Were any DTCs stored? | Go to Step 10 | Go to Step 5 | |||||||
Command the MIL OFF utilizing the scan tool Engine Output - Gasoline for the MIL. Does the MIL turn OFF? | Go to Step 6 | ||||||||
6 | Crank the engine for at least ten seconds. Does the engine start? | Go to Step 7 | Go to Step 9 | ||||||
Determine which fuel system the engine is operating on by observing the fuel indicator lamp and the scan tool AFO Signal and AFO Signal Command parameters.
Is the engine operating on Alternative Fuel? | Go to Step 8 | ||||||||
Are the values normal or within typical range? | Go to Step 13 | Go to applicable DTC diagnosis | |||||||
9 |
Does the engine start and continue to run with the CNG 20 amp fuse removed? | Go to Ignition Relay Diagnosis | |||||||
Does the PCM have a DTC P1665 stored? | Go to Step 11 | Go to Step 12 | |||||||
11 | Inspect the PCM for the correct BI-FUEL software and calibration. Is the PCM programmed with BI-FUEL software and calibration? | ||||||||
Have all DTCs been diagnosed? | Go to Step 17 | Go to applicable DTC diagnosis | |||||||
13 | Is there a driveability symptom? | Go to Step 14 | Go to Step 15 | ||||||
14 | Perform the fuel system diagnosis and return to this step. Refer to Fuel System Diagnosis . Did the fuel system diagnosis locate and repair a concern? | Go to Step 17 | |||||||
15 | Is there a fuel gage, fuel gage selector switch or fuel indicator lamp operation concern? | Go to Fuel Gauge Relay Diagnosis - Alternative Fuels , Fuel Indicator Lamp Always On - Alternative Fuels or Fuel Indicator Lamp Inoperative - Alternative Fuels | Go to Step 16 | ||||||
16 |
Does the engine start and run on gasoline? | Go to Step 17 | |||||||
17 |
Does the vehicle operate on CNG with normal driveability, no FIL/MIL illumination and no stored DTCs? | System OK | Go to Step 1 |