Diagnostic System Check

DESCRIPTION
The Diagnostic System Check is an organized approach to identifying a condition that is created by a malfunction in the powertrain 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.

DIAGNOSTIC AIDS

IMPORTANT: System voltage must remain between 9-16 volts for serial data communication.

When performing diagnostic procedures in the J Platform service manual always review the wiring schematics within the Bi-Fuel Supplement in order to determine if an Alternative Fuels circuit or component may be effecting the vehicle.

Always store control module (AF ECU, 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.

TEST DESCRIPTION

Steps 1-9:

Steps 10-15:

The number(s) below refer to the step number(s) on the diagnostic table.
1. This step inspects for proper initial MIL operation.
2. 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 two ground circuits (terminals 4 and 5). All three circuits are required for proper scan tool operation.
4. Both control modules simultaneously test certain DTCs regardless of the fuel system that is in operation.
5. Checking for a shorted MIL circuit and proper control module operation of the MIL circuit.
7. The Fuel Indicator Lamp (FIL) located within the fuel gauge select switch button will illuminate when the engine is operating on gasoline.

In addition the NGO Enable Output scan tool parameter will indicate the AF ECU's commanded state of the NGO Enable circuit while the NGO Enable Input parameter will indicate the gasoline control modules monitored state of the NGO Enable circuit. Regardless of the fuel system in operation these two data parameters will always indicate the same data unless an NGO Enable circuit fault is present.
8. 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.
10. Check for applicable bulletins before proceeding with DTC diagnosis.

After determining which control module has stored DTCs, diagnose DTCs in the following order:
^ PCM MEMORY or INTERNAL FAULT DTCs (PCM Memory, PCM Not Programmed, etc.)
^ SYSTEM and IGNITION VOLTAGE DTCs (System Voltage High or Low, etc.)
^ COMPONENT / CIRCUIT DTCs (TP Sensor Circuit Voltage High or Low, Oxygen Sensor Circuit Insufficient Activity, etc.)
^ SYSTEM DTCs (MAP System Performance, TP Sensor Performance, etc.)