System Description

All vehicles are equipped with either a 3.9L (V-6) engine, a 5.2L (V-8) engine, two different 5.9L (V-8) engines, two different 8.0L (V-10) engines, or a 5.9L Cummins in-line 6-cylinder, four-valve-per-cylinder diesel engine.

The 3.9L (V-6) and 5.2L (V-8) engines will be referred to as: LDC (Light Duty Emission Cycle) engines.
- The 5.9L (V-8) gas powered engine will be referred to as either: LDC (Light Duty Emission Cycle) or HDC (Heavy Duty Emission Cycle) engine.
- The 8.0L (V-10) engine will be referred to as either: MDC (Medium Duty Emission Cycle) or HDC (Heavy Duty Emission Cycle) engine.
- The diesel engine will be referred to as: HDC (Heavy Duty Emission Cycle) engine.

Either of the HDC gas powered engines can be easily identified by the use of an engine mounted air injection pump. The 3.9L5.2L/5.9L LDC gas engines, the 8.0L MDC V-10 engine or the diesel engine will not use an air injection pump.

Maintenance requirements for LDC, MDC and HDC emission systems differ because of different load and operating conditions.

SYSTEM DESCRIPTION
The Powertrain Control Module (PCM) monitors many different circuits in the fuel injection, ignition, emission and engine systems. If the PCM senses a problem with a monitored circuit often enough to indicate an actual problem, it stores a Diagnostic Trouble Code (DTC) in the PCM's memory. If the problem is repaired or ceases to exist, the PCM cancels the code after 40 warm-up cycles. Diagnostic trouble codes that affect vehicle emissions illuminate the Malfunction Indicator (check engine) Lamp. Refer to Malfunction Indicator Lamp.

Certain criteria must be met before the PCM stores a DTC in memory. The criteria may be a specific range of engine RPM, engine temperature, and/or input voltage to the PCM.

The PCM might not store a DTC for a monitored circuit even though a malfunction has occurred. This may happen because one of the DTC criteria for the circuit has not been met. For example, assume the diagnostic trouble code criteria requires the PCM to monitor the circuit only when the engine operates between 750 and 2000 RPM. Suppose the sensor's output circuit shorts to ground when engine operates above 2400 RPM (resulting in 0 volt input to the PCM). Because the condition happens at an engine speed above the maximum threshold (2000 rpm), the PCM will not store a DTC.

16-Way Data Link (Diagnostic) Connector Location:

There are several operating conditions for which the PCM monitors and sets DTC's. Refer to Monitored Systems, Components, and Non-Monitored Circuits.

Technicians must retrieve stored DTC's by connecting the DRB scan tool (or an equivalent scan tool) to the 16-way data link connector. Refer to Diagnostic Trouble Codes.

NOTE: Various diagnostic procedures may actually cause a diagnostic monitor to set a DTC. For instance, pulling a spark plug wire to perform a spark test may set the misfire code. When a repair is completed and verified, connect the DRB scan tool to the 16-way data link connector to erase all DTC's and extinguish the MIL (Check Engine Lamp).

MALFUNCTION INDICATOR LAMP (MIL)
As a functional test, the MIL (check engine) illuminates at key-on before engine cranking. Whenever the Powertrain Control Module (PCM) sets a Diagnostic Trouble Code (DTC) that affects vehicle emissions, it illuminates the MIL If a problem is detected, the PCM sends a message to the instrument cluster to illuminate the lamp. The PCM illuminates the MIL only for DTC's that affect vehicle emissions. There are some monitors that may take two consecutive trips, with a detected fault before the MIL is illuminated. The MIL stays on continuously when the PCM has entered a Limp-In mode or identified a failed emission component. Refer to the Diagnostic Trouble Code charts for emission related codes.

Also, the MIL either flashes or illuminates continuously when the PCM detects active engine misfire. Refer to Misfire Monitoring.

Additionally, the PCM may reset (turn off) the MIL when one of the following occur:
- PCM does not detect the malfunction for 3 consecutive trips (except misfire and Fuel system Monitors).
- PCM does not detect a malfunction while performing three successive engine misfire or fuel system tests. The PCM performs these tests while the engine is operating within ± 375 RPM of and within 10% of the load of the operating condition at which the malfunction was first detected.

SERVICE REMINDER INDICATOR (SRI) LAMP
The Service Reminder Indicator (SRI) lamp is used with the 5.9L HDC V-8 gasoline powered engine only. The lamp is displayed on the instrument panel as the MAINT REQ'D lamp.

The SRI system is incorporated into the powertrain control module (PCM). The PCM records the vehicles mileage and stores it into memory. At that time, the PCM checks for certain mileage trip points. When the current mileage matches one of the trip points, the SRI lamp is activated.

Certain parts are to be replaced, or certain maintenance must be performed at either an indicated mileage or when the SRI lamp remains on when the key is in the ON position. After performing the part replacement or required maintenance, the SRI lamp must be reset to turn the lamp off. Use the DRB scan tool to reset the SRI lamp.

Failure to perform the part replacement or required maintenance and only reset the SRI lamp may be a violation of federal law. Only after performing the part replacement or required maintenance, should the SRI lamp be reset.

STATE DISPLAY TEST MODE
The switch inputs to the Powertrain Control Module (PCM) have two recognized states; HIGH and LOW. For this reason, the PCM cannot recognize the difference between a selected switch position versus an open circuit, a short circuit, or a defective switch. If the State Display screen shows the change from HIGH to LOW or LOW to HIGH, assume the entire switch circuit to the PCM functions properly. Connect the DRB scan tool to the data link connector and access the state display screen. Then access either State Display Inputs and Outputs or State Display Sensors.

CIRCUIT ACTUATION TEST MODE
The Circuit Actuation Test Mode checks for proper operation of output circuits or devices the Powertrain Control Module (PCM) may not internally recognize. The PCM attempts to activate these outputs and allow an observer to verify proper operation. Most of the tests provide an audible or visual indication of device operation (click of relay contacts, fuel spray, etc.). Except for intermittent conditions, if a device functions properly during testing, assume the device, its associated wiring, and driver circuit work correctly. Connect the DRB scan tool to the data link connector and access the Actuators screen.