System Description

Fig 1 16 -- Way Data Link (Diagnostic) Connector:

SYSTEM DESCRIPTION
The Jeep/Truck Engine Controller (JTEC) monitors many different circuits in the fuel injection, ignition, emission and engine systems. If the JTEC senses a problem with a monitored circuit often enough to indicate an actual problem, it stores a Diagnostic Trouble Code (DTC) in the JTEC's memory. If the problem is repaired or ceases to exist, the JTEC 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 JTEC stores a DTC in memory The criteria may be a specific range of engine RPM, engine temperature and/or input voltage to the JTEC.

The JTEC 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 JTEC 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 JTEC). Because the condition happens at an engine speed above the maximum threshold (2000 rpm), the JTEC will not store a DTC.

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

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 (Fig. 1) to erase all DTC's and extinguish the MIL.

Technicians can display stored DTC's by two different methods. Refer to Diagnostic Trouble Code Descriptions.

MALFUNCTION INDICATOR LAMP (MIL)
As a functional test, the MIL (check engine) illuminates at key-on before engine cranking. Whenever the Jeep/Truck Engine Controller (JTEC) sets a Diagnostic Trouble Code (DTC) that affects vehicle emissions, it illuminates the MIL. If a problem is detected, the JTEC sends a message to the instrument cluster to illuminate the lamp. The JTEC 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 JTEC has entered a Limp-In mode or identified a failed emission component. Refer to Trouble Code Descriptions.

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

Additionally, the JTEC may reset (turn off) the MIL when one of the following occur:
- JTEC does not detect the malfunction for 3 consecutive trips (except misfire and Fuel system Monitors).
- JTEC does not detect a malfunction while performing three successive engine misfire or fuel system tests. The JTEC 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.