Grand Cherokee 2WD V8-5.2L VIN Y (1998)
Malfunction Indicator Lamp: Description and Operation
SYSTEM OPERATION
The Check Engine or Malfunction Indicator Lamp (MIL) gives an indication when the Powertrain Control Module (PCM) has recorded a
Diagnostic Trouble Code (DTC) for an On-Board Diagnostics II (OBDII) emissions-related circuit or component malfunction. The lamp is turned
on by the instrument cluster circuitry for about three seconds when the ignition switch is turned to the On position as a bulb test. After the bulb
test, the lamp is controlled by the instrument cluster circuitry based upon a message received from the PCM on the Chrysler Collision Detection
(CCD) data bus.
The PCM uses inputs from many emissions-related circuits and sensors, along with its internal programming, to decide whether a condition exists
that requires the MIL lamp to be turned on. The PCM then sends the proper message to the instrument cluster on the CCD data bus to turn the
lamp on or off. When the instrument cluster circuitry receives a MIL lamp-on message from the PCM, it sends a chime request message to the
Body Control Module (BCM) on the CCD data bus for a single chime tone to sound.
The MIL lamp can also be used to display a stored DTC by flashing on and off.
As a functional test, the Malfunction Indicator Lamp (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.
Also, the MIL either flashes or illuminates continuously when the PCM detects active engine misfire.
Additionally, the PCM may reset (turn off) the MIL when one of the following occur:
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PCM does not detect the malfunction for 3 consecutive trips (except misfire and Fuel system Monitors).
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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.
