Jeep 1996 Grand Cherokee Service Manual - page 1439
detected, the PCM sends a message to the instru-
ment cluster to illuminate the lamp. The PCM illu-
minates 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 continu-
ously when the PCM has entered a Limp-In mode or
identified a failed emission component. Refer to the
Diagnostic Trouble Code charts in this group for
emission related codes.
Also, the MIL either flashes or illuminates contin-
uously when the PCM detects active engine misfire.
Refer to Misfire Monitoring in this section.
Additionally, the PCM may reset (turn off) the MIL
when one of the following occur:
• PCM does not detect the malfunction for 3 con-
secutive trips (except misfire and Fuel system Moni-
tors).
• PCM does not detect a malfunction while per-
forming three successive engine misfire or fuel sys-
tem tests. The PCM performs these tests while the
engine is operating within
6 375 RPM of and within
10 % of the load of the operating condition at which
the malfunction was first detected.
STATE DISPLAY TEST MODE
The switch inputs to the Powertrain Control Mod-
ule (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 cor-
rectly. Connect the DRB scan tool to the data link
connector and access the Actuators screen.
DIAGNOSTIC TROUBLE CODES
A Diagnostic Trouble Code (DTC) indicates the
PCM has recognized an abnormal condition in the
system.
The technician can display a DTC in three differ-
ent ways:
• a two-digit number flashed on the Malfunction
Indicator (Check Engine) Lamp
• a two-digit number displayed on the vehicle
odometer
• a description of the DTC can be read using the
DRB scan tool
Diagnostic trouble codes are the results of a system
or circuit failure, but do not directly identify the
failed component or components.
NOTE: For a list of DTC’s, refer to the charts in this
section.
OBTAINING DIAGNOSTIC TROUBLE CODES
USING DRB SCAN TOOL
WARNING:
APPLY
PARKING
BRAKE
AND/OR
BLOCK WHEELS BEFORE PERFORMING ANY TEST
ON AN OPERATING ENGINE.
(1) Connect DRB scan tool to the data link (diag-
nostic) connector located in the passengers compart-
ment, below the center of instrument cluster on the
drivers side (Fig. 1).
(2) Turn the ignition switch on, access Read Fault
Screen. Record all the DTC’s shown on the DRB scan
tool.
Observe
the
malfunction
indicator
(check
engine) lamp on the instrument panel. The lamp
should light for 2 seconds then go out (bulb check).
(3) To erase DTC’s, use the Erase Trouble Code
data screen on the DRB scan tool.
Fig. 1 Data Link (Diagnostic) Connector Location
25 - 2
EMISSION CONTROL SYSTEMS
ZJ
DESCRIPTION AND OPERATION (Continued)