Malfunction Indicator Lamp Mil Description; Operation - Jeep Wrangler 2001 Service Manual

8Js - 26 INSTRUMENT CLUSTER
LOW FUEL INDICATOR (Continued)
• Less Than Empty Percent Tank Full Mes-
sage - Each time the cluster receives a message from
the PCM indicating the percent tank full is less than
empty, the low fuel indicator is illuminated immedi-
ately. This message would indicate that the fuel tank
sender input to the PCM is a short circuit.
• More Than Full Percent Tank Full Message
- Each time the cluster receives a message from the
PCM indicating the percent tank full is more than
full, the low fuel indicator is illuminated immedi-
ately. This message would indicate that the fuel tank
sender input to the PCM is an open circuit.
• Actuator Test - Each time the cluster is put
through the actuator test, the low fuel indicator will
be turned on, then off again during the bulb check
portion of the test to confirm the functionality of the
LED and the cluster control circuitry.
The PCM continually monitors the fuel tank send-
ing unit, then sends the proper messages to the
instrument cluster. For further diagnosis of the low
fuel indicator lamp or the instrument cluster cir-
cuitry that controls the LED, (Refer to 8 - ELECTRI-
CAL/INSTRUMENT CLUSTER - DIAGNOSIS AND
TESTING). For proper diagnosis of the fuel tank
sending unit, the PCM, the PCI data bus, or the mes-
sage inputs to the instrument cluster that control the
low fuel indicator, a DRBIII
Refer to the appropriate diagnostic information.
MALFUNCTION INDICATOR
LAMP MIL
DESCRIPTION
A Malfunction Indicator Lamp (MIL) is standard
equipment on all instrument clusters. The MIL is
located near the lower edge of the instrument cluster
overlay, to the right of the odometer/trip odometer
Vacuum-Fluorescent Display (VFD). The MIL con-
sists of a stencil-like cutout of the International Con-
trol and Display Symbol icon for "Engine" in the
opaque layer of the instrument cluster overlay. The
dark outer layer of the overlay prevents the indicator
from being clearly visible when it is not illuminated.
An amber lens behind the cutout in the opaque layer
of the overlay causes the icon to appear in amber
through the translucent outer layer of the overlay
when it is illuminated from behind by a Light Emit-
ting Diode (LED) soldered onto the instrument clus-
ter electronic circuit board. The MIL is serviced as a
unit with the instrument cluster.

OPERATION

The Malfunction Indicator Lamp (MIL) gives an
indication to the vehicle operator when the Power-
train Control Module (PCM) has recorded a Diagnos-
2001 JEEP WRANGLER
scan tool is required.
tic Trouble Code (DTC) for an On-Board Diagnostics
II (OBDII) emissions-related circuit or component
malfunction. The MIL is controlled by a transistor on
the instrument cluster electronic circuit board based
upon cluster programming and electronic messages
received by the cluster from the PCM over the Pro-
grammable Communications Interface (PCI) data
bus. The MIL Light Emitting Diode (LED) receives
battery current on the instrument cluster electronic
circuit board through the fused ignition switch out-
put (run-start) circuit whenever the ignition switch is
in the On or Start positions; therefore, the LED will
always be off when the ignition switch is in any posi-
tion except On or Start. The LED only illuminates
when it is provided a path to ground by the instru-
ment cluster transistor. The instrument cluster will
turn on the MIL for the following reasons:
• Bulb Test - Each time the ignition switch is
turned to the On position the indicator is illuminated
for about seven seconds as a bulb test.
• PCM Lamp-On Message - Each time the clus-
ter receives a lamp-on message from the PCM, the
indicator will be illuminated. The indicator can be
flashed on and off, or illuminated solid, as dictated
by the PCM message. For some DTC's, if a problem
does not recur, the PCM will send a lamp-off message
automatically. Other DTC's may require that a fault
be repaired and the PCM be reset before a lamp-off
message will be sent. For more information on the
PCM and the DTC set and reset parameters, (Refer
to 25 - EMISSIONS CONTROL - OPERATION).
• Communication Error - If the cluster receives
no lamp-on or lamp-off message from the PCM for
twenty seconds, the MIL is illuminated by the instru-
ment cluster and a "no BuS" message will appear in
the odometer/trip odometer Vacuum Fluorescent Dis-
play (VFD) to indicate a loss of bus communication.
The indicator remains controlled and illuminated by
the cluster until a valid lamp-on or lamp-off message
is received from the PCM.
• Actuator Test - Each time the cluster is put
through the actuator test, the MIL will be turned on,
then off again during the bulb check portion of the
test to confirm the functionality of the LED and the
cluster control circuitry.
The PCM continually monitors the fuel and emis-
sions system circuits and sensors to decide whether
the system is in good operating condition. The PCM
then sends the proper lamp-on or lamp-off messages
to the instrument cluster. For further diagnosis of
the MIL or the instrument cluster circuitry that con-
trols the LED, (Refer to 8 - ELECTRICAL/INSTRU-
MENT CLUSTER - DIAGNOSIS AND TESTING). If
the instrument cluster turns on the MIL after the
bulb test, it may indicate that a malfunction has
occurred and that the fuel and emissions system may
TJ