Jeep 1997 Grand Cherokee Service Manual page 237

HEADLAMP SWITCH DATA (LEFT SIDE
ZG
DIAGNOSIS AND TESTING (Continued)
If the problem is sensed in a monitored circuit
often enough to indicate an actual problem, a DTC is
stored. The DTC will be stored in the PCM memory
for eventual display to the service technician. If the
problem is repaired or ceases to exist, the PCM can-
cels the DTC after 51 engine starts.
Certain criteria must be met for a DTC to be
entered into PCM memory. The criteria may be a
specific range of engine rpm, engine temperature
and/or input voltage to the PCM.
A DTC indicates that the PCM has recognized an
abnormal signal in a circuit or the system. A DTC
may indicate the result of a failure, but never iden-
tify the failed component directly.
It is possible that a DTC for a monitored circuit
may not be entered into memory even though a mal-
function has occurred. Refer to On- Board Diagnos-
tics (OBD) in Group 25, Emission Control Systems
for additional information.
ACCESSING DIAGNOSTIC TROUBLE CODES
A stored Diagnostic Trouble Code (DTC) can be dis-
played by cycling the ignition key On-Off-On-Off-On
within three seconds and observing the malfunction
indicator lamp. This lamp is displayed on the instru-
ment panel as the CHECK ENGINE lamp (Fig. 14).
They can also be displayed through the use of the
Diagnostic Readout Box (DRB) scan tool. The DRB
connects to the data link connector, left of the steer-
ing column above the brake pedal (Fig. 15). For oper-
ation of the DRB, refer to the appropriate Powertrain
Diagnostic Procedures service manual.
Fig. 15 Data Link Connector Location
EXAMPLES:
• If the lamp (Fig. 14) flashes 1 time, pauses and
flashes 2 more times, a flashing Diagnostic Trouble
Code (DTC) number 12 is indicated. If this code is
ABOVE BRAKE PEDAL) LINK OF CONNECTOR COLUMN
observed, it is indicating that the battery has been
disconnected within the last 50 key-on cycles. It
could also indicate that battery voltage has been dis-
connected to the PCM. In either case, other DTC's
may have been erased.
• If the lamp flashes 1 time, pauses and flashes 7
more times, a flashing Diagnostic Trouble Code
(DTC) number 17 is indicated.
After any stored DTC information has been
observed, the display will end with a flashing DTC
number 55. This will indicate the end of all stored
information.
ERASING TROUBLE CODES
After the problem has been repaired, use the DRB
scan tool to erase a DTC. Refer to the appropriate
Powertrain Diagnostic Procedures service manual for
operation of the DRB scan tool.
DRB SCAN TOOL
For operation of the DRB scan tool, refer to the
appropriate Powertrain Diagnostic Procedures ser-
vice manual.
WATER PUMP TESTS
LOOSE IMPELLER
DO NOT WASTE reusable coolant. If solution is
clean, drain coolant into a clean container for reuse.
WARNING: DO NOT REMOVE THE CYLINDER
BLOCK DRAIN PLUGS OR LOOSEN THE RADIATOR
DRAINCOCK WITH THE SYSTEM HOT AND UNDER
PRESSURE. SERIOUS BURNS FROM THE COOL-
ANT CAN OCCUR.
(1) Drain the cooling system. Refer to Draining
Cooling System in this group.
(2) Loosen the fan belt. Refer to Belt Service in the
Engine Accessory Drive Belt section of this group.
(3) Disconnect the lower radiator hose from the
water pump.
(4) Bend a stiff welding rod or similar device as
shown in (Fig. 16). To prevent breakage of rod, min-
imum thickness should be 3/16 inch (.187 inches).
(5) Position the rod in the water pump inlet and
attempt to hold the impeller while turning the fan
pulley. If equipped with a thermal viscous fan drive,
rotate the water pump shaft with a wrench attached
to one of the fan pulley mounting nuts. If the impel-
ler is loose and can be held with the rod while the
fan blades are turning, the pump is defective. Do not
use excessive force when rotating pump shaft. If the
impeller turns, the pump is OK.
(6) Connect the hose and install the coolant, or
proceed with repairs.
COOLING SYSTEM 7 - 9