General Information; Description And Operation; Powertrain Control Module (Pcm); Servo Cable - Jeep 1997 Grand Cherokee Service Manual

8H - 2 VEHICLE SPEED CONTROL SYSTEM
DESCRIPTION AND OPERATION (Continued)

SERVO CABLE

The speed control servo cable is connected between
the speed control vacuum servo diaphragm and the
throttle body control linkage. This cable causes the
throttle control linkage to open or close the throttle
valve in response to movement of the vacuum servo
diaphragm.

POWERTRAIN CONTROL MODULE (PCM)

The speed control electronic control circuitry is
integrated into the Powertrain Control Module
(PCM). The PCM is located in the engine compart-
ment behind the coolant recovery tank. The PCM
speed control functions are monitored by the
On-Board Diagnostics (OBD). All OBD-sensed sys-
tems are monitored by the PCM. Each monitored cir-
cuit is assigned a Diagnostic Trouble Code (DTC).
The PCM will store a DTC in electronic memory for
certain failures it detects. See On-Board Diagnostic
Test For Speed Control System in this group for more
information. The PCM cannot be repaired and must
be replaced if faulty.
VACUUM RESERVOIR
The vacuum reservoir is mounted below the bat-
tery tray. The reservoir contains a one-way check
valve to trap engine vacuum in the reservoir. When
engine vacuum drops, as in climbing a grade while
driving, the reservoir supplies the vacuum needed to
maintain proper speed control operation. The vacuum
reservoir cannot be repaired and must be replaced if
faulty.
VEHICLE SPEED SENSOR
The Vehicle Speed Sensor (VSS) is a pulse genera-
tor mounted to an adapter near the transmission out-
put shaft. The sensor is driven through the adapter
by a speedometer pinion gear. The VSS pulse signal
is monitored by the PCM speed control circuitry to
determine vehicle speed and to maintain speed con-
trol set speed. Refer to the appropriate Powertrain
Diagnostic Procedures manual for diagnosis and test-
ing of this component. Refer to Group 14, Fuel Sys-
tem for removal/installation procedures.

DIAGNOSIS AND TESTING

ROAD TEST
Perform a vehicle road test to verify reports of
speed control system malfunction. The road test
should include attention to the speedometer. Speed-
ometer operation should be smooth and without flut-
ter at all speeds.
Flutter in the speedometer indicates a problem
which might cause surging in the speed control sys-
tem. The cause of any speedometer problems should
be corrected before proceeding. Refer to Group 8E,
Instrument Panel and Gauges for speedometer diag-
nosis.
If a road test verifies a system problem and the
speedometer operates properly, check for:
• A Diagnostic Trouble Code (DTC). If a flash
lamp code 15, 34 or 77 exists at the Check Engine
Lamp (MIL), conduct tests per the Powertrain Diag-
nostic Procedures service manual.
• A misadjusted brake (stop) lamp switch. This
could also cause an intermittent problem.
• Loose or corroded electrical connections at the
servo. Corrosion should be removed from electrical
terminals and a light coating of Mopar MultiPurpose
Grease, or equivalent, applied.
• Loose or leaking vacuum hoses or connections.
• Secure attachment of both ends of the speed con-
trol servo cable.
• Smooth operation of throttle linkage and throttle
body air valve.
• Failed speed control servo. Do the servo vacuum
test.
CAUTION: When test probing for voltage or conti-
nuity at electrical connectors, care must be taken
not to damage connector, terminals or seals. If
these components are damaged, intermittent or
complete system failure may occur.

ON-BOARD DIAGNOSTICS TEST FOR SPEED

CONTROL SYSTEM

GENERAL INFORMATION

The Powertrain Control Module (PCM) monitors
critical input and output circuits of the speed control
system, making sure they are operational. A Diagnos-
tic Trouble Code (DTC) is assigned to each input and
output circuit monitored by the On-Board Diagnostic
(OBD) system. Some circuits are checked continu-
ously and some are checked only under certain con-
ditions.
If the OBD system senses that a monitored circuit
is bad, it will put a DTC into electronic memory. The
DTC will stay in electronic memory as long as the
circuit continues to be bad. The PCM is programmed
to clear the DTC's from memory after 40 engine war-
m–up cycles if the problem does not occur again. The
DRB scan tool may also be used to erase a DTC.
Diagnostic trouble codes are the results of a
system or circuit failure, but do not directly
identify the failed component or components.
DIAGNOSTIC TROUBLE CODES
The technician can display a DTC in three differ-
ent ways:
ZG