Description And Operation; Vacuum Reservoir; Vehicle Speed Sensor; Diagnosis And Testing - Dodge ram truck 3500 1996 Service Manual

8H - 2 VEHICLE SPEED CONTROL SYSTEM
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DESCRIPTION AND OPERATION (Continued)
SERVO CAILE
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
The speed control electronic control circuitry is
integrated Into the Powertrain Control Module
(PCM). The PCM Is located In the right-rear area of
the engine compartment. The PCM speed control
functions are monitored by the On-Board Diagnostics
(OBD). All OBD-sensed systems are monitored by the
PCM. Each monitored circuit is assigned a Diagnos­
tic Trouble Code (DTC). The PCM will store a DTC in
electronic memory for certain failures it detects. See
On-Board Diagnostic Tests in this group for more
information. The PCM cannot be repaired and must
be replaced i f faulty.
VACUUM RESERVOIR
The vacuum reservoir is mounted under the cowl
intake air screen in the cowl plenum. The reservoir
contains a one-way check valve to trap engine vac­
uum in the reservoir. When engine vacuum drops, as
in climbing a grade while driving, the reservoir sup­
plies the vacuum needed to maintain proper speed
control operation. The vacuum reservoir cannot' be
repaired and must be replaced i f 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
to the speedometer/odometer is monitored by the
PCM speed control circuitry to determine vehicle
speed and to maintain speed control set speed. Refer
to the appropriate Powertrain Diagnostic Procedures
manual for diagnosis and testing of this component.
Refer to Group 14, Fuel System for removal/installa­
tion procedures.
D I A G N O S I S A N D T E S T I N G
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 i n 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.
I f a road test verifies a system problem and the
speedometer operates properly, check for:
• A mis adjusted 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.
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
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.
I f the OBD system senses that a monitored circuit
is bad, i t 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 memory after 50 engine starts i f the
problem does not occur again.
DIAGNOSTIC TROUBLE CODES
Diagnostic Trouble Codes (DTCs) are two-digit
numbers flashed on the Malfunction Indicator (Check
Engine) Lamp that identify which circuit is bad. A
DTC description can also be read using the DRB scan
tool. Refer to Group 25, Emission Control System for
more DTC information.
A DTC does not identify which component in a cir­
cuit is bad. Thus, a DTC should be treated as a
symptom, not as the cause for the problem. In some
cases, because of the design of the diagnostic test
procedure, a DTC can be the reason for another DTC
to be set. Therefore, i t is important that the test pro­
cedures be followed in sequence, to understand what
caused a DTC to be set.
Refer to the following Speed Control Diagnostic
Trouble Code chart for DTCs which apply to the