Dodge RAM TRUCK 1500 1995 Service Manual page 455

B R — • WEHICIE SPEED .CONTROL SYSTEM 8H - 1
C O N T E N T S
page page
DIAGNOSIS . . . . . . . . . . . . . . . . . . . . 3 SERWiCE PROCEDURES. . ... 11
GENERAL INFORMATION 1
G E N E R A L I N F O R M A T I O N
The vehicle speed control system (Fig. 1) is an
available option on Ram t r u c k models. The system is
electronically controlled and vacuum operated. Fol­
lowing are general descriptions of the major compo­
nents i n the speed control system. Refer to Group 8W
- W i r i n g Diagrams for complete circuit descriptions
and diagrams.
SPEED CONTROL SERVO
The speed control servo is mounted to a bracket
under the battery t r a y on the left side of the engine
compartment. The servo u n i t consists of a solenoid
valve body, a vacuum servo and the mounting
bracket. The P C M controls the solenoid valve body.
The solenoid valve body controls the application and
release of vacuum to the diaphragm of the vacuum
servo. The servo u n i t cannot be repaired and is ser­
viced only as a complete assembly.
SPEED CONTROL SWITCH
The speed control switch module is mounted to the
center of the steering wheel below the driver's airbag
module. The P C M monitors the state of the speed
control switches. The individual switches are labeled:
OFF, ON CRUISE," RESUME/ACCEL, SET/COAST.
Refer to the owner's manual for more information on
speed control switch functions and setting proce­
dures. The individual switches cannot be repaired. I f
one switch fails, the entire switch module must be re­
placed.
STOP LAMP SWITCH
Vehicles w i t h the speed control option use a dual
function stop lamp switch. The switch is mounted i n
the same location as the conventional stop lamp
switch, on the brake pedal mounting bracket under
the instrument panel. The P C M monitors the state of
the dual function stop lamp switch. Refer to Group 5
- Brakes for more information on stop lamp switch
service and adjustment procedures.
SERVO CABLE
The speed control servo cable is connected between
the speed control vacuum servo diaphragm and the
throttle control linkage. This cable causes the throt­
tle control linkage to open or close the throttle valve
i n response to movement of the vacuum servo dia­
phragm.
POWERTRAIN CONTROL MODULE
The speed control electronic control circuitry is i n ­
tegrated into the Powertrain Control Module (PCM).
The P C M is located i n the engine compartment on
the right side dash panel. The P C M speed control
functions are monitored by the On-Board Diagnostics
(OBD). A l l OBD-sensed systems are monitored by the
P C M . Each monitored circuit is assigned a Diagnos­
tic Trouble Code (DTC). The P C M w i l l store a DTC i n
electronic memory for any failure i t detects. See Us­
ing On-Board Diagnostic System i n this group for
more information. The P C M cannot be repaired and
must be replaced i f faulty.
VACUUM RESERVOIR
The vacuum reservoir is mounted under the cowl
screen i n the r i g h t side cowl plenum. The reservoir
contains a one-way check valve to trap engine vac­
u u m i n the reservoir. When engine vacuum drops, as
i n climbing a grade while driving, the reservoir sup­
plies the vacuum needed to m a i n t a i n proper speed
control operation. Diesel engine models do not use a
vacuum reservoir. 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
(two-wheel drive) or transfer case (four-wheel drive)
output 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 P C M speed control circuitry to determine vehicle
VEHICLE SPEED CONTROL SYSTEM