From my 2001 factory service manual:
Diesel With Manual Trans.
The speed control system is fully electronically controlled
by the Engine Control Module (ECM). A
cable and a vacuum controlled servo are not
used if the vehicle is equipped with a manual
transmission and a diesel engine. This is a servo-
less system. The controls consist of two steering
wheel mounted switches. The switches are labeled:
ON/OFF, RES/ACCEL, SET, COAST, and CANCEL.
The system is designed to operate at speeds above
30 mph (50 km/h).
Diesel With Manual Trans.
The Vehicle Speed Sensor (VSS) is no longer used
for any Dodge Truck.
Vehicle speed and distance covered are measured
by the Rear Wheel Speed Sensor. The sensor is
mounted to the rear axle. A signal is sent from this
sensor to the Controller Antilock Brake (CAB) computer.
A signal is then sent from the CAB to the
Engine Control Module (ECM) to determine vehicle
speed and distance covered. The ECM will then
determine strategies for speed control system operation.
Diesel Engine With Manual Trans.
Vacuum is not used for any part of the speed control
system if equipped with a diesel engine and a
manual transmission.
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.
If a road test verifies a system problem and the
speedometer operates properly, check for:
² A Diagnostic Trouble Code (DTC). If a DTC
exists, conduct tests per the Powertrain Diagnostic
Procedures service manual.
² A misadjusted brake (stop) lamp switch. This
could also cause an intermittent problem.
² Loose, damaged 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.
² Leaking vacuum reservoir.
² Loose or leaking vacuum hoses or connections.
² Defective one-way vacuum check valve.
² Secure attachment of both ends of the speed control
servo cable.
² Smooth operation of throttle linkage and throttle
body air valve.
² Failed speed control servo. Do the servo vacuum
test.
Diesel With Manual Trans.
There are two separate switch pods that operate
the speed control system. The steering-wheelmounted
switches use multiplexed circuits to provide
inputs to the Engine Control Module (ECM) for ON,
OFF, RESUME, ACCELERATE, SET, DECEL and
CANCEL modes. Refer to the owner's manual for
more information on speed control switch functions
and setting procedures.
The individual switches cannot be repaired. If one
switch fails, the entire switch module must be
replaced.
Diesel With Manual Trans.
When speed control is selected by depressing the
ON switch, the Engine Control Module (ECM) allows
a set speed to be stored in ECM RAM for speed control.
To store a set speed, depress the SET switch
while the vehicle is moving at a speed between 35
and 85 mph. In order for the speed control to engage,
the brakes cannot be applied. The speed control can
be disengaged manually by:
² Stepping on the brake pedal
² Depressing the OFF switch
² Depressing the CANCEL switch.
² Depressing the clutch pedal
NOTE: Depressing the OFF switch or turning off the
ignition switch will erase the set speed stored in
the ECM.
For added safety, the speed control system is programmed
to disengage for any of the following conditions:
² A rapid increase of rpm (indication that the
clutch has been disengaged)
² Excessive engine rpm (indicates that the transmission
may be in a low gear)
² The speed signal increases at a rate of 10 mph
per second (indicates that the coefficient of friction
between the road surface and tires is extremely low)
² The speed signal decreases at a rate of 10 mph
per second (indicates that the vehicle may have
decelerated at an extremely high rate)
Once the speed control has been disengaged,
depressing the RES/ACCEL switch (when speed is
greater than 30 mph) restores the vehicle to the target
speed that was stored in the ECM.
While the speed control is engaged, the driver can
increase the vehicle speed by depressing the RES/ACCEL
switch. The new target speed is stored in the
ECM when the RES/ACCEL is released. The ECM also
has a 9tap-up9 feature in which vehicle speed increases
at a rate of approximately 2 mph for each momentary
switch activation of the RES/ACCEL switch.
A “tap down” feature is used to decelerate without
disengaging the speed control system. To decelerate
from an existing recorded target speed, momentarily
depress the COAST switch. For each switch activation,
speed will be lowered approximately 1 mph.
Diesel Engine With Manual Trans.
Vacuum is not used for any part of the speed control
system if equipped with a diesel engine and a
manual transmission.
I cut and pasted all the sections from the manual for Diesel engines with manual trans. I hope it halfway makes sense.
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