Cruise Control: Description and Operation
SPEED CONTROL SYSTEMGas Engines and/or Diesel With Automatic Transmission
The speed control system is operated by the use of a cable and a vacuum controlled servo. Electronic control of the speed control system is integrated into the Powertrain Control Module (PCM). 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 kph).
WARNING: THE USE OF SPEED CONTROL IS NOT RECOMMENDED WHEN DRIVING CONDITIONS DO NOT PERMIT MAINTAINING A CONSTANT SPEED, SUCH AS IN HEAVY TRAFFIC OR ON ROADS THAT ARE WINDING, ICY, SNOW COVERED, OR SLIPPERY.
When speed control is selected by depressing the ON switch, the PCM allows a set speed to be stored in PCM 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, nor can the gear selector be indicating the transmission is in Park or Neutral.
The speed control can be disengaged manually by:
- Stepping on the brake pedal.
- Depressing the OFF switch.
- Depressing the CANCEL switch.
- Depressing the clutch pedal (if equipped).
NOTE: Depressing the OFF switch or turning OFF the ignition switch will erase the set speed stored in the PCM.
For added safety, the speed control system is programmed to disengage for any of the following conditions:
- An indication of Park or Neutral.
- A rapid increase of rpm (indication that the clutch has been disengaged).
- Excessive engine rpm (indicates that the transmission may be in a low gear).
- 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).
- 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 PCM.
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 PCM when the RES/ACCEL is released. The PCM also has a tap-up 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 Transmission
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 kph).
WARNING: THE USE OF SPEED CONTROL IS NOT RECOMMENDED WHEN DRIVING CONDITIONS DO NOT PERMIT MAINTAINING A CONSTANT SPEED, SUCH AS IN HEAVY TRAFFIC OR ON ROADS THAT ARE WINDING, ICY, SNOW COVERED, OR SLIPPERY.
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:
- Stepping on the brake pedal.
- Depressing the OFF switch.
- Depressing the CANCEL switch.
- 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:
- A rapid increase of rpm (indication that the clutch has been disengaged).
- Excessive engine rpm (indicates that the transmission may be in a low gear).
- 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).
- 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 tap-up 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.
VEHICLE SPEED INPUT
Gas Engines and/or Diesel With Automatic Transmission
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 Powertrain Control Module (PCM) to determine vehicle speed and distance covered. The PCM will then determine strategies for speed control system operation.
Diesel Engine With Manual Transmission
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.
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.
SPEED CONTROL SERVO
A speed control servo is not used if equipped with both a diesel engine and a manual transmission.
The servo unit consists of a solenoid valve body, and a vacuum chamber. The solenoid valve body contains three solenoids:
- Vacuum
- Vent
- Dump
The vacuum chamber contains a diaphragm with a cable attached to control the throttle linkage.
A speed control servo is not used if equipped with both a diesel engine and a manual transmission.
The Powertrain Control Module (PCM) 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 unit cannot be repaired and is serviced only as a complete assembly.
Power is supplied to the servo's by the PCM through the brake switch. The PCM controls the ground path for the vacuum and vent solenoids.
The dump solenoid is energized anytime it receives power. If power to the dump solenoid is interrupted, the solenoid dumps vacuum in the servo. This provides a safety backup to the vent and vacuum solenoids.
The vacuum and vent solenoids must be grounded at the PCM to operate. When the PCM grounds the vacuum servo solenoid, the solenoid allows vacuum to enter the servo and pull open the throttle plate using the cable. When the PCM breaks the ground, the solenoid closes and no more vacuum is allowed to enter the servo. The PCM also operates the vent solenoid via ground. The vent solenoid opens and closes a passage to bleed or hold vacuum in the servo as required.
The PCM duty cycles the vacuum and vent solenoids to maintain the set speed, or to accelerate and decelerate the vehicle. To increase throttle opening, the PCM grounds the vacuum and vent solenoids. To decrease throttle opening, the PCM removes the grounds from the vacuum and vent solenoids. When the brake is released, if vehicle speed exceeds 30 mph to resume, 35 mph to set, and the RES/ACCEL switch has been depressed, ground for the vent and vacuum circuits is restored.
SWITCH
Gas Engines and Diesel With Automatic Transmission
There are two separate switch pods that operate the speed control system. The steering-wheel-mounted switches use multiplexed circuits to provide inputs to the Powertrain Control Module (PCM) 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.
When speed control is selected by depressing the ON, OFF switch, the Powertrain Control Module (PCM) allows a set speed to be stored in its RAM for speed control. To store a set speed, depress the SET switch while the vehicle is moving at a speed between approximately 35 and 85 mph. In order for the speed control to engage, the brakes cannot be applied, nor can the gear selector be indicating the transmission is in Park or Neutral.
The speed control can be disengaged manually by:
- Stepping on the brake pedal.
- Depressing the OFF switch.
- Depressing the CANCEL switch.
The speed control can be disengaged also by any of the following conditions:
- An indication of Park or Neutral.
- The vehicle speed signal increases at a rate of 10 mph per second (indicates that the co-efficient of friction between the road surface and tires is extremely low).
- Depressing the clutch pedal.
- Excessive engine rpm (indicates that the transmission may be in a low gear).
- The vehicle speed signal decreases at a rate of 10 mph per second (indicates that the vehicle may have decelerated at an extremely high rate).
- If the actual speed is not within 20 mph of the set speed.
- The previous disengagement conditions are programmed for added safety.
- Once the speed control has been disengaged, depressing the ACCEL switch restores the vehicle to the target speed that was stored in the PCM's RAM.
NOTE: Depressing the OFF switch will erase the set speed stored in the PCM's RAM.
If, while the speed control is engaged, the driver wishes to increase vehicle speed, the PCM is programmed for an acceleration feature. With the ACCEL switch held closed, the vehicle accelerates slowly to the desired speed. The new target speed is stored in the PCM's RAM when the ACCEL switch is released. The PCM also has a tap-up feature in which vehicle speed increases at a rate of approximately 2 mph for each momentary switch activation of the ACCEL switch.
The PCM also provides a means to decelerate without disengaging speed control. To decelerate from an existing recorded target speed, depress and hold the COAST switch until the desired speed is reached. Then release the switch. The ON, OFF switch operates two components: the PCM's ON, OFF input, and the battery voltage to the brake switch, which powers the speed control servo.
Diesel Engine With Manual Transmission
There are two separate switch pods that operate the speed control system. The steering-wheel-mounted 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.
When speed control is selected by depressing the ON, OFF switch, the Engine Control Module (ECM) allows a set speed to be stored in its RAM for speed control. To store a set speed, depress the SET switch while the vehicle is moving at a speed between approximately 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:
- Stepping on the brake pedal.
- Depressing the OFF switch.
- Depressing the CANCEL switch.
The speed control can be disengaged also by any of the following conditions:
- The vehicle speed signal increases at a rate of 10 mph per second (indicates that the co-efficient of friction between the road surface and tires is extremely low).
- Depressing the clutch pedal.
- Excessive engine rpm (indicates that the transmission may be in a low gear).
- The vehicle speed signal decreases at a rate of 10 mph per second (indicates that the vehicle may have decelerated at an extremely high rate).
- If the actual speed is not within 20 mph of the set speed.
- The previous disengagement conditions are programmed for added safety.
- Once the speed control has been disengaged, depressing the ACCEL switch restores the vehicle to the target speed that was stored in the ECM's RAM.
NOTE: Depressing the OFF switch will erase the set speed stored in the ECM's RAM.
If, while the speed control is engaged, the driver wishes to increase vehicle speed, the ECM is programmed for an acceleration feature. With the ACCEL switch held closed, the vehicle accelerates slowly to the desired speed. The new target speed is stored in the ECM's RAM when the ACCEL switch is released. The ECM also has a tap-up feature in which vehicle speed increases at a rate of approximately 2 mph for each momentary switch activation of the ACCEL switch.
The ECM also provides a means to decelerate without disengaging speed control. To decelerate from an existing recorded target speed, depress and hold the COAST switch until the desired speed is reached. Then release the switch. The ON, OFF switch operates two components: the ECM's ON, OFF input, and the battery voltage to the brake switch, which powers the speed control servo.
VACUUM RESERVOIR
Gasoline Powered Engines
A vacuum reservoir is used to supply the vacuum needed to maintain proper speed control operation when engine vacuum drops, such as in climbing a grade while driving. A one-way check valve is used in the vacuum line between the reservoir and the vacuum source. This check valve is used to trap engine vacuum in the reservoir. On certain vehicle applications, this reservoir is shared with the heating/air-conditioning system. The vacuum reservoir cannot be repaired and must be replaced if faulty.
Diesel Powered Engines With Automatic Transmission
A vacuum reservoir is not used if equipped with a diesel powered engine. Instead, an engine driven pump (vacuum pump) is used to supply vacuum for speed control operation. This vacuum pump is used with the diesel engine only if it is equipped with an automatic transmission.