Cruise Control: Description and Operation

SPEED CONTROL SYSTEM

Speed Control Switch:

The speed control system is electronically controlled and vacuum operated. The electronic control is integrated into the Powertrain Control Module which is located in the engine compartment. The controls are located on the steering wheel and consist of five switches. The ON, OFF and SET buttons are located on the left side of the airbag module. The RESUME, ACCEL, COAST and CANCEL buttons are located on the right side of the airbag module.

The system is designed to operate at speeds above 25 mph (40 km/h).

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.

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 VSS 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 VSS 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.

Inputs Required for Operation
The inputs required by the PCM to operate the Speed Control System include:
- Speed Control switches
- Brake switch
- Park/Neutral switch
- Vehicle speed signal
- Engine speed
- CCD bussed message from TCM

SPEED CONTROL SERVO - PCM OUTPUT
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.

The 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 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 25 mph to resume, 30 mph to set, and the RES/ACCEL switch has been depressed, ground for the vent and vacuum circuits is restored.

SPEED CONTROL SWITCHES - PCM INPUT
There are two separate switch pods that operate the speed control system. The steering-wheel-mounted switches use multiplexed circuits to provide five separate voltages (inputs) to the PCM. The voltages correspond to the ON, OFF, SET, RESUME, ACCEL, 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 speed control ON voltage informs the PCM that the speed control system has been activated and allows a set speed to be stored in RAM for speed control. The speed control SET voltage informs the PCM that a fixed vehicle speed has been selected. The vehicle must be moving at a speed between 25 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.

Once the speed control has been disengaged, depressing the ACCEL switch generates the speed control RESUME voltage, indicating that the previous fixed speed is requested. This restores the vehicle to the target speed that was 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 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.

The speed control CANCEL voltage tells the PCM to deactivate but retain set speed in memory (same as depressing the brake pedal). When depressing the OFF switch, the speed control OFF voltage tells the PCM that the speed control system has been deactivated and will erase the set speed stored in the PCM's RAM.

Multiplexing
The PCM sends out 5 volts through a fixed resistor and monitors the voltage change between the fixed resistor and the switches. If none of the switches are depressed, the PCM will measure 5 volts at the sensor point (open circuit). If a switch with no resistor is closed, the PCM will measure 0 volts (grounded circuit). Now, if a resistor is added to a switch, then the PCM will measure some voltage proportional to the size of the resistor. By adding a different resistor to each switch, the PCM will see a different voltage depending on which switch is pushed.

On most vehicles another resistor has been added to the at rest circuit causing the PCM to never see 5 volts. This was done for diagnostic purposes. If the switch circuit should open (bad connection) then the PCM will see the 5 volts and know the circuit is bad. The PCM will then set an open circuit fault.

STOP LAMP SWITCH
The switch is mounted on the brake pedal mounting bracket under the instrument panel.

Vehicles equipped with the speed control option use a dual function stop lamp switch. The PCM monitors the state of the dual function stop lamp switch.

The brake switch is equipped with three sets of contacts, one normally open and the other two normally closed (brakes disengaged). The PGM sends a 12 volt signal to one of the normally closed contacts in the brake switch, which is connected to a ground. With the contacts closed, the 12 volt signal is pulled to ground causing the signal to go low. The low voltage signal, monitored by the PCM, indicates that the brakes are not applied. When the brakes are applied, the contacts open, causing the PCM's output voltage to go high, disengaging the speed control also grounding the dump solenoid.

The second set of normally closed contacts is supplied battery voltage any time speed control is selected. From the brake switch, current is routed to the speed control servo solenoids. The speed control solenoids (vacuum, vent and dump) are provided this current any time the speed control is ON and the brakes are disengaged. When the driver applies the brakes, the contacts open and current is interrupted to the solenoids. The normally open contacts are fed battery voltage. When the brakes are applied, battery voltage is supplied to the stop lamps.

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.

VACUUM RESERVOIR
The reservoir stores 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 generator mounted to an adapter near the transmission 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 PCM speed control circuitry to determine vehicle speed and to maintain speed control set speed.