Cruise Control: Description and Operation

Speed Control Switches:

SPEED CONTROL
The speed control system is electronically controlled and vacuum operated. The electronic control is integrated into the powertrain control module, located on the driver's side under the speed control servo. The controls are located on the steering wheel. The ON, OFF, and SET buttons are located on the left side of the airbag module. The RESUME, ACCEL, CANCEL and COAST buttons are located on the right side of the airbag module.

The system is designed to operate at speeds above 30 mph (48 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 activated by depressing the ON switch, the PCM allows a set speed to be stored in RAM for speed control. To store a set speed, depress and release the SET switch while the vehicle is moving at a speed between 30 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

NOTE: Turning the system oft by 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 rpm (indicates 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 co-efficient 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)
- If the actual speed is greater than 20 mph over the set speed.

Once the speed control has been disengaged, depressing the RESUME switch when speed is greater than 25 mph allows the vehicle to resume control 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 ACCEL switch. The new target speed is stored in the PCM when the ACCEL switch is released. The PCM also has a "tap-up" feature in which target speed increases by 2 mph for each momentary switch activation of the ACCEL switch. The PCM also provides a means to decelerate to a new lower target speed without disengaging speed control. Depress and hold the COAST switch until the desired speed is reached, then release the switch.

The PCM also has a "Tap Down" feature in which target speed decreases at 1 mph for each momentary switch activation of the coast switch.

SERVO
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 by 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 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.

SWITCH
There are two separate switch pods that operate the speed control system and are located on the steering wheel.

The speed control system has five separate resistive switches that provide a single multiplexed (MUX) voltage inputs to the PCM. The switch names are: ON, OFF, SET COAST, RESUME, ACCEL, TAP-UP, COAST, and CANCEL. Based on conditions when the buttons are pushed (and released), the five voltages ranges provided to the PCM result in the following functions: ON, OFF, SET, COAST RESUME, ACCEL, TAP-UP TAP-DOWN, COAST and CANCEL. Refer to the Speed Control for more information.

Also the PCM receives an input from the brake switch to sense whether the brake pedal has been depressed. When the PCM receives the brake depressed input, it turns off power to the speed control servo and disengages speed control. Also the power to the servo is supplied through the brake switch, which opens the circuit when the brake pedal is depressed.

The individual switches cannot be repaired. If one switch fails, the entire switch module must be replaced.

VACUUM RESERVOIR
The vacuum reservoir is located under the left front headlamp, in front of the battery. It is made of plastic and does not contain any other parts, such as a check valve.

The reservoir stores engine vacuum. Manifold vacuum is supplied from the brake booster check valve. The speed control vacuum supply hose has a check valve at the source (brake booster) to maintain the highest available vacuum level in the servo, reservoir and vacuum hoses. 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.