Principles of Operation

PRINCIPLES OF OPERATION

The speed control system is controlled by the powertrain control module (PCM). The speed control system is designed to maintain a selected vehicle speed between 48 and 200 km/h (30 and 124 mph). The speed control system is controlled by the steering wheel mounted switch(es) (ON, OFF, SET+ [ACCEL], SET- [COAST], RESUME), the stoplamp switch and the speed control deactivator switch (part of the stoplamp switch). The steering wheel mounted switch(es) are hardwired to the PCM through the clockspring.

The speed control functions include:
- Turning on the speed control system
- Setting and maintaining the desired vehicle speed
- Accelerating the vehicle speed
- Decelerating the vehicle speed
- Turning the vehicle speed control system off
- Canceling the speed control

Pressing and releasing the ON switch turns the speed control system on. Pressing and releasing the SET+ or SET- switch while the vehicle is traveling at the desired speed activates the speed control system.

Tapping the SET+ or the SET- switch while in the set mode respectively, increases or decreases the maintained vehicle speed by 1.6 km/h (1 mph) per tap. If the respective button is pressed and held, the vehicle speed continues to accelerate or decelerate until the button is released.

Pressing and releasing the OFF switch, or switching the ignition switch to the OFF position, turns the speed control system off. Applying the brake pedal puts the speed control system into the STANDBY mode. Pressing the RESUME button, when the speed control system is in the STANDBY mode causes the vehicle to accelerate to the last set speed. Resume does not function if the OFF button is pressed, the ignition switch is in the OFF position, or if the current vehicle speed is below the minimum operational speed.

The clockspring provides the electrical interface between the steering column wiring and the speed control switches in the steering wheel.

The speed control deactivator switch (part of the stoplamp switch) is provided as an additional safety feature. When the brake pedal is applied, the speed control deactivator switch opens and removes the voltage signal from the PCM input circuit, deactivating the speed control system.

Whenever the speed control system is engaged and active, a speed control icon on the instrument cluster is illuminated.

The inputs to the PCM are:
- Transmission output shaft speed (OSS) sensor
- Digital transmission range (TR) sensor
- Speed control switch(es)
- Stoplamp switch
- Speed control deactivator switch (part of stoplamp switch)
- Accelerator pedal position (APP) sensor

The outputs of the PCM for the speed control system are:
- Speed control indicator lamp
- Throttle command

The speed control system throttle position is completely controlled by the PCM through the electronically controlled throttle body. Speed control electronics are contained entirely within the PCM.

When the speed control system is active, the PCM corrects for deviations in the actual vehicle speed by proportionally moving the throttle plate. The PCM modulates the throttle to minimize error between the actual vehicle speed and the desired speed.

The PCM strategy uses the throttle control for smooth accelerations.

The PCM sends a message over the controller area network (CAN) to the instrument cluster whenever the speed control telltale should be turned on or off.

In the event of an off command or a deactivation request from any source, the speed control system carries out a deactivation and immediately returns the throttle to the idle position.

The speed control system provides self-diagnostics. Speed control is disabled anytime an error is detected in the system. No cluster telltale or message center messages are displayed when faults occur. DTCs are logged by the PCM.

An electronically controlled throttle system fault also causes the speed control system to be disabled and the malfunction indicator lamp (MIL) illuminates, or a message center message is displayed.

Additionally, the following conditions cause the speed control system to deactivate:
- Transmission gear selector in a position other than D or OD.
- Speed control set speed is over-ridden with the accelerator pedal for a period longer than 5 minutes.
- Vehicle speed loss from set speed of greater than 16 km/h (10 mph) occurs.
- Vehicle speed falls below the minimum allowable limit of 48 km/h (30 mph).
- Speed control is pressed or stuck longer than 2 minutes.