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 201 km/h (30 and 125 mph).

The speed control system is controlled by the steering wheel-mounted switches (ON, OFF, SET ±, COAST, RESUME), and the brake pedal. The steering wheel-mounted switches are hardwired to the PCM.

The speed control functions include:
- Turning on the speed control system
- Setting and maintaining the desired vehicle speed
- Accelerating the vehicle speed
- Tapping-up/tapping-down the vehicle speed
- Resuming the prior vehicle speed
- Coasting the vehicle speed
- Turning off the vehicle speed control system
- Canceling the speed control

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

Tapping the SET ± or the COAST 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 switch in the steering wheel.

The speed control deactivator 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 for the speed control system are the:
- Transmission output shaft speed (OSS) sensor
- Digital transmission range (TR) sensor
- Speed control switch
- Stoplamp switch
- Speed control deactivator switch
- Accelerator pedal position sensor
- Parking brake

The outputs from the PCM for the speed control system are the:
- Instrument cluster
- Throttle command

The speed control system throttle position is completely controlled by the PCM through the electronic throttle body (ETB). Speed control electronics are contained entirely within the electronic throttle control (ETC) subsystem.

When the speed control 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) vehicle speed control set speed status (enable/disable) 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 system carries out a hard 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. Fault codes are logged by the PCM.

An ETC system fault causes the speed control system to be disabled and the ETC warning lamp illuminates, or a message center message is displayed.

Additionally, the following conditions will cause the speed control system to deactivate:
- The transmission gear selector is in a position other than D or OD.
- The speed control set speed is over-ridden with the accelerator pedal for longer than 5 minutes.
- A vehicle speed loss from set speed of greater than 10 mph occurs.
- The vehicle speed falls below the minimum allowable limit (48 km/h [30 mph]).
- The parking brake is applied.
- A speed control switch is pressed or stuck longer than 2 minutes.