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 switches (ON, OFF, SET+, SET-, and RESUME), the stoplamp switch, the clutch pedal position (CPP) switch (if equipped), and the speed control deactivator switch. The steering wheel mounted switches 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.
- coasting down to a lower speed.
- resuming the prior vehicle speed.
- turning off the vehicle speed control system.

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 turning 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 CPP switch is used on vehicles equipped with manual transmissions. When the clutch pedal is applied with the vehicle speed control system engaged, the normally closed switch opens and signals the PCM to deactivate the speed control.

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

The speed control deactivator switch is provided as an additional safety feature. Normally, when the brake pedal is applied, an electrical signal from the stoplamp circuit to the PCM deactivates the system. Under increased brake pedal effort, the speed control deactivator switch opens and removes the voltage from the PCM input circuit, releasing the throttle.

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

The inputs to the PCM are:
- Output shaft speed (OSS) sensor
- Digital transmission range (TR) sensor
- Speed control switch(es)
- CPP switch
- Speed control deactivator switch
- Stoplamp switch
- Accelerator pedal position sensor

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

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

When the speed control system is active, the PCM corrects for deviations in the actual vehicle 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 serial message over the controlled area network (CAN) to the instrument cluster whenever the speed control telltale is turned on or off.

In the event of an OFF command or a deactivation request from the speed control deactivator switch, the speed control 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 electronic throttle control (ETC) system fault also causes the speed control system to be disabled. In this case, an ETC system warning telltale (wrench light) 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 (automatic transmission).
- clutch pedal is pressed.
- speed control set speed is over-ridden with the accelerator pedal for a period longer than 5 minutes.
- parking brake is applied (manual transmission).
- 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 switch pressed or stuck longer than 2 minutes.