Principles of Operation

Speed Control

Principles of Operation

The speed control system is controlled by the PCM. The speed control system is designed to maintain a selected vehicle speed between 40 km/h (25 mph) and the maximum limited vehicle speed. The speed control system is controlled by the steering wheel mounted switches (ON, OFF, SET/ACCEL, COAST and RES), the stoplamp switch (gas engine), the brake pressure switch (diesel engine), and the speed control deactivator switch (integral to the stoplamp switch). The steering wheel mounted switches are hardwired to the PCM through the clockspring.

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

Tapping the ACCEL 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 switch is pressed and held, the vehicle speed continues to accelerate or decelerate until the switch 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 in the standby mode. Pressing the RES switch when the speed control system is in the standby mode causes the vehicle to accelerate to the last set speed. The resume feature does not function if the OFF switch is pressed, the ignition switch is in the OFF position or if the current vehicle speed is less than the minimum operational speed.

For gasoline engines, the speed control deactivator switch is provided as an additional safety feature. When the brake pedal is applied, a voltage signal from the stoplamp switch to the PCM deactivates the system. Under increased brake pedal effort the speed control deactivator switch (integral to the stoplamp switch) opens up and removes the voltage input to the PCM.

For diesel engines, the brake pressure switch is provided as an additional safety feature. When the brake pedal is applied, a voltage signal from the stoplamp switch to the PCM deactivates the system. Under increased brake pedal effort the brake pressure switch opens and removes the voltage signal supplied to the PCM.

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

The inputs to the PCM are:

- Stoplamp switch
- Output Shaft Speed (OSS) sensor
- Transmission Range (TR) sensor
- Speed control switches
- Speed control deactivator switch (integral to the stoplamp switch (gasoline engines))
- Brake pressure switch (diesel engine)
- Accelerator pedal position sensors

The outputs of the PCM are:

- Speed control indicator lamp
- Throttle command

For gasoline engines, 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 actual vehicle speed and the desired speed.

For diesel engines, the speed control is completely controlled by the PCM through the electronically controlled fuel injection. When the speed control system is active, the PCM corrects for deviations in the actual vehicle speed by proportionally adjusting the fuel delivery. The PCM modulates the fuel injectors to minimize error between actual vehicle speed and the desired speed.

The PCM sends a message over the High Speed Controller Area Network (HS-CAN) to the IC 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 hard deactivation and immediately returns the throttle to the idle position.

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

An electronically controlled throttle system fault also causes the speed control system to be disabled and a warning telltale (wrench light) or a message center message is displayed.

Additionally, the following conditions can 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 40 km/h (25 mph)
- Speed control switch is pressed or stuck longer than 2 minutes
- Park brake switch engaged