Principles of Operation

WARNING: This page is about a different car, the 2008 Ford RV Cutaway, 2008 Ford Econoline, 2008 Ford E450 Super Duty, and 2008 Ford Cutaway. However, it is still accessible from the selected car via links, so may be relevant.

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 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 in 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. The resume feature does not function if the OFF button is pressed, the ignition switch is in the OFF position or if the current vehicle speed is less than the minimum operational speed.

NOTE: 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 switch to the PCM deactivates the system. Under increased brake pedal effort the speed control deactivator switch opens up and removes the voltage input to the PCM.

NOTE: 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
Transmission output shaft speed (OSS) sensor
Digital transmission range (TR) sensor
Speed control switches
Speed control deactivator switch
Accelerator pedal position sensors
Parking brake

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 electronically controlled throttle. Speed control electronics are contained entirely within the electronic throttle control (ETC) subsystem.

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.

The PCM strategy uses the throttle control for smooth accelerations. In instances where the vehicle tends to exceed the set speed, the PCM invokes an engine braking strategy to help maintain the desired vehicle set speed.

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 cluster telltale or message center messages are displayed when faults occur. Fault codes are logged by the PCM.

An ETC system fault also causes the speed control system to be disabled. In this case, an ETC system warning telltale (wrench light) illuminates.

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 48 km/h (30 mph)
Speed control switch is pressed or stuck longer than 2 minutes
Parking brake applied