Speed Control

The speed control subsystem is designed to automatically maintain vehicle speed within prescribed limits above 25 miles per hour (mph). The function is integrated into the Powertrain Control Module (PCM) and includes the following: Brake On/Off (BOO) switch, Brake Pressure Applied (BPA) switch, Clutch Pedal Position (CPP) switch and Parking Brake Applied (PBA) switch.

The speed control driver command switches, located on the steering wheel, provide the features ON, OFF, SET/ACCEL/TAP-UP, COAST/TAP-DOWN, and RESUME.

COMPONENTS
The vehicle speed control subsystem consists of the following components:
- Steering column and controls
- Brake On/Off (BOO) switch
- Clutch Pedal Position (CPP) switch
- Horn relay assembly
- Vehicle Speed Sensor (VSS) assembly
- Brake Pressure Applied (BPA) switch
- Wiring harness
- PCM

PCM INPUTS
The PCM inputs that are used for speed control consist of the following:
- VSS signal - A sine wave with frequency proportional to speed (variable reluctance sensor).
- VSS signal ground - The signal provides the ground reference for the vehicle speed sensor signal.
- Speed control driver command switch signal - This is a variable DC voltage determined by driver selection of steering wheel control switches, fed through slip ring and brush contacts.
- Speed control driver command switch signal return - This signal provides the ground reference for the command switch signal.
- BOO switch signal - Provides the PCM with an on/off signal based on brake pedal position.
- BPA switch signal - Provides the PCM with an additional redundant on/off signal based on brake pedal position.
- CPP switch signal - Provides the PCM with an on/off signal based on clutch pedal position.

Set Speed
The PCM controls fuel to minimize the error between the actual vehicle speed and this target speed. The SET SPEED can be modified by the SET ACCEL, COAST, TAP-UP, TAP-DOWN, RESUME and OFF commands.

Stand By
In this mode the PCM is waiting for SET, COAST or RESUME to enter the ACTIVE mode. STAND-BY mode is entered under the following conditions:
- The first ON command after power has been applied to the PCM.
- The first ON command after a previous OFF command.
- The PCM receives a brake signal while in the ACTIVE mode.
- The PCM receives a manual transmission clutch pedal position switch signal while in the ACTIVE mode.
- The vehicle speed drops approximately 10mph (16kph) below the SET SPEED while in the ACTIVE mode.
- The vehicle speed drops below 25mph (39kph).
- An attempt is made to set speed above 125mph (204kph).

Set Speed Error
The difference between the actual vehicle speed and the set speed.

SYSTEM FUNCTION

Ignition Off
Speed control system is off with no current supplied to the PCM. The engine must be running to operate speed control.

Ignition On
The PCM is operating, waiting for an ON command to put the system into the STAND-BY mode. The engine must be running.

On
If the system is in the OFF mode the first time the ON button is pressed it will cause the control module to enter STAND-BY mode.

Set/Accel/Tap-Up
If the system is in STAND-BY mode, a SET command will cause the PCM to maintain vehicle speed at the SET SPEED (vehicle speed at the moment the SET/ACCEL/TAP-UP switch is released).

SPEED CONTROL OPERATION RANGE
The speed control system operates between 25-125 mph (39-204 kph).

SPEED CONTROL SYSTEM OPERATION

Active mode
PCM is controlling speed to the set point by controlling fuel.

Off Mode
The speed control system is in the OFF mode under the following circumstances:
- Prior to receiving an ON command after the PCM has been powered on.
- After receiving an OFF command.
- Upon hard failures to speed control components or critical powertrain control components.

In the OFF mode, the PCM is waiting for an ON command to enter STAND-BY mode.

If the speed control system is in the STAND-BY mode and the vehicle speed is greater than 131 mph (210 kph), a SET command will not be recognized and the speed control system will remain in the STAND-BY mode.

If the system is in ACTIVE mode and the vehicle speed is less than 3 mph (4.8 kph) of the SET SPEED, then each momentary tap (greater than 50 milliseconds) of the SET/ACCEL/TAP-UP switch will cause the vehicle to smoothly increase in speed 1.0 mph (1.6 kph).

If the system is in ACTIVE mode and the vehicle speed is greater than 3 mph (4.8 kph) of the SET SPEED, then each momentary tap of the SET/ACCEL/TAP-UP switch will cause the vehicle speed to be controlled at the higher vehicle speed attained at the time the switch is released.

If the system is in ACTIVE or STAND-BY mode, the vehicle will accelerate, within acceleration limits, for as long as the ACCEL switch is held. When the ACCEL switch is released, the vehicle speed will be controlled, within set speed error limits and vehicle speed operating limits, at the higher vehicle speed attained at the time the switch is released.

If the system is in ACTIVE mode, depressing the SET/ACCEL/TAP-UP switch for more than 50 milliseconds will cause a minimum of 1.0 mph (1.6 kph) speed increase.

Vehicle speed will be limited to MAX SPEED when using ACCEL and TAP-UP commands.

Coast/Tap-Down
If the system is in STAND-BY mode, a COAST/TAP-DOWN command will cause vehicle speed to be maintained within limits.

If the system is in ACTIVE mode, each momentary tap (greater than 50 milliseconds) of the COAST/TAP-DOWN switch will cause the vehicle to smoothly decrease in speed 1.0 mph (1.6 kph). The vehicle speed will decrease as long as the COAST/TAP-DOWN switch is held. When the COAST/TAP-DOWN switch is released the vehicle speed will be controlled at the lower vehicle speed attained at the time the switch is released.

If the system is in ACTIVE mode, depressing the COAST/TAP-DOWN switch for more than 50 milliseconds will cause the system to revert to STAND-BY mode.

Resume
A momentary tap of the RESUME switch (greater than 50 milliseconds) will cause the system to re-acquire a previously set speed, provided the system had been previously in ACTIVE mode. Acceleration occurring during RESUME mode will be smooth and continuous.

No resume will occur if vehicle speed is reduced below minimum speed lockout.

The RESUME function is overridden by actuating the BOO switch, CPP switch, BPA switch, COAST, SET, or OFF command switches.

Brake Disable
A brake disable signal causes the engine rpm to return to idle. The system will then revert to STAND-BY mode.

Clutch Disable
A clutch disable signal has the same response as a brake disable.

Off
An OFF command will cause the engine rpm to return to idle. The system then reverts to OFF mode.

Redundant Brake
A vehicle speed decrease of approximately 10 mph (16 kph) below the set speed causes the engine to return to idle speed. The system will then revert to STAND-BY mode.

Minimum Speed Lockout
If the system is presently in STAND-BY mode and the vehicle speed is less than 23 mph (37 kph), the system will not enter ACTIVE mode.

If the system is presently in ACTIVE mode and the vehicle speed drops below 23 mph (37 kph), the system enters STAND-BY mode.

Maximum Speed Lockout
Vehicle speed will be limited to 131 mph (210 kph) when using the ACCEL and TAP-UP commands. If the system is in STAND-BY mode and the vehicle speed is greater than 131 mph (210 kph), a SET command will cause the subsystem to remain in STAND-BY mode.

Pedal Override
Pedal may override speed control. Releasing the pedal will cause speed control to resume control.