Cruise Control: Description and Operation

CIRCUIT OPERATION

The Cruise Control System maintains a driver selected speed by operating the throttle through the Cruise Control Servo. The Powertrain Control Module (PCM) controls the servo based on input data from driver-operated controls and other inputs like vehicle speed and engine rpm.

System Enable/Disable
The Cruise Control System is enabled when the Cruise On/Off Switch is in the on position. Voltage from Fuse D5 is now applied to the PCM's cruise enable input (terminal 1D2 4.9L or terminal 2D14 Northstar) the Set Coast Switch, Resume/Accel Switch and the cruise brake switch input (terminal 1C2 4.9L or terminal 2D12 Northstar). The PCM now can enable the cruise function once the driver sets the desired speed.

The Cruise Control System is disabled when the Cruise On/Off Switch is in the off position. This removes voltage from the PCM's cruise enable input (terminal 1D2 4.9L or terminal 2D14 Northstar). Various PCM codes will also disable the Cruise Control System for the entire ignition cycle when set.

Cruise Disengagement
The PCM has two disengage inputs. One is the cruise brake switch input (terminal 1C2 4.9L or terminal 2D12 Northstar) and the other is the TCC brake switch input (terminal 1C4 4.9L or 2D13 Northstar). The PCM will disengage the Cruise Control System when battery voltage is not applied to one or both of the brake inputs. Voltage is applied to the TCC brake switch input (terminal 1C4 4.9L or 2D13 Northstar) when the ignition is in RUN through Fuse D1, CKT 750, the TCC Brake Switch and CKT 420. Depressing the brake pedal opens this circuit. Voltage is applied to the cruise brake switch input (terminal 1C2 4.9L or terminal 2D12 Northstar) when the ignition is in RUN through Fuse D5, CKT 250, the Cruise Control Switch, CKT 397, the Stop/BTSI/Cruise Brake Switch and CKT 86. Depressing the brake pedal or placing the Cruise Control Switch in the OFF position will open this circuit.

One other disengage for the system is the Cruise Vacuum Release Valve. The valve is part of the TCC Brake Switch and is designed to vent servo vacuum to atmosphere when the brake pedal is depressed. This disengages the cruise system by preventing the servo from pulling in the throttle cable.

Set/Coast
The Set/Coast Switch serves two main functions: 1) to set the cruise speed. and 2) to reduce the cruise set speed once the system has been engaged.

To set the cruise speed the vehicle speed must be above 24 mph and the Set/Coast Switch must be closed momentarily at the desired speed. When this occurs voltage from Fuse D5 is applied to the PCM set/coast input (terminal 1D3 4.9L or 2D15 Northstar) through the Cruise On/Off Switch and the Set/Coast Switch. The PCM reacts by storing the vehicle speed that it was monitoring at its vehicle speed input and begins operating the Cruise Control Servo to maintain the set speed.

Once a speed has been set, single momentary closures of the Set/Coast Switch will cause the PCM to decrease the set speed 1 mph per closure (tap down). Holding the Set/Coast Switch closed will cause the PCM to release the throttle and the car will coast until the switch is released. The PCM will maintain the vehicle speed when the switch was released if it is above 24 mph.

Resume/Accel
The Resume/Accel Switch serves two main functions: 1) to increase the cruise set speed once the system has been engaged, and 2) to return the vehicle to a previously set speed after the system has been disengaged. When the Resume/Accel Switch is closed, Fuse D5 applies voltage to the PCM resume accel input (terminal 1C3 4.9L or 2D16 Northstar) through the closed Cruise On/Off Switch and the Resume/Accel Switch. The PCM now returns the vehicle to its previously set speed by simultaneously monitoring the vehicle speed input and operating the Cruise Control Servo.

If the Resume/Accel Switch is held closed with a speed already set, the PCM will increase the vehicle speed until the switch is released. Once the switch is released the PCM will maintain that speed as the set speed.

If the Resume/Accel is tapped with cruise engaged the PCM will increase the vehicle speed 1 mph per switch closure.

Cruise Control Servo
The Cruise Control Servo is a vacuum-operated diaphragm that moves the throttle cable in response to the PCM commands. The PCM controls the servo by using two solenoid valves, a vent valve solenoid and the vacuum valve solenoid. The PCM energizes the solenoid valves by applying voltage to the valves from the PCM valve control output terminals (3F3, 3F2 4.9L or 1A9, 1A12 Northstar). The solenoid valves are grounded at ground G301 (4.9L) or ground G102 (Northstar).

Increasing Throttle Angle
When the PCM needs to increase speed, the PCM energizes the Vacuum Valve Solenoid in the servo to apply vacuum to the diaphragm and increase the throttle angle. The PCM also energizes the Vent Valve Solenoid to close and seal the vacuum circuit in the servo. Once the desired speed is reached, the Vacuum Valve Solenoid is de-energized. The valve now moves to the closed position, sealing the vacuum circuit. The Vent Valve remains energized and closed to maintain vacuum and speed.

Decreasing Throttle Angle
When the PCM needs to reduce speed, the Vent Valve Solenoid is de-energized, the vent valve opens, and vacuum is reduced in the servo, allowing the throttle angle to decrease. The Vacuum Valve Solenoid remains de-energized when a speed reduction is required.

Servo Position Sensor
The PCM uses the Servo Position Sensor to monitor throttle movement to ensure smooth acceleration and deacceleration. The Servo Position Sensor is a variable inductor. As the servo's vacuum diaphragm moves, a steel rod attached to the throttle cable inside the Cruise Control Servo moves in and out of a coil of wire. When this occurs, the inductance of the wire coil changes. Inductance is an electrical property that opposes the flow of alternating (AC) current. Inductance is similar to resistance but it cannot be measured with a standard voltmeter.

The PCM's servo position sensor input terminal (2B3 4.9L or 3F9 Northstar) applies a 5 volt alternating signal to the Servo Position Sensor and monitors the change in the inductance across the sensor coil as the throttle cable moves.