GM Celebrity/Century/Ciera/6000 1982-1996 Repair Guide

General Description


Cruise control is a speed control system which maintains a desired vehicle speed under normal driving conditions. Steep grades either up or down, may cause some variation in the set speed. Speed is maintained only by mechanically holding the throttle open or releasing it. The system cannot apply braking to slow the vehicle.

Most systems use vacuum to operate a throttle servo unit. The servo unit maintains a desired speed by trapping vacuum when the servo is at the desired position. The position of the servo and the vehicle speed are overseen by the controller or Cruise Control Module (CCM), this unit controls the input or release of vacuum within the servo, thus adjusting the throttle to maintain speed.

Some models may use an ElectroMotor Cruise (EMC) system. The EMC uses an electric motor and connecting strap to vary the throttle angle according to directions from the CCM. The system is completely independent of vacuum and allows smoother throttle transitions.

Although the systems vary widely across GM's model line (component location, circuitry, interaction of computers, etc.) all cruise control units share certain common traits:

The system will not engage until the vehicle exceeds a minimum speed, usually 25 mph (40 kph).
The system provides several modes of operation including cruise, coast, resume/accelerate, most allow "tap up" and "tap down", an incremental increase or decrease in set speed.
All have a system ON/OFF switch, the system must be turned ON before it will engage. Many have a pilot light on the dash which illuminates when the cruise control is engaged.
All systems will disconnect if the brake and/or clutch pedal is depressed when the system is engaged. The vacuum sustained systems release vacuum as well as interrupting the electrical signal to the controller.
The accelerator may be depressed at any time to override the chosen cruising speed. If the system receives no new orders, it will remember the previous cruising speed and return to it when the driver finishes accelerating.
Failures in the cruise control may stem from electrical, mechanical and/or vacuum problems within the cruise control system or any combination of these.

The use of the speed control is not recommended when driving conditions do not permit maintaining a constant speed. These conditions may include heavy or varying traffic, winding roads or slippery surfaces.

The main parts of the vacuum systems are the:

Mode control switches
Cruise Control Module (CCM)
Servo unit
Vehicle Speed Sensor (VSS)
Vacuum supply
Electrical and vacuum release switches
Electrical harness


For any of the following conditions to occur, the vehicle must be moving above 25 mph (40 kph) and the cruise control system switch must be switched ON . If either of these basics change, the system will cease to operate.

Set and Coast

When the vehicle is at the desired speed, engage the SET/COAST button momentarily. This button is usually at the end of the stalk and is pushed inward to engage it. Vehicle speed will be maintained within 1 mph (2 kph) of the speed the vehicle was traveling when the button was pushed. The system will maintain this speed until:

The brake or clutch pedals move enough to disengage the system.
A system control switch is used to send a new message to the controller.
The system switch is turned OFF .
The ignition is switched OFF .

If only the brake or clutch pedal is moved, the system disconnects the cruise function but remembers the set speed and can return to it. Pushing a system control button or turning the system off removes memory of the previous speed.

Once the vehicle is holding the desired speed, the SET/COAST button may be used to slow the vehicle. Holding the button in will disconnect the system as long as the button is held. Once the car has slowed to the new desired speed, releasing the button will cause the system to hold the new, slower speed.

If the driver accelerates beyond the set cruising speed, pushing the SET/COAST button causes the system to maintain the new higher speed. If the button is not engaged at the higher speed, the system will maintain the previous set speed after the vehicle slows.

Resume and Accelerate

The RESUME/ACCEL switch is used after braking to tell the system to return to the previous speed. The switch must be moved momentarily into engagement, less than one second for most systems. If held too long, the system enters the ACCEL mode.

The ACCEL mode, when engaged, will accelerate the vehicle from the present set speed, when the switch is released, the new (higher) speed will be set and maintained.

Tap-Up and Tap-Down

Many of the GM systems are programmed to obey momentary signals from the SET/COAST and RESUME/ACCEL switches. If the SET/COAST switch is tapped or engaged for less than one second, the controller will reduce the car speed by 1 mph (2 kph) per tap. Similarly, if the RESUME/ACCEL switch is momentarily engaged, the set speed will increase by 1 mph (2 kph) per tap. This is a particularly handy function for fine tuning vehicle speed to prevailing limits, weather or traffic conditions.

If either switch is held too long, the system enters either the COAST or ACCEL mode. When the driver next releases the switch, the system will note and maintain the new speed.


Vacuum Servo Unit

The servo operates the throttle in response to signals from the controller. The servo consists of a vacuum operated diaphragm, a normally open solenoid valve to vent the diaphragm chamber to atmosphere, a normally closed solenoid valve to connect the diaphragm chamber to the vacuum source, and a variable inductance position sensor. Under normal operating conditions, the controller reacts to 3 vehicle or system states.


Both vacuum and vent valves are closed or sealed. The servo has a constant vacuum on the diaphragm and places no flow requirements on the vacuum source. Vacuum is trapped in diaphragm chamber.


The controller energizes the vacuum solenoid, opening the vacuum valve to the vacuum source. This increases the vacuum level in the servo to increase the throttle opening. The vent remains closed.


The controller de-energizes the vent solenoid, opening the vent valve to the atmosphere. This reduces vacuum in the servo and allows the throttle return spring to decrease the throttle opening. The vacuum inlet valve remains closed. When the controller senses an over or underspeed condition, it will pulse the opening of the vent or vacuum valve. The controller pulse will be repeated, as required, until the speed correction brings the vehicle to the set speed.

Under normal road conditions, the vacuum valve will remain in a completely open position when vehicle speed has dropped 5 mph (8 kph) below set speed. Likewise, if vehicle speed is 3 (5 kph) or more mph over the set speed, such as down a steep grade, the vent will go into full open position.

When the cruise system is on and operating with no input from the driver through the control switches, no correction to the speed will be made until the vehicle deviates 1 / 2 mph (.8 kph) from the set speed.

The servo incorporates a steel core which moves within a coil. Its resulting variable inductance provides a continuous voltage signal to the controller. The servo position signal is constantly compared to the vehicle speed signal. This comparison determines if the pulses issued have corrected the speed error or if additional pulses are required. This comparison is also used to lengthen the average pulse when it is not enough to compensate for the speed error, such as a steep grade.

EMC Controller/Actuator

The Electro-Motor Cruise (EMC) system eliminates the vacuum servo and incorporates the controller and actuator into a single unit. The control assembly consists of the electronic controller board, a solenoid operated clutch, the drum gear and strap (connected to the throttle linkage) and a stepper motor with 3-phase wiring and 4-pole stator.

When the vehicle speed is set or changed, the controller assembly adjusts throttle position by activating the stepper motor. The motor provides rotation in either direction through the use of 3 drive transistors. As the rotor turns, a pinion gear drives the drum gear assembly, causing the strap to wind or unwind on the drum. The strap pulls or releases the throttle linkage accordingly.

If the system is turned off or deactivated for any reason, the controller de-energizes the solenoid operated clutch. The spring on the clutch arm pushes the pinion away from the drum gear and a spring within the drum unwinds the strap. The throttle linkage is released and returns to the idle position until a new speed is set by the operator.

Electronic Controller (Module)

The controller interprets the position of the servo or electric motor, the position of the control switches and the output of the speed sensor. In response to these inputs, the controller electrically signals the opening or closing of the vent and vacuum solenoid valves in the servo or the rotation of the actuator motor.

The control functions may be performed by either a separate controller (stand alone system) or through the engine control module (integrated system). The stand alone controller uses a quartz crystal to produce clock signals for the integrated circuits within. The ECU/PCM can perform the same functions to control the system. Integrated systems also receive input from the neutral safety switch, many of these systems receive a throttle position signal instead of servo position signal.

Vehicle Speed Sensor (VSS)

There are 2 types of VSS buffer amplifiers, single or dual output. The optic head portion of the VSS is located in the speedometer frame. A reflective blade is attached to the speedometer cable/head assembly. The blade spins like a propeller, with its blades passing through a light beam from a LED in the optic head. As each blade enters the LED light beam, light is reflected back to a photocell in the optic head, causing a low power speed signal to be sent to the buffer for amplification and signal conditioning. The switching transistor within the buffer provides the speed signal to the ECM/PCM and/or cruise control module.


See Figure 1

This device supplies the vehicle speed input to the controller on some cars. Vehicle speed information is provided to the controller by a Permanent Magnet (PM) generator driven by the transmission/transaxle. The PM generator produces an AC sine wave signal. Through use of an integrator and Schmitt Trigger, the signal is converted to square wave DC. Since cruise control is not the only function requiring VSS input, these signals are sent to various control units on the vehicle. Depending on the model, the speed signal may be used by the speedometer.

Click image to see an enlarged view

Fig. Fig. 1: The VSS permanent magnet generator (PM) electrical schematic

Vacuum Accumulator

Found on most systems (except those with diesel engines), the familiar ball-shaped vacuum accumulator is used to maintain satisfactory cruise control performance in vehicles with low manifold vacuum at normal road speeds. The accumulator is connected to the intake manifold through a one-way check valve and "stores" the high vacuum level that is available during periods of low engine loading. The system operates on accumulator vacuum when it is higher than manifold vacuum.