Cruise Control: Description and Operation

NOTE: Unlike model 202 vehicles with Engine 104, the cruise control system on Engine 111 is independent of the ISC and has a separate actuator (M16) and control module (N4/2).

The cruise control module is ready for operation as soon as the ignition is switched on. However, the vehicle must be travelling at above approx. 40 km/h before the unit can be actively engaged.

Once the vehicle has attained the desired cruising speed, the driver need only briefly move the pushbutton switch in the appropriate direction for "Accelerate/Set" or "Decelerate/Set". The speed thus selected is then maintained until such time as the driver enters a new speed or the cruise control is switched off.

To modify the stored speed, e.g. to adapt to changing traffic conditions, hold the pushbutton switch in ether the "Accelerate/Set" or "Decelerate/Set" position until the vehicle assumes the desired speed.

It is possible to raise or lower the speed in increments of 1 km/h by briefly tapping the pushbutton switch in the direction for "Accelerate/Set" or "Decelerate/Set". The vehicle responds with controlled acceleration at a rate of 0.7 m/s2. When the pushbutton switch is subsequently released, the new speed is maintained.

To switch off the cruise control, the driver can either briefly tap the pushbutton switch toward "Off" or depress the brake pedal; the cruise control reverts to the idle mode.

Switching off the unit with the pushbutton switch activates the servo motor responsible for returning the control module to the idle position.

In contrast the system responds to the application of pressure at the brake pedal by immediately disengaging the actuator; it interrupts the flow of current to the electromagnetic clutch, and a return spring retracts a gear in the actuator's engagement mechanism.

Return springs bring the unit back into the idle position. Once deactivated, the system remains ready for renewed operation for as long as the ignition remains switched on.

When the cruise control has been deactivated, either through application of the brake pedal or at the pushbutton switch, if the pushbutton switch is briefly pushed in the "Memory" direction at vehicle speeds of over 40 km/h, the vehicle then accelerates at a rate of 1 m/s2 until it reaches the last speed programmed with "Set". The last programmed speed remains stored in the memory until the ignition is switched off.

When the vehicle exceeds the programmed speed in response to pressure on the accelerator, for instance, during overtaking maneuvers, it automatically returns to the preset speed as soon as the pedal is again released.

On uphill stretches the engine's output may not be adequate for maintaining the programmed speed, and the vehicle will slow down. The preset speed is resumed as soon as the road levels out, provided that vehicle speed has not fallen below 40 km/h and/or the brakes have not been applied in the meantime.

Should the engine's braking effect be too small to maintain the preset speed when descending gradients, the vehicle will accelerate, and it may be necessary to apply the brakes. If the brakes are not applied, the vehicle will resume the programmed speed as soon as the road again levels out.

CAUTION: Never place the transmission range selector lever in position "N" when the cruise control is operating, as this would cause the engine to race.

Safety Switches
The last Speed programmed into the control module is erased when the ignition is switched off; the speed memory is reset to "0 km/h" when the ignition is switched on. This feature ensures that no unknown speed is stored in the memory when the vehicle is started again, for instance, after a change of drivers.

The control module continually monitors vehicle speed, with its processor recognizing and evaluating every variation.

When the pushbutton switch is used to change the vehicle's speed, the processor in the control module ensures that the specified acceleration rates of 0.7 m/s2 for "Accelerate/Set", or 1 m/s2 for "Memory" are maintained.

When the brakes are applied, the current supply to the actuator's electromagnetic clutch is interrupted. At the same time, a return spring retracts a gear within the actuator's engagement mechanism, to ensure that positive engagements ceases immediately. The return springs bring the connecting rod back to the idle mode. Should, for example, a vehicle with a faulty stop lamp switch be decelerated at a rate exceeding 1.5 m/s2, the control module's processor will recognize this condition and respond by interrupting the flow of current to the electromagnetic clutch, disengaging the actuator for the duration of the braking maneuver.

The amount of time which elapses until cut-off depends upon the specific rate of deceleration.

If, for any reason, the programmed speed should be exceeded by more than approx. 6 km/h, a response is triggered at the speed monitor. The current to the actuator's electromagnetic dutch is switched off, the unit disengages and the connecting rod returns to the idle position. If the vehicle speed then drops below the threshold of approx. 6 km/h, i.e. the vehicle exceeds the programmed speed by only 5 km/h, the electromagnetic clutch is reactivated and the actuator engages again.

The control module monitors the operation of the actuator's electromagnetic clutch during all braking maneuvers.

Should a defect in the clutch unit prevent the actuator from disengaging normally, the control module tells the actuator to return the connecting rod to the idle position. The control module records this condition and deactivates the system until the ignition is again switched off.

When the ignition is subsequently switched back on, the system will function normally until such time as the brakes are applied again, at which time the defective electromagnetic dutch will once again deactivate the system.

The control module incorporates a lockout function to prevent two commands from being entered at once.

For instance, if a defect at the pushbutton switch results in simultaneous entry of the commands for "Accelerate" and "Memory", the control module responds by switching off the system, which remains inoperative until the ignition is switched off and then on again, cancelling the lockout.

The cruise control consists of the following components, which are combined in a single electrical circuit:

Pushbutton Switch:

Press position "1" or "2" = Set speed
Hold position "1" = Increase set speed
Hold position "2" = Reduce set speed
Press position "3" = Switch off cruise control
Press position "4" = At Speeds above approx. 40 km/h, vehicle returns to the speed that was programmed before the unit was switched off.

Control Module, Reference Resistor:

The cruise control (CC) control module with reference resistor (N4/2) compares the actual and the specified speed. It reacts to deviations from the specified speed, by transmitting the appropriate control pulses to the cruise control (CC) actuator (M16) to bring the specified and actual speeds back into agreement.

The cruise control reference resistor (R29) serves to adapt the cruise control control module (N4/2) to the various vehicle models.

Vehicle Speed Signal:

The cruise control control module receives the vehicle speed signal for comparing the actual and specified speed from the ABS control module.

The cruise control control module is connected to this via the connector, rear speed signal (1-pin) (X62/13).

The connector, rear speed signal (1-pin) (X62/13) is located in the cable conduit, footwell, front right.

Actuator:

The cruise control (CC) actuator (M16) receives its control impulses from the control module and actuates the throttle control by way of connecting rods. It consists of an electric motor with gear drive, a one-way clutch, a potentiometer and an electromagnetic clutch.
The actuator's electric motor transmits motion to the output shaft with the one-way clutch via the gear unit.

The one-way clutch makes it possible to over-ride the activated cruise control, e.g. when using the accelerator pedal to increase vehicle speed during overtaking maneuvers.

The potentiometer monitors the output shaft's position for the control module.

The electromagnetic clutch controls the gear providing positive engagement between the electric motor and the output shaft.

The actuator's electromagnetic clutch is activated at the following switch positions:
Accelerate - Set
Decelerate - Set
Memory

The electromagnetic clutch is deactivated when the brakes are applied, as well as when the control rod assumes the idle position after the unit is switched off with the pushbutton switch.

When the electromagnetic clutch is deactivated, a return spring pulls back a gear to immediately disengage the mechanism.