Cruise Control Description and Operation Adaptive Cruise Control (ACC)

The adaptive cruise control system (ACC) is an enhanced cruise control system with the ability to sense and react to forward traffic. Compared to the common cruise control system found on many vehicles, the main functional enhancement of the ACC system is the ability to detect the presence of a vehicle in the path of the ACC vehicle. The ACC retains the existing cruise control feature that controls the vehicle speed to the driver selected speed. However, ACC allows a driver to set and maintain a following distance to the preceding nearest vehicle in the ACC vehicle path. The distance sensing cruise control (DSCC) module calculates a follow speed limit to ensure an acceptable distance is maintained to the preceding vehicle in front, should one be present. The ACC system automatically adjusts the speed of an ACC vehicle when the vehicle comes up behind a slower travelling vehicle in front. The ACC system applies limited automatic braking and throttle control without driver input when necessary to maintain the set following distance of the ACC vehicle. The preceding vehicle's speed and acceleration along with the ACC vehicle speed and acceleration and the distance between the two vehicles are factors used by the distance sensing cruise control (DSCC) module to determine the ACC follow speed limit. The ACC vehicle speeds up to the original driver selected set speed when the pathway becomes clear without driver input.

The adaptive cruise control system (ACC) depends on various modules on the vehicle to function and each module performs a function that is critical to the proper operation of the ACC system. ACC will not operate if any components fail. Communication between modules is via the High-Speed general motors local area network (GMLAN) and the class 2 serial data circuit. The body control module (BCM) provides a translating gateway function between the class 2 and the GMLAN data links. The GMLAN utilizes the controller area network (CAN) communications protocol. The following is a functional description of the distance sensing cruise control (DSCC) module and the other associated components:

Distance Sensing Cruise Control (DSCC) Module

The distance sensing cruise control (DSCC) module contains the forward looking sensor (FLS) radar and the DSCC controller. The DSCC module utilizes the FLS radar to identify and classify objects in the road environment. The FLS radar scans the road environment to detect targets within its specified field of view. The DSCC controller then makes throttle and/or brake commands to the engine control module (ECM) and electronic brake control module (EBCM) via GMLAN serial data circuit to control vehicle acceleration/deceleration based on data from the FLS radar. The ECM and EBCM provide throttle control and automatic braking needed for proper cruise speed adjustment. An internal yaw rate sensor is used by the DSCC module to determine the predicted path of the ACC vehicle. The main power feed for the DSCC module is the battery and this signal is always at battery potential. However, the ACC system only activates when the ignition 1 input is active and in the RUN position. The ignition 1 voltage signal to the DSCC module has to be 9.2 volts or more to be considered active by the DSCC module. When the ignition 1 signal is not active, all ACC functions are disabled. The following is a list of the DSCC module functions:

Body Control Module (BCM)

The following are the ACC system functions provided by the BCM:

Engine Control Module (ECM)

The following are the ACC system functions performed by the ECM:

Antilock Brake System (ABS) Module

The following are the ACC system functions performed by the ABS module:

Refer to ABS Description and Operation in Antilock Brake System.

Transmission Control Module (TCM)

The TCM provides a down shift to protect the brakes. When the TCM detects some driver braking activity or ACC automatic braking activity, the TCM down shifts the vehicle transmission to increase engine braking. This helps reduce the ABS braking activity, extend brake pad life and reduce brake overheating.

Driver Information Center (DIC)

The DIC is a part of the instrument panel cluster (IPC) and displays the ACC system warning messages. The DSCC module requests messages to be displayed on the DIC by sending a GMLAN request to the BCM. The BCM sends a class 2 serial data request to the DIC demanding the display of the warning message. When the message is acknowledged by the driver and the cause of the message resolved, the DIC turns the message off. These displays are accompanied by a chime warning internal to the DIC. The ACC system will not operate if the DIC fails. The following are the three ACC messages that can be displayed on the DIC:

Head Up Display (HUD)

The HUD displays ACC system telltales and status information. The DSCC module sends a message to the BCM via the GMLAN serial data circuit requesting the display of a HUD telltale. The HUD displays the telltale when it receives a class 2 message from the BCM demanding illumination of the telltale. Feedback to a driver action takes precedence over other status information sent to the HUD by the BCM. The HUD is enabled in power mode RUN. The driver may have to adjust the position of the HUD or the intensity of the display to see the information being displayed. The ACC system will not operate if the HUD fails. The HUD displays the following telltales and ACC status indicators:

Radio Amplifier

The amplifier receives audible warning commands via a class 2 message from the BCM. The chime alerts the vehicle driver to certain ACC vehicle conditions. The audible chime accompanies a visual indicator as a warning to the driver. The ACC system will not operate if the radio amplifier fails.

The Brake Pedal Position (BPP) Sensor

The brake pedal position (BPP) sensor helps the BCM determine when pressure is being applied to the brake by the driver stepping on the brake pedal. The BPP sensor receives a low reference signal and a 5 volt reference from the BCM. When the brake is applied, a signal is applied from the BPP sensor through the BPP sensor signal circuit to the BCM. The BCM utilizes this signal in deciding when to command the activation of the stop lamps. The BCM continuously sends signals via the GMLAN serial data circuit to the EBCM and the ECM reflecting the brake pedal position status. This will include the initial brake travel signal required for the activation of the adaptive cruise control (ACC) system. The ACC system will not engage unless the initial brake pedal travel signal is valid and received by the BCM. When the BCM brake pedal position signal indicates that brake pedal pressure is being applied by the driver when the ACC system is engaged, the ECM immediately disengages the ACC system. Re-engaging the ACC system will require the driver pressing the set or resume switch. Refer to Brake Pedal Position Sensor Calibration and Exterior Lighting Systems Description and Operation in Lighting Systems for more information on the brake pedal pressure sensor.

Accelerator Pedal

Deceleration of the ACC vehicle to maintain a certain distance and speed while a slower moving vehicle is in its path is achieved through throttle control by the ECM and the application of light automatic braking by the ABS module. During automatic braking the ECM is commanded to release the throttle by the DSCC module. The DSCC module via the GMLAN serial data circuit requests the ECM to release the throttle from the currently controlled cruise position. At the end of automatic braking, the ECM will control the vehicle speed to the current set vehicle speed. Driver throttle override occurs when the driver of the ACC vehicle depresses the accelerator pedal with the intention of requesting greater speed than the current throttle position. An example is an ACC vehicle driver stepping on the accelerator for a passing maneuver. The ECM continuously monitors the throttle and reports a throttle override condition. When no throttle override is present, automatic braking is allowed. When throttle override is detected by the ECM, automatic braking in not allowed to affect the vehicle acceleration.

The cruise control switch, functionally is a common feature that is shared between the adaptive cruise control system (ACC) and the regular cruise control system. The ACC system will not operate if any cruise switch fails. The cruise control switch comprises the following cruise control function switches:

The cruise control function switches are arranged in a resistive ladder design whereby each switch function is set up with different resistance values. The body control module (BCM) through the cruise control switch signal circuit detects a predetermined voltage value when any cruise control switch function is activated. The associated cruise control function signal detected by the BCM is then sent to the engine control module (ECM) as a GMLAN serial data circuit message. The ECM on receiving the message provides the cruise control function requested by the BCM. The ECM is responsible for recognizing and responding to cruise control switch requests sent by the BCM. The cruise control function switches are used by the ECM to communicate to the distance sensing cruise control (DSCC) module the driver selected vehicle speed. The driver selected vehicle speed is communicated through GMLAN serial data circuit to the DSCC module and the BCM. The ACC system engages and adjusts vehicle speeds based on the activation of the following cruise control function switches:

On/Off Switch

The BCM monitors the On/Off switch status and makes a determination that the On/Off switch is activated based on a predetermined voltage value at the cruise control switch signal circuit. The On/Off switch state is then relayed to the ECM via the GMLAN serial data circuit. The ECM will recognize and communicate the On/Off switch input to the DSCC module. When the On/Off switch is turned ON, it will cause the ACC system to enter either STANDBY ENABLED or STANDBY DISABLED state. The STANDBY ENABLED state indicates that every condition required for the ACC system to function have been met but the ACC is not yet engaged. When in the STANDBY DISABLED state, all conditions necessary for the ACC to function has not been met. When the On/Off switch is turned OFF, ACC will enter the DISABLED state. ACC will not be active in the DISABLED state. When automatic braking is active and the ACC On/Off switch is turned OFF, ACC will delay entering the DISABLED state.

The Set/Decrease Switch

The ACC system is engaged when the vehicle driver presses and releases the set/decrease switch. The engaged state of the ACC system can only be entered when the On/Off switch is turned ON. When the set/decrease switch is pressed, the driver selected speed is set to the current vehicle speed by the ECM. The vehicle speed must be above 40 km/h (25 mph). The current driver selected speed is displayed by the HUD. While in the engaged state, the driver selected vehicle speed and following distance can be adjusted. Pressing the set/decrease switch when the ACC system is engaged allows the driver to decrease the driver selected vehicle speed. Pressing and holding the set/decrease switch will allow the vehicle to decelerate from the current set vehicle speed without deactivating ACC. The Tap-Down function occurs by momentarily pressing and releasing the set/decrease switch when ACC is engaged. With the Tap-Down function, the driver selected speed is decreased by 1 km/h (1 mph) each time the set/decrease switch is pressed.

The Resume/Increase Switch

When in the resume/increase state, ACC is active and the previously set driver selected speed has not been cleared. The resume/increase switch is used to increase the driver selected speed when ACC is active. The extent to which the driver selected speed can be increased from the resume/increase switch depends on how long the switch is pressed. The presence of a slower moving vehicle in the path of the ACC vehicle will limit the extent to which the driver selected speed can be achieved. If there is no preceding vehicle in front limiting the ACC vehicle acceleration, the current vehicle speed attained is the new driver selected speed. The current driver selected speed is displayed by the HUD. Acceleration is terminated when the resume/increase switch is released. Momentarily pressing and releasing the resume/increase switch will result in the Tap-Up function. With the Tap-Up function, the driver selected speed is increased in increments of 1 km/h (1 mph).

The gap up/down switch allows the driver to determine how closely the adaptive cruise control system (ACC) vehicle follows a target vehicle while ACC is engaged. When the ACC vehicle speed is being limited due to a slower travelling vehicle, the ACC vehicle speed is automatically controlled to the follow speed limit. The gap switch has six following distance selections that range from 1 second to 2 seconds. The gap switch following distance between the ACC vehicle and the target vehicle is expressed in time as opposed to actual distance. The distance maintained for a selected gap will vary based on vehicle speed. The faster the vehicle speed, the further back you will follow. The gap up/down switch is hard-wired to the body control module (BCM) and voltage is provided to the switch from the BCM via the gap up/down ignition 1 voltage circuit. Based on voltage variations, the BCM is able to read the selected gap up/down switch selection and communicates the switch status on the GMLAN serial data circuit to the distance sensing cruise control (DSCC) module. The gap up/down switch is a three position momentary switch - Center, Gap Up and Gap Down - with the following valid signal values:

The initial push of the gap switch recalls the current setting and activates the display. Subsequent pushes of the gap switch will change the gap setting. Momentarily pressing the gap up/down switch will adjust the switch to the desired gap setting. When the voltage from the gap up/down switch circuit is in an invalid range, the BCM will default to increasing the gap value.

SERVICE RADAR CRUISE

The distance sensing cruise control (DSCC) module commands the display of the SERVICE RADAR CRUISE message in the driver information center (DIC) when it detects a malfunction in the adaptive cruise control system (ACC) system. The DSCC module sends a request to the BCM via the GMLAN serial data circuit to display the SERVICE RADAR CRUISE message in the DIC. The DIC displays the SERVICE RADAR CRUISE message when it receives a class 2 serial data request from the body control module (BCM) . The SERVICE RADAR CRUISE is displayed in the DIC only when a diagnostic condition is present, the cruise switch is in On position and the vehicle speed is above 40 km/h (25 mph). When the message is displayed in the DIC the ACC system disengages. The DIC sounds an internal chime anytime it displays the SERVICE RADAR CRUISE message.

CRUISE NOT READY

The DSCC module commands the display of the CRUISE NOT READY message in the DIC when ACC cannot engage due to a temporary condition. The ACC vehicle conditions that prompt the display of the CRUISE NOT READY message are temporary and do not require service. If no target is identified by the FLS radar at the time of vehicle ignition, the CRUISE NOT READY message will be displayed in the DIC until the forward looking sensor (FLS) radar is able to identify a target. CRUISE NOT READY message is also displayed in the DIC when the DSCC transceiver is too hot and the brakes are too hot. The DIC sounds an internal chime anytime it displays the CRUISE NOT READY message.

CLEAN RADAR

The CLEAN RADAR message displays in the DIC when the DSCC module detects a blockage of the forward looking sensor (FLS) radar. The FLS radar may be blocked by snow, ice, dirt, heavy rain or road spray. In rare instances, the target characteristics or a vertically misaligned FLS radar may trigger the activation of the CLEAN RADAR message in the DIC. The DSCC module commands the display of the CLEAN RADAR message via a GMLAN serial data circuit request to the BCM. The DIC displays the CLEAN RADAR message when it receives a class 2 serial data request from the BCM. When the CLEAN RADAR message is displayed in the DIC, safely park the vehicle and turn OFF the ignition. Clean the driver side front fascia lens to eliminate the blockage. It may be necessary to clean both the outside surface of the lens, the inside surface of the lens and the DSCC module surface. Refer to Distance Sensing Cruise Control Maintenance for more cleaning information. It is important to turn the ignition OFF before cleaning any of the named surfaces. This is because the CLEAN RADAR message may still display in the DIC after cleaning the three surfaces. Cycling the ignition to OFF for 10 seconds is necessary to clear the CLEAN RADAR indicator if the indicator remains ON in the DIC. The DIC sounds an internal chime anytime it displays the CLEAN RADAR message.

Cruise Engaged Indicator

The ACC engaged indicator is displayed in the head up display (HUD) when the ACC is engaged by pressing the set/decrease switch. When the set/decrease switch is pressed by the driver, the BCM forwards the switch state to the engine control module (ECM) over the GMLAN serial data circuit. The ECM will use the switch input information from the BCM to determine the driver intended vehicle speed. The ECM communicates to the DSCC module over the GMLAN serial data circuit the ACC engaged state and the driver selected speed. When the DSCC module receives the ECM message indicating that ACC is active, the DSCC module commands the display of the ACC engaged indicator via a GMLAN serial data message to the BCM. The HUD displays the ACC engaged indicator when it receives a class 2 serial data message from the BCM requesting the display. The driver may have to adjust the position of the HUD or the intensity of the display to see the information being displayed properly.

Set Speed

The ACC vehicle set speed is displayed at all times in the primary or secondary display of the HUD when ACC is engaged. The set speed is displayed in the primary HUD display for a few seconds after the initial engagement of ACC or a change in the set speed has just occurred. When, the set speed display time in the primary HUD display elapses, the set speed display is moved to the secondary HUD display. The set speed is also moved to the secondary display when a new ACC related message needs to be displayed. The DSCC module set speed display request is sent to the BCM via the GMLAN serial data circuit. The HUD displays the set speed when it receives a class 2 message from the BCM requesting the set speed displayed in the HUD. The HUD controls when the set speed is displayed in the primary or secondary HUD display.

Follow Distance

The HUD displays the driver selected following distance when ACC is engaged and the gap up/down switch is active. The current follow distance setting is displayed in the HUD for a few seconds after the gap up/down switch is pressed to increase or decrease the following distance. The DSCC module follow distance display request is sent to the BCM via the GMLAN serial data circuit. The HUD displays the follow distance when it receives a class 2 message from the BCM requesting the follow distance displayed in the HUD. The HUD will not display the follow distance if an indicator with a higher priority needs to be displayed in the HUD.

CRUISE SPEED LIMITED

The CRUISE SPEED LIMITED indicator displays in the HUD when the DSCC module detects that the ACC vehicle is travelling at a speed outside the operational range of the FLS radar and the vehicle speed is being reduced below the driver set speed. The DSCC module limits the vehicle speed in order to stay within the acceptable speed limitations of the FLS radar. The DSCC module also limits the vehicle speed in order to adjust the vehicle speed for the chosen gap switch setting. The high speed limit of the ACC system is 186.7 km/h (116 mph). The DSCC module requests the display of the CRUISE SPEED LIMITED indicator via a GMLAN serial data message to the BCM. The HUD displays the CRUISE SPEED LIMITED indicator when it receives a class 2 serial data message from the BCM. With a maximum following distance setting, the maximum operating speed will be limited to 160.9 km/h (100 mph). Selecting a smaller following distance will allow greater operating speeds up to a maximum of 186.7 km/h (116 mph).

Vehicle Ahead Indicator

The vehicle ahead indicator is displayed in the HUD when the FLS radar identifies an in-path vehicle. The vehicle ahead indicator is a warning to the driver that a vehicle is ahead within a distance of 100 meters (109.3 yards). The indicator also serves as a feedback to the driver that the FLS radar is functioning properly. The DSCC module commands the display of the vehicle ahead indicator via a GMLAN serial data message to the BCM. The HUD displays the vehicle ahead indicator when it receives a class 2 serial data message from the BCM. The vehicle ahead indicator only displays with ACC active and may sometimes display for stationary road objects.

TIGHT CURVE

The TIGHT CURVE indicator is displayed in the HUD when the DSCC module reduces the vehicle speed for a curve in the road. The tight curve situation exists when the curve of the road extends outside the field of view of the FLS radar such that the FLS radar cannot see 100 meters (109.3 yards) ahead. The degree to which the vehicle speed is reduced depends on the extent the curve on the road reduces the usable range of the FLS radar field of view. In a tight curve situation, the ACC vehicle speed may be reduced even when there is a target vehicle in the path of the ACC vehicle. The DSCC module uses an internal algorithm to control the vehicle speed to the radius of the curvature of the road. The DSCC requests the display of the TIGHT CURVE indicator via a GMLAN serial data message to the BCM. The HUD displays the TIGHT CURVE indicator when it receives a class 2 serial data message from the BCM. Since the TIGHT CURVE indicator is only displayed when the vehicle speed is reduced for a curve, the driver may notice a delay between entering the curve and the display of the TIGHT CURVE indicator.

Driver Alert

The driver alert indicator warns the driver of a certain road or vehicle conditions that require his attention. When the driver alert indicator flashes in the HUD it is accompanied by an audible warning from the audio amplifier. The driver alert indicator flashes in the HUD and the audible warning sounds when one of the following occurs:

The DSCC module requests the driver alert indicator and the audible warning via a GMLAN serial data message to the BCM. The BCM then sends a class 2 message to the HUD and to the audio amplifier requesting the driver alert indicator and the audible warning to be activated.

The adaptive cruise control (ACC) system will engage and adjust vehicle speeds based on the activation of the following cruise switches:

ACC will not engage if any inhibit criteria is active. Also, the driver must step on the brakes at each ignition cycle before attempting to engage ACC. The engine control module (ECM) looks to the body control module (BCM) for the brake initial travel achieved signal indicating that the driver has stepped on the brakes before attempting to engage ACC.

The vehicle speed must be above 40 km/h (25 mph) and below 188.3 km/h (117 mph ) for the ACC system to be engaged. In the engaged state, the ACC system allows the driver to override the controlled vehicle speed temporarily while still remaining in the engaged state.

DSCC Module Inhibits

The distance sensing cruise control (DSCC) module inhibits the adaptive cruise control (ACC) system when any of the following conditions exists:

ECM Inhibits

The engine control module (ECM) inhibits the ACC system operation when any of the following conditions exists:

The adaptive cruise control (ACC) system disengages when any of the following occurs:

Pressing the brake pedal while ACC is engaged will disengage the ACC system.

Automatic Alignment

The adaptive cruise control (ACC) system is equipped with an internal forward looking sensor (FLS) radar automatic alignment feature. For proper operation of the ACC system, the FLS radar must be well aligned to the vehicle. The FLS radar alignment to the vehicle is continuously corrected and adjusted as necessary during the ACC vehicle operation. To achieve a complete automatic FLS radar alignment, the ACC vehicle has to be driven on a road with stationary objects like mail boxes, road signs, trees, ramps and poles etc. The distance that the vehicle needs to be driven to achieve a complete automatic alignment, depends on the number of stationary objects encountered on the road that the vehicle is travelling. The more stationary objects encountered, the quicker the FLS radar will achieve automatic alignment. When the distance sensing cruise control (DSCC) module is replaced with a new module, it is necessary for the vehicle to be driven until the internal automatic alignment is complete. The scan tool is equipped to display the automatic alignment status and the degree of alignment error or skew of the FLS radar. A DTC C1002 sets in the DSCC module when the degree of error or skew is 3.9 degrees or over.

It is important when performing a wheel alignment service on a vehicle equipped with the ACC system, to change the Auto Alignment Learn Status parameter in the DSCC module scan tool data list to Not Aligned. The Auto Alignment Learn Status parameter of the DSCC module needs to read Not Aligned to allow the FLS radar to automatically align to the vehicles new wheel alignment setting. Driving the vehicle in an area with stationary objects will cause the FLS radar to automatically align to the vehicles new wheel alignment setting.

Mechanical Alignment

When the alignment of the FLS radar is not achieved by automatic alignment and an out of alignment diagnostic, DTC C1002 is set by the DSCC module. At this point, it becomes necessary to mechanically align the FLS radar using the special alignment tool. The typical alignment value of the DSCC module is 0° +/- 2°. The DSCC module diagnostic, DTC C1002 sets when 3.9 degrees of skew or error of the FLS radar is present. Refer to Distance Sensing Cruise Control Aiming for information on how to aim and align the FLS radar. An out of alignment condition may be as a result of tampering or damage to the DSCC module mounting structures or the adjuster settings. An out of alignment condition may also be as a result of damage to the front end of the vehicle or merely as a result of wear and tear . Refer to Distance Sensing Cruise Control Module Replacement and Distance Sensing Cruise Control Bracket Replacement on how to properly mount and adjust the DSCC module. Performance of the ACC system may be degraded after a DSCC module replacement. Normal vehicle driving will correct the performance of the ACC system.