About I-CAR - Current Events/News: e-newsletters: Advantage Online: February 16, 2004

February 16, 2004 - A new type of speed control, called adaptive cruise control, is being used on some new model vehicles. Adaptive cruise control is basically an extension of conventional cruise control systems. These systems allow you to set a following distance, or time interval, between your vehicle and the vehicle ahead, as well as a maximum speed. This effectively does away with the familiar RESUME setting. You simply set the parameters once and go. When you come up to a slower moving vehicle in front, or if a vehicle changes into your lane, the system automatically closes the throttle and/or applies brake pressure to slow the vehicle and maintain the set following distance. There is no need to press the brake pedal to slow for a vehicle ahead, and then push the RESUME button to continue at the pre-set speed. The adaptive cruise control system is truly “hands-off” and performs these functions automatically. As with conventional speed control systems, manual inputs from the driver, both to the accelerator and brake, take precedence over the adaptive cruise control system.

Applications

Current vehicle makers that are offering adaptive cruise control systems as options on some of their vehicles include, but may not be limited to, Audi (2004 A8), BMW (2003–2004 7 Series and 2004 5 Series), General Motors (2003–2004 Cadillac XLR), Infiniti (2003–2004 Q45 and FX), Jaguar (2003–2004 XKR), Lexus (2001–2004 LS430), and Mercedes-Benz (2000–2004 S-Class and CL-Class, 2003–2004 E-Class and SL-Class).

Figure 1–This is the display just after setting the speed and following distance on the Distronic system on a 2001 Mercedes-Benz CL600.

Figure 2–This is the display on the Mercedes-Benz Distronic system when another vehicle is ahead in the same lane at a distance of about 91 m (300 ft).

Figure 3–The radar signal on this Distronic adaptive cruise control system is narrow enough to only encompass the lane that the vehicle is in. (Courtesy of Mercedes-Benz)

Figure 4–The radar sensor and control unit on this 2004 BMW 745i is located behind the front bumper with the radar sensor visible through an opening in the bumper cover.

Figure 5–This radar sensor on a 2001 Mercedes-Benz CL600 with Distronic adaptive cruise control is behind the Mercedes-Benz star emblem in the center of the grille when the hood is closed.

The Mercedes-Benz adaptive cruise control system is called Distronic. When the Distronic system is set, the instrument panel display shows a digital reading of the set speed. The following distance that is set is shown with an arrow on a graduated scale in front of a vehicle icon (see Figure 1). When another vehicle is in the same lane in front, it is shown as a second vehicle icon, with the distance that it is ahead graphed on the same graduated scale (see Figure 2).

BMW calls their system active cruise control (ACC). With ACC, the driver sets a time interval rather than a distance between vehicles. There are four time intervals that can be set from 1.5–2.8 seconds. The system control module then sets the following distance based on the speed of the vehicle. The faster the vehicle is traveling, the greater the following distance will be for a given time interval.

Operation

So how do these systems work? They have many of the same parts as conventional speed control systems. Added is a radar unit, which is used to determine the distance between the vehicle and other vehicles in front (see Figure 3). The control module is also integrated with other vehicle systems such as anti-lock brakes, traction and stability control systems, and body, engine, and transmission control modules. As with conventional speed control systems, the adaptive cruise control system maintains the vehicle speed at the pre-set amount, as long as the vehicle path is unobstructed. The control module uses information from the radar sensor, vehicle speed sensors, and vehicle steering and yaw rate sensors to determine if the path is obstructed and to adjust vehicle speed and maintain the pre-set following distance from a slower moving vehicle ahead.

The radar may be a separate stand-alone unit, as in the Mercedes-Benz system, or it may be included with the system control module, as in the BMW and Cadillac systems. Because these radar units need an unobstructed line of sight to operate properly, they are located in front, directly in harm’s way in a frontal collision. They may be located under or behind the front bumper (see Figure 4), or behind the grille (see Figure 5).

Collision Repairs

When a radar unit is damaged and requires replacement, alignment of the unit on the vehicle is critical to proper system operation. BMW states that horizontal misalignment of as little as 1° will cause system performance degradation and amounts greater than 1° may cause system malfunction. Alignment of the BMW radar is only possible using a vehicle-specific BMW scan tool and special ACC adjustment tools (BMW part# 81-10-0-021-292).

The radar alignment specification on the 2004 Cadillac XLR is listed as 0° +/- 2°. A diagnostic trouble code (DTC) sets when the radar unit is misaligned by 3.9° or more. If there is misalignment between 2.1° and 3.8°, performance will be affected but the unit will still operate. If the unit misalignment is not severe, the system is capable of automatic alignment. To perform the automatic alignment, a scan tool is required to place the system into the alignment mode. The vehicle must then be driven on a road with stationary objects such as mailboxes, road signs, trees, and poles. The vehicle must be driven until the scan tool displays the alignment procedure as complete. The more stationary objects there are on the road, the quicker the internal automatic alignment is achieved. If the radar unit is too far out of alignment, automatic alignment is not possible, and the out-of-alignment DTC sets. The radar unit then requires manual alignment. To perform the manual alignment or radar aiming procedure, the J-45442 Distance Sensing Cruise Control aiming tool is required. This tool is equipped with a laser and is installed to the front of the radar and control unit. The vehicle needs to be on a level, unobstructed area, 12.8 m (42 ft) by 3.6 m (12 ft) with a vertical wall at one end. Tape or painted lines are used to mark the floor and wall to position the vehicle. A 51 mm (2") circle painted on the wall in the correct location is used as a target for the laser. The horizontal and vertical aiming screws on the back of the radar unit are adjusted until the laser beam is in the center of the target.

Conclusion

While adaptive cruise control systems use many of the same parts as conventional cruise control systems, the addition of the radar unit to the front of the vehicle adds some challenges when repairing vehicles with front-end damage. Service manual information is required to access the proper procedures for alignment and aiming of the radar unit. The necessity of having vehicle-specific scan tools and other special service tools for proper alignment of the radar unit means that many of these vehicles may have to be sublet to a dealership service department to ensure that the cruise control systems function properly.

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