Auto Electronics

Under the Hood of the 2005 Mercedes Benz SLK350 Roadster

This month, WAE looks under the hood and all the rest of the sheet metal of a vehicle that has four in-vehicle networks. The Mercedes-Benz SLK350 Roadster has a high-speed and low-speed CAN bus, a LIN bus and a fiber-optic MOST bus.

The dual-wire, high-speed CAN bus, which operates at speeds up to Mbits per second, provides the communication link for powertrain control and other high-speed real-time control systems. The electronic throttle must communicate with the engine management system for integrated sequential multipoint fuel injection and ignition control of individual cylinders for spark timing and knock. In addition, communication is required between the optional seven-speed automatic transmission that provides electronically controlled shifting.

A low-speed CAN bus, which operates at speeds up to 125 kbits/s, connects central body control functions in the car. A LIN bus, which operates at speeds up to 20 kbits/s provides a subnetwork for CAN and is used for mirrors, doors and window motors. These networks work together to control standard and optional features linking sensors and switches to provide control and power functions.

From the power side, the power front side windows operate with a one-touch express-up/down. When the power top is activated, the same button lowers or raises all four windows. The top can be remotely activated by an infrared control to open or close the roof of the SLK. The memory system retains settings for the eight-way power seats with three-position memory, the four-way power steering column, and the side mirrors. The SLK-350 even has a rest mode to ventilate the passenger compartment or help keep it warmer using residual heat for up to 30 minutes after car is turned off.

From the sensor standpoint, a low-tire-pressure sensor alerts the driver to any loss of pressure in the tires via the SLK350's multifunction display. The display also indicates when the heated front seats are activated. The digital climate control provides independent left and right controls for temperature and airflow, plus automatic digital control of fan speed and air distribution. The system's charcoal filter can sense an increase in smog and automatically shut off incoming air. Finally, an infrared sensor detects the amount of rainfall on the windshield for automatic adjustment of the frequency of the intermittent wipers.

The MOST network uses a fiber-optics bus to operate at speeds up to 25 Mbits/s for optimized transmission of audio systems. The optional entertainment package has a Harman/Kardon Logic7 premium sound system. The quality of the fiber-optically linked system is enhanced by monitoring the sound in the cabin and compensating for unwanted noise and digital dynamic volume control that provides sound balance under all driving conditions.

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