Radio reception

The following control modules are involved in the radio function:

• Antenna tuner with antenna amplifier and diversity unit
  
• Audio system controller
  
• Audio amplifier (HiFi or stereo option) or amplifier (Top HiFi)
  
• Control Display
  

Brief description of components

Rear window antenna

The rear-window antenna is subdivided into a number of reception areas in order to improve reception. Division into left and right areas reflects the arrangement of the amplifiers.

Main functions

Antenna tuner

The antenna tuner is a control module for the radio and TV reception functions.

The antenna tuner has a separate power supply.

• In the AM band, the antenna tuner converts the HF signals from the AM antenna amplified by the diversity unit and feeds the LF signals onto the MOST bus.
  
• In the FM band, the antenna diversity unit connects the antenna with the best FM reception to the tuner. The tuner amplifies the HF signals and converts them. They are passed to the ASK as LF signals.
  

The following are connected to the antenna diversity unit:

• one antenna for the AM (amplitude modulation) band; this is used for amplifier function only;
  
• two antenna circuits (right and left) for the FM band with two antennas each; Signal processing for the two left antennas and amplification is performed by the antenna diversity unit; the right-hand antenna signals are combined into one by the antenna amplifier and passed to the antenna diversity unit as a single signal.
  

The antenna diversity unit selects the best antenna circuit by the following process:

• The antenna tuner checks the strength of the signal being received on each antenna group by continually switching back and forth between the left and right antenna group on the antenna diversity unit.
  
• The antenna diversity unit patches through the antenna with the best reception quality on the basis of the signal received from the antenna tuner.
  That antenna group is used for the next check cycle.
  
• If there is no feedback on the reception signal strength on the antenna diversity unit, e.g. due to a break in the ribbon cable between the antenna tuner and the antenna diversity unit, the antenna diversity unit automatically switches to the left antenna group.
  If either the CONTROL or DIAG signal is lost, no antenna group can be selectively activated through the diagnosis system.
  

Service functions

Checking FM reception

FM reception is best checked by driving around a circular route. The antenna diversity unit connects the antenna with the best reception to the antenna tuner.

Checking the antenna diversity unit

Connect the vehicle to the diagnostic system and select the control-module functions.

• Select a local FM station with good reception.
  
• Select control-module functions and read off the reception characteristics.
  
• Select the left antenna path and read off the reception characteristics.
  
• Select the right antenna path and read off the reception characteristics.
  
• For the remainder of the fault finding process follow the circuit diagram
  

Diagnosis request