Rear Window Defogger With Manually Regulated A/C System, Checking

A/C Control Module -J301- activation 

• By pressing the button -E- with the ignition switched on, the Vehicle Electrical System Control Module -J519- is activated (by the A/C control module via the data bus). As soon as the vehicle electrical system control module switches on the rear window defroster (only when the engine is running), the indicator lamp in the button -E- comes on (the vehicle electrical system control module sends the request via the data bus). Rear window heater is switched off by pressing button -E- again and by switching ignition off. Refer to refer to the appropriate SYSTEM WIRING DIAGRAMS information.
  Fig 1: Identifying A/C Control Module

  Courtesy of AUDI OF AMERICA, LLC
• If the voltage measured on the vehicle electrical system control module to terminal 30 is below the valued stored in the vehicle electrical system control module, then the activation of the Rear Window Defogger -Z1- will be switched off completely and this will relieve the load on the generator. Check using Vehicle Diagnosis, Testing and Information System VAS 5051 in "Guided Fault Finding" function.
• The rear window defogger is controlled by the vehicle electrical system control module only when the engine is running. If the engine is not running, there is no control and there is no request from the vehicle electrical system control module. The LED in the button -E- does not come on, or will go out shortly after the button is pressed. Refer to Owner's manual and refer to the appropriate SYSTEM WIRING DIAGRAMS information.
• If rear window defogger must be switched off for a short time by vehicle electrical system control module due to insufficient voltage, indicator lamp in heated rear window button -E- remains switched on, if switch-off is prolonged however, vehicle electrical system control module switches off the indicator lamp.
• Function test of heated rear window activation is described in Guided Fault Finding for vehicle electrical system control module and for A/C system. Use Vehicle Diagnosis, Testing and Information System VAS 5051 in "Guided Fault Finding" function.
• From MY 2010, this vehicle has a Start/Stop System for specific engines as optional equipment. On vehicles with the Start/Stop System, the rear window defogging is switched off whenever the Stop function is active (to protect the battery). The setting for the rear window defogger remains stored. After the engine is started again by the Start function, rear window defogging is switched on again. Check using Vehicle Diagnosis, Testing and Information System VAS 5051 under "Guided Fault Finding".