Overvoltage troubleshooting

This document provides the necessary information and instructions for troubleshooting on vehicles where overvoltage has occurred.

In the vehicle network, the control units are configured in such a way that they should withstand short voltage peaks above the maximum operating voltage of 16 V without function limitations. However, if voltage peaks while the vehicle is being driven exceed times of more than 300 ms, typically the following possible sporadic symptoms can be observed by the customer:

• the on-board monitor goes dark.
  
• the interior lighting becomes brighter or flickers.
  
• failure of the ASC/DSC system and ASC+T/DSC indicator and warning lamps light up in the instrument cluster.
  
• Failure of SZL: problems with windscreen wipers, indicators, etc.
  

The possible causes for sporadic occurrences of overvoltage are:

• a load shedding on load-current consumer units or high-current consumer units due to loose contacts.
  
• a defective alternator.
  

Procedure

• Read all fault code memories with a short test and display all stored fault code memory entries ”Overvoltage” with the help of the test module ”Overvoltage troubleshooting”. If present, additional information such as kilometre reading and voltage values are displayed.
  
• Check battery contact at the positive and negative terminals: Are the connection terminals tight enough?
  
• Check earth/ground points and earth connections.
  
• Check plug-in connections and contacts of the high-current consumer units of the vehicle:
  
  • electrical auxiliary heater based on PTC principle.
    
  • combustion preheating control unit
    
  • electric fan.
    
  • other high-current consumer units that might be fitted in the vehicle.
    
• Check the alternator using the alternator diagnosis: function, plug-in connections and contacts, BSD cable, etc.