The voltage for the preheater plugs is supplied via the preheater relay. The relay is switched by the DDE control unit.
The heater plugs are designed as sheathed-element glow plugs.
The preheating duration is dependent on the engine coolant temperature and battery voltage and is indicated by the preheating/DDE indicator.
At temperatures below 5 °C, preheating takes place for at least 4 seconds. The preheating time increases as the temperature decreases.
After-heating takes place after engine start at temperatures below 20 °C in order to improve idle and exhaust characteristics.
Start preheating takes place for approx. 10 seconds if after the preheating time has elapsed the engine is not started and the ignition remain on.
The DDE control unit itself detects a short/break in relay activation (type of fault "activation short-circuit ..." or "activation break ..."). The DDE control unit receives a fault signal from the preheater relay in the form of the S_GRS signal if there is a fault in the preheater plugs, preheater relay or in the voltage supply of the relay (type of fault "preheater system fault"). Fault code 3505 "Preheating system" is stored in both cases.