Voltage is supplied to the preheater plugs via two preheater relays. Both relays are activated by the DDE master control unit.
The heater plugs are designed as sheathed-element glow plugs. The cylinders are allocated to the two relays as follows:
The preheating duration is dependent on the coolant temperature and battery voltage and is indicated by the preheater/DDE indicator.
At temperatures below 5 °C, preheating takes place for at least 4 seconds. The preheating time increases as the temperature decreases.
After engine start, after-heating takes place at a temperature below 51 °C in order to improve idling and exhaust gas characteristics. The duration is dependent on the characteristic curve, e.g. 80 s after-heating at 20 °C.
If, after the preheating time has elapsed, the engine is not started and the ignition remains switched on, start preheating takes place for a maximum of 6 seconds.
The master DDE control unit itself detects a short/break in relay activation (type of fault "activation short-circuit ...", "activation break ..."). If there is a defect in the preheater plugs, a preheater relay or in the voltage supply of a preheater relay, the DDE master control unit receives an error message via the S_GRS1 or S_GRS2 signals from the respective preheater relay (type of fault "preheater system fault"). In the event of a defect, fault codes 3505 "preheating system 1" or 3506 "preheating system 2" are stored.