Glow Plug System: Description and Operation

GF15.20-P-3005OG Preheat, Function
ENGINE 642.8 in MODEL 204.0 /2 /9, 207.3 /4, 212.0 /2, 218

Function requirements for preheating, general points
^ Circuit 15 ON
^ Preglow system functional

Preheating, general points
The preglow system warms up the engine combustion chambers to the temperature required for ignition of the fuel-air mixture.

At cold temperatures the vent line heater element (R39/1) is actuated by the CDI control unit (N3/9). This prevents the crankcase ventilation system from freezing and therefore associated engine damage.

Function sequence for preheating
The function sequence is subdivided into the following steps:
^ Function sequence for preglowing
^ Function sequence for start-ready glowing
^ Function sequence for start-up glowing
^ Function sequence for afterglowing
^ Function sequence for diagnostic glowing
^ Function sequence for diesel particulate filter (DPF) preheating, with code (474) Particulate filter
^ Function sequence for emergency glowing

Function sequence for preglowing
The CDI control unit receives the message "circuit 15 ON" from the electronic ignition lock control unit (N73) via the chassis CAN (CAN E). The CDI control unit reads in the engine oil temperature via the oil temperature sensor (B1) and then actuates, via the drive LIN (LIN C1), depending on the coolant temperature, the glow output stage (N14/3) and the preglow indicator lamp (A1e16) via the chassis CAN. The glow output stage then actuates the glow plugs (R9) on the positive side.

If an error occurs in the preglow system, at the glow plugs or in the lines, this is shown by the preglow indicator lamp and is stored in the CDI control unit.

Function sequence for start-ready glowing
Start-ready glowing ensures an adequately high temperature until the engine finally begins to run. If the CDI control unit receives the information "circuit 50 ON" from the electronic ignition lock control unit via the chassis CAN, it will continue to actuate the glow output stage for a certain time irrespective of the engine oil temperature, and the glow output stage in turn actuates the glow plugs. After the start-ready temperature of 1250°C has been reached, the preglow indicator lamp goes out.

Function sequence for start-up glowing
Start-up glowing is used to stabilize the cold start engine speed. If the CDI control unit receives the "circuit 61 ON" message from the alternator (G2) over the drive LIN, the glow output stage is actuated over the drive LIN. The glow output stage then actuates the glow plugs. In this way the first working cycles and the eventual running of the engine are supported.

Function sequence for afterglowing
Afterglowing improves engine running after a cold start and the engine warm-up characteristics.
After a successful engine start, the CDI control unit reads in the engine oil temperature via the oil temperature sensor and actuates the glow plugs via the glow output stage depending on the engine oil temperature.

Function sequence for diagnostic glowing
This function is used to energize the glow plugs to a low temperature level and a system diagnosis is performed. It is activated for troubleshooting purposes and when the preglow system has not be activated for a long period (no preheating required owing to high ambient temperature).

Function sequence for DPF glow function, with code (474)
Particulate filter
The DPF glow function is used to raise the load and to stabilize combustion and thus to assist regeneration of the DPF.

To do this the CDI control unit receives the message "circuit 50 ON" via the chassis CAN from the electronic ignition lock control unit. The CDI control unit reacts by actuating the glow output stage.

Function sequence for emergency glowing
If a problem in communication occurs between the CDI control unit and the glow output stage so that the glow output stage does not recognize any activity for 2 s after activation via circuit 87, the emergency glowing function is actuated for 180 s. If communication is missing during a glowing process for more than 250 ms then the glowing process will be continued for 180 s.

If no new glow request is received from the CDI control unit then emergency glowing stops after 180 s.