Glow system

Glow system

The glow system of the N47 4-cylinder diesel engine consists of the following components:

• DDE control unit
  
• Electronic combustion preheating control unit
  
• Ceramic glow plugs
  
• LIN bus as interface between the DDE control unit and combustion preheating control unit
  

The first time the combustion preheating control unit is switched on, the electrical resistance of the glow plugs is evaluated at the start of the glow process. A glow plug that is already hot has a significantly higher resistance than a glow plug that is cold. If the resistance leads to the detection of hot glow plugs, the glow plugs are supplied with less energy at the start of glowing. If, on the other hand, cold glow plugs are detected, the glow plugs are supplied with the maximum energy at the start of glowing. This function is referred to as dynamic repeat glowing. It prevents a glow plug that is already hot from being overheated by a second glow process that follows shortly after the first glow process, thus supplying too much energy.

To check the glow system, there are two adapter leads:

- 1-pin, order number 13 6 470

- 12-pin, order number 13 6 460

Function

The required glow plug temperature is determined by the DDE control unit on the basis of the following operational parameters:

• Engine speed
  
• Intake air temperature
  
• Fuel injection rate
  
• Ambient pressure
  
• Vehicle voltage
  
• Status signal for starter enable
  

To activate glowing, the DDE sends the required glow plug temperature to the combustion preheating control unit.

Across the LIN bus, the DDE also sends the following operating values to the combustion preheating control unit:

• Vehicle voltage
  
• Fuel injection rate per stroke
  
• Engine speed
  

The combustion preheating control unit uses these three operating values to determine the required current for the glow plugs and supplies the glow plugs with current.

Types of glowing

Glowing is activated in various operating states:

• Preglowing:
  Preglowing is activated after switching on terminal 15. Preglowing is terminated when:
  
  • The engine speed threshold of 42 rpm is exceeded (starter motor is operated)
    or
    
  • The preglow time has elapsed. The preglow time depends on the coolant temperature and is specified in a characteristic curve as follows:
    

• Start standby glowing:
  When preglowing has been terminated by the preglow time elapsing, start standby glowing is activated. Start standby glowing is terminated:
  
  • After 10 s of start standby glowing
    or
    
  • When the engine speed threshold of 42 rpm is exceeded.
    

• Start glowing:
  Start glowing is activated at every engine start if the coolant temperature is below 75 °C. Start glowing starts after an engine speed threshold of 42 rpm has been exceeded.
  Start glowing is terminated:
  
  • After the maximum start glow time of 60 s has elapsed
    or
    
  • After termination of the starting operation
    or
    
  • When the coolant temperature exceeds 75 °C.
    
• Emergency glowing:
  In the event of a failure in the communication between the DDE and combustion preheating control unit for more than 1 second, emergency glowing is triggered for 3 minutes. Here, the combustion preheating control unit uses safe values to prevent damage to the glow system.
  
• Hidden glowing:
  Up to a coolant temperature of 30 °C, preglowing and start standby glowing are activated as so-called hidden glowing. Hidden glowing is triggered by the following signals:
  
  • Driver seat occupancy
    
  • Driver belt buckle
    
  • Valid key
    
  • Terminal R
    
  • Brake operated
    
  • Clutch operated
    

Hidden glowing is triggered a maximum of four times and then only enabled again with the engine start.

Communication

The DDE control unit and combustion preheating control unit communicate across the LIN bus.

The LIN bus is a bidirectional data interface based on the master-slave principle. The DDE control unit is the master controller.

Brief description of components

The following components are described for the glow system:

Combustion preheating control unit

The combustion preheating control unit contains the power output stages for activation of the glow plugs. The combustion preheating control unit does not have its own fault code memory. Faults in the glow system that are detected by the combustion preheating control unit are reported to the DDE by the combustion preheating control unit across the LIN bus. The faults are then stored in the DDE fault code memory.

If the permitted operating temperature of the combustion preheating control unit is exceeded, the combustion preheating control unit shuts down any glow process to prevent damage.

Glow plugs

The ceramic glow plugs are designed for a voltage of 7.0 to 10.0 Volts. During glowing, on-board supply voltage can be applied briefly for rapid heating up. To maintain the glow plug temperature, the glow plugs are supplied with a PWM signal. This sets an effective voltage at the glow plugs that is lower than the on-board supply voltage.

Fig. 1: sectional view of ceramic glow plug

Notes for Service department

General information

Important!

The ceramic glow plugs are sensitive to impacts and bending. Glow plugs that are dropped can be damaged.