Startup of heater booster operation, function

S83.70-2525-06

1 Casing

2 Coolant inlet

3 Coolant outlet

4 Heat exchanger

5 Burner housing

A90 Heater booster

A90.1 Connector for main wiring harness on heater booster

A75 Heater booster control module

A75.3 Connector for heater booster in electrics box

B94 Coolant temperature switch

B95 Overheating protection device

B96 Flame sensor

M44 Combustion air blower

M54 Fuel metering pump for heater booster

R18 Glow plug

Function sequence

1 The heater booster (A90) is activated via the timer (accessory) or manually using the switch on the heater control panel.

2 The combustion air blower (M44) in the heater is actuated by the control module (A75) and starts up. In "part load mode" the combustion air blower (M44) delivers with its lowest output.

3 The interrogation of the flame monitor (B96) is started in the control module (A75) of the heater booster (A90). The flame monitor reports:

"cold" = no flame, starting is continued

"hot" = flame, starting is aborted due to a defect in the flame monitor (B96) (fault shutdown).

4 The preheating time of approx. 45 s starts. The glow plug (R18) is switched on.

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5 If the preheating time has finished, the fuel metering pump (M54) delivers the fuel. The fuel and the combustion air form the fuel/air mixture in the combustor housing.

6 The safety time (approx. 90 seconds) for flame formation commences. The fuel/air mixture is ignited by the glow plug (R18), the flame monitor (B96) monitors the combustion process which has started.

7 If the safety time is over, the flame monitor reports:

"hot" =sufficient flame formation, the glow plug (R18) is switched off

"cold" = no, or insufficient, flame formation the fuel metering pump (M54) is switched off, and starting is repeated once only.

8 The stabilizing time (approx. 20 seconds) for combustion starts and the fuel metering pump (M54) is switched to "part load mode" via the control module (A75). Combustion has stabilized.

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Function sequence (continued)

When the heater operation has started and after the stabilization period, a distinction must be made between three function sequences depending on the existing coolant temperature:

 Start heater operation, coolant temperature < 79 °C

 Start heater operation, coolant temperature 79 to 86 °C

 Start heater operation, coolant temperature > 86 °C

10a) Start heater operation, coolant temperature < 79 °C

If the coolant temperature is below 79 °C and the heat requirement is greater than the instantaneous heat output, the combustion air blower (M44) and the fuel metering pump (M54) is switched to "full load mode" by the control module (A75). The combustion air blower (M44) operates at maximum speed.

The heat exchange process starts up in the heat transfer unit , the coolant temperature increases.

10b) Start heater operation, coolant temperature 79 to 86 °C

If the coolant temperature is between 79 and 86 °C and the heat requirement is less than the instantaneous heat output, the combustion air blower (M44) and the fuel metering pump (M54) continue to be operated in"part load mode" by the control module (A75).

The temperature of the coolant either remains constant or drops as the heat requirement increases.

10 Start heater operation, coolant temperature > 86 °C

 If the coolant temperature is above 86 °C and the heat requirement is less than the instantaneous heat output or there is no heat requirement at all, then the fuel metering pump (M54) is switched off by the control module (A75) and combustion is interrupted.

 The run-on time for cooling the heater and for purging the combustion chamber is started for approx. 120 seconds. The combustion air blower (M44) runs for approx. 25 seconds in part load mode and approx. 95 seconds in full load mode.

 The run-on time has finished and the combustion air blower (M44) is switched off.

 The heater is in a combustion pause and the coolant temperature drops.

 If the coolant has reached a certain temperature (approx. 77 °C), the heater booster (A90) is again switched over automatically into heating mode. Heating mode starts with a preheating time (approx. 45 seconds), the safety time and the stabilization time start (approx. 20 seconds).

For the heater D 5W (diesel), in the run-on period after-glowing continues for a certain period of time (approx. 30 seconds). This after-glowing prevents white smoke during combustion.