Starting heater mode, function

S83.70-2521-06

1 Jacket

2 Coolant inlet

3 Coolant outlet

4 Heater core

5 Burner housing

A34 Hot water auxiliary heater

A34.1 Main wiring harness connector to auxiliary heater

A34.2 Water pump auxiliary heater connector

A87 Hot water auxiliary heater with heater booster function

A87.1 Main wiring harness connector to auxiliary heater with heater booster function

A87.2 Water pump connector auxiliary heater

A72 Auxiliary heater control module

A72.3 Auxiliary heater connector in electric compartment

A74 Auxiliary heater control module with heater booster function

A74.3 Auxiliary heater connector with heater booster function in electric compartment

B94 Coolant temperature switch

B95 Overheating fuse

B96 Flame monitor

M19 Auxiliary heater fuel metering pump

M44 Combustion air blower

R18 Glow plug

M54 Heater booster fuel metering pump

Function sequence

1 The heating mode can be started with the timer (A7) for the hot water auxiliary heater (A34/A87) when the engine is switched off. A signal is transferred from the timer (A7) to the control module (A72/A74).

2 The combustion air blower (M44) in the heater and the heater auxiliary pump (M10/M45) are actuated by the control module (A72/A74) and start running. The combustion air blower (M44) operates at "partial load"
at its lowest output.

3 The control module (A72/A74) switches on the heater blower (M40) via the air conditioning pushbutton control module (A20) or the auxiliary heater relay (K29) at a coolant temperature of > 30 °C. The control module (A72/A74) receives this temperature signal from the temperature switch (B94).

4 The flame monitor (B96) is checked by the control module (A72/A74) for the hot water auxiliary heater (A34/A87). The flame monitor signals:

"cold" = no flame, start-up is continued

"hot" =flame, start interrupted due to defective flame monitor (B96) (malfunction shut-off).

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

6 When the preglowing time is completed, the fuel metering pump (M19/M45) starts pumping fuel. The fuel and the combustion air form a fuel/air mixture in the burner housing.

7 The safety time (approx. 90 s) starts for the flame formation. The fuel/air mixture is ignited by the glow plug (R18), the flame monitor (B96) monitors the starting combustion.

8 When the safety time is completed, the flame monitor indicates:

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

"cold" = no or insufficient flame formation, the fuel metering pump (M19/M45) is switched off and the start-up operation is repeated once.

9 The stabilization time (approx. 20 s) for the combustion starts and the fuel metering pump (M19/M45) is switched to "partial load" by the control module (A72/A74). The combustion has stabilized.

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Function sequence (continuation)

When starting up the heater it is necessary to distinguish between three function sequences following the stabilization time corresponding to the coolant temperature present:

 Heating mode started, coolant temperature < 79 °C

 Heating mode started, coolant temperature 79 to 86 °C

 Heating mode started, coolant temperature > 86 °C

10a) Heating mode started, coolant temperature < 79 °C

If the coolant temperature is below 79 °C and the heat requirement is greater than the momentary heater output the combustion air blower (M44) and the fuel metering pump (M19/M45) are switched to "full load" by the control module (A72/A74). The combustion air blower (M44) operates at its highest speed.

The heater core starts to exchange heat and the coolant temperature increases.

10b) Heating mode started, coolant temperature between 79
and 86 °C

If the coolant temperature is between 79 °C and 86 °C and the heat requirement is less than the momentary heater output the combustion air blower (M44) and the fuel metering pump (M19/M45) are switched to "partial load" by the control module (A72/A74).

The coolant temperature remains either constant or drops with increasing heat requirement.

10c) Heating mode started, coolant temperature > 86 °C

 If the coolant temperature is above 86 °C and the heat requirement is less than the momentary heater output or there is no heat requirement at all, the fuel metering pump (M19/M45) is switched off by the control module (A72/A74) and the combustion is interrupted.

 The shut-off delay for cooling the heater and purging the combustion chamber starts for approx. 120 s. The combustion air blower (M44) runs for approx. 25 s at partial load and for approx. 95 s at full load.

 The shut-off delay is completed, the combustion air blower (M44) is switched off, the heater auxiliary pump (M10/M45) and the heater blower (M10) continue to run.

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

 When the coolant has reached a certain temperature (approx. 77 °C) the hot water auxiliary heater (A34/A87) is automatically switched to the heating mode. The heating mode starts with the preglowing time (approx. 45 s), then the safety time and stabilization time (approx. 20 s).

 On the D 5W heater (diesel) the glow plug continues to receive power for a certain time (approx. 20 s) during the shut-off delay period. This prevents white smoke during combustion.