Webasto DBW 46 Service Manual download pdf (Page 10)

BBW 46 / DBW 46

3 Functional Description

301

3. Functional Description

3.1 Sequence of Operation

3.1.1 Switch On

Upon switch on positive control voltage is applied from the

battery across F2 and terminal A5 to the electronics of the

control unit.

– operating indicator light H1 illuminates.

– glowing is activated via relay K1 and circulation pump

via relay K2.

– after pre-glowing (approx. 30 sec) a cycled voltage is

applied to connector C2 via transistor V108. From

connector C2 dosing pump Y1 is triggered across

temperature fuse F3.

– the safety period of approx. 90 sec commences.

– after fuel priming time (approx. 5 sec) the combustion

air fan motor M1 is started in full load mode via relay

K4.

If there is a "flame" condition during the safety period,

flame sensor B1 signals "bright" and relay K1 de-

energises. The safety period is reset and the glow plug is

switched off. This procedure requires approx. 15 sec for

normal ignition.

Repeat start:

If there is a "no flame" condition during the safety period,

there will be an automatic restart attempt with a 30 sec

run-down involving glowing, pre-glowing (approx. 30 sec)

and safety period (approx. 90 sec). This start-up pattern is

also applicable for a re-activation during controlled

operation.

NOTE

With control units

order no. 105 499 12 Volt

105 507 24 Volt

230 04_ 12 Volt

230 05_ 24 Volt

after switch-on and after a control idle period there will be

a glow plug condition sampling.

Glow plug filament interrupts or fusing defects in the glow

plug power circuit will cause a run-down of 60 seconds

with a subsequent error lockout.

For control units performing glow plug condition sampling

the installation of an external glow cycling relay is not

permitted, as the control unit would interpret a glow plug

open power circuit when removing the voltage.

During start-up and combustion operation also a voltage

sampling is performed. In case of a low voltage which is

9.5 V ± 0.5 V (for 12 V heaters) or 19 V ± 1 V (for 24 V

heaters) over a period of 20 seconds there will be an error

lockout with a 60 second run-down.

The control units with order no. 230 04_ and 230 05_ have

the glow cycling integrated with no influence on the

sampling within the control unit.

3.1.2 Combustion Operation

After activation the heater first operates with full load.

The temperature sensor increases its resistance with

rising temperature dependent on the heating circuit

temperature.

When reaching the individual switching thresholds the

control unit initiates the following operating conditions

(see functional diagram).

Vehicle air fan:

When exceeding a water temperature of 55 °C the vehicle

air fan is switched on via relays K5, K6 and connection A6.

When the temperature drops below 50 °C the vehicle air

fan is deactivated.

Part load operation:

When exceeding a water temperature of 78 °C the part

load resistor R1 is connected in series with the

combustion air fan motor M1 (relay K4 de-energises, relay

K3 energises). At the same time the dosing pump

frequency drops by about 50 %.

Full load operation:

When in part load operation the water temperature drops

to 70 °C, full load operation is resumed (relay K3 de-

energises, relay K4 energises). The dosing pump

operates at full load frequency.

Control idle

When in part load operation the water temperature

exceeds 86 °C, the dosing pump is switched off. Run-

down operation commences.

Run-down

Run-down operation provides for venting and degassing

of the combustion chamber as well as for cooling down

the heat exchanger to avoid overheat damage. After

"flame-out" the flame sensor signals "dark" (for optical

run-down refer to Fig. 301).

During run-down (approx. 60 sec) the combustion air fan

runs in full load and is deactivated afterwards. The run-

down period (for electronic run-down refer to Fig. 301) is

always 60 sec and is not abbreviated by switch-off or a

rapid drop in temperature.

After run-down only the circulation pump and the vehicle

air fan continue their operation.

Start after control idle:

When the temperature drops below 70 °C during the

control idle period, an automatic re-start is initiated (see

3.1.1).

As soon as the flame sensor signals "bright", combustion

operation has resumed and the heater enters part load

operation when the temperature rises above 78 °C

(otherwise full load operation).

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