Windshield Wiper B747-400

[Spanner Turner]

Mate, not sure if you had access to this section, but this is about the best explanation I can find in words.
Otherwise you'll be up for pulling one apart to satisfy your curiosity.
The key to the park position is the mechanical park switch which has to be activated to cut power to the motor.




Low Speed Operation
When the "LOW" setting is selected, relay "R1" is energised. Closing the switch, energises fields 1 and 2 in series. The motor then
starts to operate at approximately 160 strokes per minute.


High Speed Operation
Setting the switch to the "HIGH" position, energises both relays "R1" and "R2". With both relays energised, field 1 and field 2 are energised in parallel. The circuit is completed
and the motor operates at an approximate speed of 250 strokes per minute.



Park Operation
Setting the switch to the "OFF" position, will allow the relay contacts to return to their normal positions. However ,the wiper
motor will continue to run, until the wiper arm reaches the park
position. When both relays are open and the park switch is closed, the excitation to the motor is reversed. This causes the area swept by
the wiper to shift downward. The wiper blade goes off the lower edge of the windshield and the cam-operated park switch opens. This
de-energises the motor and releases the brake solenoid, applying the brake. This ensures that the motor will not coast and reclose the park switch.



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