[TR] 2-speed wiper wiring diagram

[Ann and Tim Buja]

I'm not familiar with the early 2-speed motor, but from what you're
describing, the wiper motor is shunt wound, with the field winding connected
in parallel with the rotor winding.  A brief google search on shunt wound
motors yielded the following at
http://www.mpoweruk.com/motorsdc.htm
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Field Weakening
The speed of a shunt wound motor can be controlled to a limited extent
without affecting the supply voltage, by "field weakening". A rheostat in
series with the field winding can be used to reduce the field current. This
in turn reduces the flux in the air gap and since the speed is inversely
proportional to the flux, the motor will speed up. However the torque is
directly proportional to the flux in the air gap so that the speed increase
will be accompanied by a reduction in torque.
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This application in the early TR wiper motor uses a fixed resistance instead
of a variable rheostat to produce the second faster speed.  My experience
with field weakening was with industrial crane motors where field weakening
was used to increase the speed of the motor when there was reduced or no
load hanging from the crane.

Hope this helps,
Tim Buja - Rockford, IL