Forum;

I ran across this old thread, and am bumping it back up to the top because I have a possible/likely cause of the fast motor, and some explanations which hopefully will help some owner: Fast DC motor running is caused by an open Field Winding

To effect a high motor speed on a "Cumulative Compound DC Motor with Long Shunt", the Field is deenergized...this is counter-intuitive (and there is a starting torque loss price to pay), but if the Field winding of your motor were to have opened up (for any reason), this would be electrically the same as placing the switch of a two-speed setup in the Fast position. (See Reference links)

If you're lucky, disassembly and internal check will reveal only an open connection to the Field coil (which can be repaired, and which will bring slower speed operation back)...if you unlucky, the visible connections are fine, but the Field coil is internally open (this is unfixable for all but motor specialists who will have special fixturing and techniques with which to wind a new Field coil of the right wire gauge and physical dimensions)...

However, the speed could STILL be brought down to slower by wiring in a PWM controller for a DC motor (alternates between applying full power and OFF at a rapid rate which translates to a slower motor speed, but still with decent torque...opposed to simply putting a Dropping Resistor in series and applying a reduced but constant voltage, which gives poor motor performance and also results in excess motor heating, not to mention the significant wasted heat in the Dropping Resistor).

Reference Links: http://www.sw-em.com/Windshield%20Wiper%20Systems.htm
...look at: Different Motors and Different Circuits, same Control:
...and: http://www.sw-em.com/Windshield%20Wiper%20Systems.htm#Reference_Information_on_Compund_DC_Motors
Info on Dropping Resistors: http://www.sw-em.com/Vintage_Volvo_6V_to12V_conversion.htm
...look down to "Dropping Resistors"

Hope this helps someone.
Cheers