Looking at the wiring diagram, there are two wires going to the windshield wiper motor.
I presume one is the full 12 volts, and one is a lesser voltage that comes from the windshield wiper switch.
Now for the question.
If a variable voltage was presented to the high speed terminal of the windshield moter, would it run the wipers at a variable speed? And is this speed linear with the voltage?
A simple rotary potentiometer on the dash or steering column could accomplish this.
What about this? Has anybody done it? Is it possible?
As my 72 Sprint is in pieces, I can confirm that it conforms to the S3/S4 wiring diagrams. Five wires to the motor - four to the switch plus a black wire to ground.
The S1/S2 wiring diagram shows four wires to the motor. One to the fuse box, two to the switch and one to ground.
Sorry guys, I was talking about the two wires carrying +12. It is true, the wiring diagram shows four wires from the wiper switch and five wires to the wiper connector.
I have subsequently gotten a reprint of a Lucas manual. It appears that the wiper motor accepts +12 on two connectors, one of which runs the motor at low speed and the other runs the motor at high speed.
So I would suspect that putting a high current rheostat in series with the high speed connector would give variable speed wipers.
I have just added a bias switch which give one wipe ,it is connected to a switched live and then to the brown/green wire going to wiper motor.This saves the wipe switch
Whatever the total number of wires going to the wiper motor, depending whether it’s single or two speed, there is normally a permanent live, which feeds in all positions except the ‘park’ position, and a switched live from the wiper switch.
Lowering the operating voltage will slow down and eventually stall the wipers, but it may also burn out the motor.
Hella make a very effective "variwipe " control which will answer all your prayers.