Up until the 1960s, most automobile windshield wipers were powered by vacuum motors. I have a 51 Willys pickup with a vac motor. The last time I registered the truck, the only way I could get the wipers to pass the inspection was to mist the windshield with a water spray bottle before turning them on. It is time to reregister the truck and the wiper motor has failed.
This Instructable describes the process I used to convert the wipers from the original vacuum motor to a 12 volt motor. If you choose to make this conversion, I suggest you read through all of the steps before starting. These instructions are a little disjointed.
Step 1: Know what you need to replace
Step 2: Determine the motor drive radius 1st step
Step 3: Deterimining the radius 2nd step
Using a protractor, the radius of the vac motor drive arc is shown to be about 140 degrees.