Speedy Jim's Windshield Wiper Page

                 WIPER MOTOR WIRING

                               Dash-mounted Wiper Switch

                    (12 Volt/2 speed)

The wiper motor is a DC permanent magnet field design.

2 speeds are obtained by adding a third brush on the commutator. This third brush is offset from the ideal running position and thus produces a lower speed.  The motor shaft drives a worm reduction gear. The worm wheel is connected to the wiper arms thru linkages. A set of sliding contacts is embedded in the worm wheel inside the wiper gearbox housing. (The sliding wiper contacts have been shown pictorially in the diagram as a cam-operated contact.)

The dash-mounted wiper switch has two sets of contacts. The first selects Low or Hi speed by feeding power to either 53 (the Low speed brush) or 53b (the Hi speed brush).

The second set of switch contacts controls the "parking" function. The drawing shows the wipers already in the "park" position. The cam contact (53a to 53) is open. No current flows to the motor.

Now, imagine that the wipers had been running and the driver turned the switch to the OFF (PARK) position while the wipers were midway across the windshield. The cam contacts 53a to 53 would now be closed, permitting power to flow from the fuse to the Low speed motor connection (53).

The cam contact thus keeps the motor running (at Low speed) even after the wiper switch is turned OFF.

When the wipers are approaching the "park" position, thecam contacts transfer. This action cuts off power to the Low speed terminal and the motor begins to coast. The motor has a lot of inertia and would probably coast right past the "park" position and go round again!

To accurately stop the wipers every time, another contact is used on the cam switch. When the cam switch transfers, 31b is connected to 53 (as shown in the drawing). If you follow the path, you will see that motor terminal 53 is now short-circuited to motor terminal 31.

The motor (which was still spinning) now acts as a generator.
(The permanent magnet keeps field magnetism supplied.)
The short-circuited brushes produce a very high current in the armature which creates a strong counter-torque. The result is that the motor is brought to an almost instantaneous stop, accurately parking the wipers precisely where the cam dictated. The process is called "Dynamic Braking".

On all 2-speed wiper motors, the wires which actually go to the brushes internally are colored Green for Low speed and Red for High speed.