Inside the Trico Wiper Motor

Many thanks to Scary Terry for publishing tremendously useful information about the Trico wiper motor used in Saturn vehicles (and other less-intended applications.) I've sacrificed a motor to the Project Gods, and I took a bunch of photos along the way. I hope that Terry and others find the information useful.

This webpage describes a procedure for removing the worm-drive gear head. If you're looking for more information about the Parking Switch, just skip down a bit. The photos show a fully disassembled motor, but all the relvant information should be there. Multiple photos are provided throughout, in an attempt to compensate for reflective objects under crummy illumination.

Removing the Gear Head

The worm-drive gear head is a casting that's integral to the motor head-casing. The rear cover plate is swaged in place. Use a drill to remove the swaged heads, then gently pry the plate off the rear of the gear head casting. This is an irreversible step, so be prepared to move forward, or be prepared to drill and tap some holes in place of the posts you're removing. In the latter case, you're on your own ...

Using a Dremel rotary tool and an abrasive cutoff wheel, section the casting as indicated in the photos below. Make sure to cut above the motor breather port (the white circular fuzzy thing.) The breather is necessary to allow pressure equalization in the motor body during thermal cycling. You'll definitely want to retain that bit.

Worm Drive Internals

With the gear head casting removed, you have access to the motor output shaft. The steel drive worm is integral to the motor shaft, and cannot be removed. The nylon driven gear is pressed onto the output shaft.

Parking Switch

The parking swtich connects via three contacts on the rear cover. Pinouts are indicated in Terry's diagram, below.

These three contacts are connected to wipers that contact a metal plate buried beneath all that wonderful grease. You can see the circular tracks left by the contacts as the driven gear rotates.

Removing the grease reveals the contact plate. Scratches in the surface indicate the P1 and P2 contact zones. The Ground contact rides in the center area, making contact with the small nub once per revolution.

Parking Switch contact P1 is always in contact with the rotary switch plate. It is normally closed with respect to the P2 contact. When the P2 contact rotates to the open area in the rotary plate, two things will happen:

That last part may seem a little odd, but in an environment where P2 is connected to the power supply, and P1 is connected to one of the motor power leads, briefly connecting P1 to Ground will act as a brake. With windshield wipers, you're trying to get the wiper blades to stop in a known location, so braking the motor will help prevent it from shooting through the intended stopping location while the motor spins down. In the following diagram, pressing the switch will start the motor rotating. Releasing the switch, the motor will continue to run until the P2 contact reaches the break in the rotary contact plate, at which point the motor will stop. This results in the motor stopping at an indexed locaiton. Note that it would be "bad" to hold the switch down for continuous operation, as once per revolution the P1 contact will briefly connect to Ground, shorting out the power supply. Also "bad": the motor tends to stop with the braking funciton active - P1 is grounded through the parking switch. This diagram is useful for explaining what's going on, but it represents a crummy implementation. Don't use it this way.

An alternative would be to place a diode in the P1-to-Motor leg. This allows the switch to be held down without shorting the power supply to Ground. The motor will stop at the index when the switch is released, but the diode will block the motor braking function. The motor will spin down gently, rather than abruptly when the braking function is used. Both methods have their applications.

I've included a short MPEG video of the indexed switch in action.

If a Single-Pole Double-Throw (SPDT) switch is available, the following circuit eliminates most of the liability while retaining the braking function. Connect the common switch terminal to either the high-speed or low-speed motor terminal. Connect motor power to both the P2 terminal and the normally-open (NO) switch contact. The normally-closed (NC) switch contact connects to P1. The diagram shows the switch in the "off" position. Power can conduct through the NC contact, through the park switch, and to the motor. When the P1 terminal is grounded for the braking function, the parking switch will disconnect P2 internally. Moving the switch to the "on" posiiton connects the power directly to the motor, and the motor can run continuously in this mode. Releasing the switch will allow the motor to continue running until the park position is reached.

The parking switch operation may seem like a liability, but it doesn't have to be. Connecting P1 to a pull-up resistor will result in a short "low" pulse on the P1 pin once per revolution. Connecting P1 to Ground and P2 to a pull-up resistor will result in a longer "high" pulse on pin P2 once per revolution.