My car is finally working well enough that I had a chance to work on a part that I'd been meaning to work on since we discussed it in 1994 - the wiper parking.
At the time, two list members described a broken part internal to the wiper motor assembly that pushes the slider around that trips the limit switch. My part was not broken.
Mark Elbers said 'I have never
seen a TR7 whose wipers park correctly'. I still haven't, but now I know why, and what to do about it.
The motor drives a worm gear which drives a cog in a circular direction, with its axis perpendicular to
the road. Onto this cog, near the outer edge, is loosely attached one end of a metal plate, about 1/2" wide and 4" long, parallel to the ground. The other end of this plate is attached to the drive cable in a
slot. Therefore, as the motor goes around, the cog goes around, and the plate pivots to drive the cable in a back-and-forth motion.
Near the driven end of the cable is attached, through a bent piece of
metal, a piece of plastic that slides back and forth in another slot, parallel to the motion of the cable. This slider has a groove in it, parallel to the direction of motion, with a varying depth. There is a plunger
from the limit switch that fits in this groove. When the slider moves, the part of the groove where the plunger sits gets shallower. The slider thus depresses the plunger, activating the limit switch. When the
stalk-mounted switch is in the park-position, power is applied to the motor through this limit switch. The limit switch is positioned such that it clicks when the back-and-forth motion is at the end of its travel.
Those of you who understand sine waves and slopes thereof and sensitivity relationships can see what the practical person understands: This design is not robust. If you just adjust the limit switch position
so that it just barely switches at the park position, tiny variations due to temperature or wear will cause it to park away from this position or not at all.
A better design would be to put the limit switch
where it is actuated by the rotating of the cog. Such a design would actuate the switch in a way such that the slope of the cog-motion-versus-switch-motion graph is not zero, thus not leading to infinite sensitivity.
But a redesign is probably not worth the effort, due partly to the fact that the connector and the limit switch are a single integrated assembly.
Anyhow, my problem was that occasionally, like 20% of the
time, the wipers wouldn't park for a few to few dozen rotations. The solution is to adjust the limit switch so that the park position is not quite at the end of the travel of the wipers. You can do this without removing
the assembly from the car, I think. (I didn't understand it before I disassembled it. Access may be too difficult without removing from the car.) Just remove the one-screw mounting strap and rubber cover to expose the
connector/limit switch combination. Mark the position of the limit-switch. Loosen the two screws that position it. Move the limit switch a millimeter closer to the body of the assembly. Retighten the mounting screws. If
your car is like mine, it will now park not quite at the bottom of travel, but reliably.
