Innards of the rear-wiper motor from a friend's Honda SUV. There is a clever
little guide attached to the output pinion that keeps the rack engaged with
it, but allows for the variable angle of the rack as one end follows the big
plastic cam. The small image links to a larger image for more detail.
A set of spring contacts bears onto some brass slip-ring tracks embedded into the plastic gear face. The upper two contacts were observed to sit farther into the housing than the lowest one, somewhat visible here, and as found were discolored and much less springy than the lower one. A slight re-bending outward seems to hit their elastic limit much sooner than expected; they're really soft. It is possible that high currents made these two wipers really hot at some point, losing their original metallurgical qualities.
If the set of contacts carries full motor current to directly switch it
on or off depending on wiper position, then a stalled motor might have
caused that. But it is more likely that these inputs are only low-voltage
position indications into a small wiper ECU, hinting that some bumbling tech
might have caused a short through the contacts in question while trying to
test something else. The motor itself still works fine; no hint of burnout.
In this particular car the wiper has a "park" position below the level of the rear window, and a smaller "running" swath it actually sweeps out on the window when moving. Obviously, allowing the motor to simply run steadily in one direction would sweep the full stroke of the cam and be constantly parking and unparking the wiper blade off the boundary of the window. The ECU monitors the position contacts and uses an internal H-bridge to reverse the motor to keep the blade in the narrower swath, until the control switch is turned off whereupon it returns down to parked before stopping.
|Select the small pic for a short video of the full-stroke action, only to illustrate how the cam and rack mechanism works.|