Power Windows & Indirect Measurement

Courtesy Mister Fixit

I was riding in the car-car with my daughter the other day (not the SUV car, or the truck car, the car-car). She was driving and I was sitting in the passenger seat. I had my elbow on the window sill and my hand on the edge of the roof and she inadvertently pushed the up button for my window. It jammed my arm pretty good before I squawked, she let up, and the window stopped.

So then I got to thinking about the one-touch-up power window on the Honda. They must have some kind of sensor in there that will halt the window if it somehow gets blocked, otherwise I think we would have been hearing about lawsuits. The simplest sensor to implement would be one that would measure the current that the motor is drawing. If something is blocking the window, the motor would slow down and presumably draw more current, which could trigger a cut off switch.

Volkswagen used something similar a long time ago on their old bug. They had a diagnostic instrument that would hook up to their car with a cable and they claimed to be able to measure all kinds of things with it, including the compression in all the cylinders. At the time I was flat out astounded, because the only way I knew to check the compression was to take out the spark plug, plug in a compression gauge and then crank the engine. And then you had to repeat it for each cylinder in the engine.

Volkswagen's explanation was that they measured the current draw of the starter while it was cranking the engine over. As each piston came up to the top of it's compression stroke, the resistance to turning would increase and the current draw of the motor would go up.

When I first read that, I thought they were insane. They were presupposing a lot of things, like the starter was in good shape, the gears driving the crank were in good shape, the engine itself was not suffering from worn or damaged components that were creating excess drag. But then I realized they were starting with a new car, where all the components are in good shape, not a worn out, beat-to-death junker like I was in the habit of dealing with. I think that was my first inkling that one doesn't have to spend your life repairing other people's old junk.

Update January 2017 replaced missing image.