This page is here to document how I fixed my car windshield wipers, for the benefit of anyone else running into the same problem and presently searching for that phrase online.
Several years ago my windshield wipers started behaving erratically. When being used on the "intermittent" setting, the blades stopped at odd places in the middle of the glass rather than returning to the bottom and then stopping there. If I ran them on their normal setting, they'd stop immediately when I turned them off (which was my workaround for getting them out of the way).
I asked my car dealer to look into the problem and they quoted me over $900 to fix it, stating that the motor was dead and needed to be replaced. I said no thanks and decided to investigate a bit more myself before committing such a large sum of money. After all, the motor was clearly running fine at full speed. Whatever the problem was, it HAD to be some kind of switching problem - absolutely not worth that much cash.
With a friend's help I removed the motor from the car and checked it. It seemed fine. The motor contained a mechanical wiper switch which would connect one terminal of the connector to 12v for the majority of travel, then ground that terminal when the motor was at the position to stop. There was no wear and tear in this part of the motor, and checking with a multimeter showed it worked as designed. Unfortunately, the wires connecting to the motor disappeared into the side of the engine bay and I couldn't find the point they came out the other side, because of all the air conditioning crap in the way. I put it back together and lived with it for a while longer.
Eventually I got my car serviced at a different place. I asked them to also give me a quote for fixing the wipers. I told the guy about how I'd checked the motor and ruled it out as the source of the problem. When I picked it up he told me that the motor was dead and needed to be replaced - it would be $705. I assume they both looked this up in the same book without even examining the wipers.
So next I started going through the Haynes manual in detail. I found one reference to a "wiper controller" on the schematic diagram but no indication of where that part was located in the car physically. It was also a black box part - no explaination of what it did or what the connections to it were for. After I started taking the whole passenger side apart I finally found it - tucked in a dark space right in the corner where you can't see it. This stupid thing was the problem, of course. You can find this wiper controller by disconnecting the wiper motor, setting the wipers to intermittent and then listening for the faint clicking of a relay behind the dashboard.
I ended up making a schematic for the wiper motor and the controller circuit as well. The thing is basically a 12v relay (not the standard automotive type, just a PCB-mounted SPDT one) with a little chip on the side to add the delay for the "intermittent"' setting. I desoldered the relay to bench-test it and after soldering it back into the board, it all worked. Total cost: nothing.I ended up spending days looking all over google for information about how windshield wipers work and I gather that most cars work like mine. The problem is everyone seems to be happy to talk about the basic gist of how they work without telling you the details. So here you go, internet, I hope you find the information useful.
When the windshield wipers are switched off or when running on the intermittent setting, one side of the motor is connected to the 12v. The other side is connected to the moveable contact of the relay on the wiper controller board, which is normally sitting on the pin-7 contact. So the negative side of the motor is directly connected to the motor's internal wiper (the moving contact). The motor will be grounded through this wiper at all parts of its travel except when it reaches the bottom of the windscreen, when it will be switched over to the 12v contact. This means the motor will continue to run until it's switched over to the 12v side, at which time the motor will be shorted and back-EMF will act as a brake. That's why the 12v contact is there.
When the wiper controller is set to intermittent, the motor will normally be sitting at rest with the motor's wiper sitting on the 12v contact. After a certain delay the controller will energise the relay, and the motor will suddenly be grounded through pin 3 and begin to run. The wiper waits a half second or so to give the motor a chance to get going and then turns the relay off. The rest of the cycle just has the wiper return to the park position in the same way as the off setting.