So the passenger window stops working, when you press the up button.
Only the up button? Yep, and so the window was stuck inside the door. I pulled the door card and tried to inspect the winder mechanism. I saw that the part where the cable is exposed is a bit slack, and I immediately think the winder spools have come undone. The problem is, I can’t get anything out with the window stuck in the door, and to get the window unstuck I have to lift it all up and out. About an hour later I have both the rails and the rubber track undone and removed. I pull out the winder motor and track, but it all appears intact.
Bench testing the window motor revealed it to be in complete working order. So I test the plug in the door, and I measure +12V and -12V as I toggle the switch. Seems legit, and after bending the plug contacts a bit to make sure of proper connections, I still have no result. So what gives?
Well, sometimes a fancy multi-meter is not the right tool for the job. If you follow the CAR WIZARD, you’ll know that he uses a test bulb, specifically an incandescent 12V type. This is so that there is actual load at the points or connectors where you want to test, because current leaks and volts don't. In layman’s terms, the potential for it working is there (volts), but in reality it might not (current).
I pulled one of my old headlights from storage and hooked that up to the plug in the door. Sure enough, pressing down lit it up, pressing up did not. But now, the volts read +12V and -0.5V. Because of the load (resistance) closing the circuit, some current was flowing through, but only enough to warrant 0.5V. So what does this mean? Basically two options:
Firstly, there is a leak to ground, and most of the current is flowing to the body. This typically happens when your loom (or a wire) has ground through from rubbing and shorts out against some metal surface. This is the harder problem of the two to solve, and might require you rewiring a significant piece of your circuit or loom. Hopefully it wasn’t that.
The second option is that, according to the math (V = IR), the rest of the “work” is being done elsewhere, meaning there is resistance somewhere else in the circuit that is drawing all the current. The only place this could be is in the switch or relay (or at the fuse, but that would most likely be an open circuit). Here you have a choice of either trying to open and repair the switch, or replace it if it is finicky or a sealed unit (like a relay). With stuff like headlights, winder motors and wiper motors, there is significant current draw because of the heavy loads and the switches and relays tend to burn carbon onto the connections as they spark. This puts a limit on their lifespans, and it also introduces resistance over time.
Fortunately for the NA miata, the window switches unit is an old, robust design from yesteryear, expensive to replace but easy to fix.
In the end, a quick and light sanding of the connectors did the job.