You are kind of on the right track but let me try to clarify. Other than on some car models where computers are involved with the window circuit, there are actually four switches involved in running a passenger window. That doesn't include the lockout switch in the driver's switch assembly.
Each wire for the motor runs through a switch, (so there's two switches), in the passenger's switch assembly. From those two switches, the wires run through the door hinges, over to the driver's door hinges, into the driver's door, then through another pair of switches in that assembly. From the driver's switch, both wires go to ground, which is almost always through another wire going through the driver's door hinges. To verify this, ground one lead of your ohm meter to a paint-free point on the body, then measure the resistance from the motor wires. One wire can read through the motor, then to ground making it falsely appear that both wires have good continuity. Instead, just unplug the connector from the motor, then read each wire separately.
The secret here is both switches are released when you're taking those continuity readings. You can expect to find a few ohms from the resistance in the wires. You just don't want to find real high resistance or an open circuit.
Now the fun starts. When you activate one of the switches, the first thing that happens is one of the two switches becomes an open circuit, then as the movable contact continues to move, it connects to the 12 volt feed contact. The other switch in that assembly doesn't do anything. So, .. One motor wire is still grounded through the relaxed contact in the passenger's switch assembly and the relaxed contact in the driver's switch assembly, AND the second motor wire has 12 volts applied from the activated switch in the assembly.
To run the motor the other way, the two switches in one switch assembly trade functions. The one that was grounded before now gets switched over to the 12 volt feed contact, and the one that had 12 volts on it before now stays relaxed in the grounded position. This way the polarity is switched for the motor and it runs the other way.
The clue here is if the motor runs one way, (either way), from the driver's switch, the wiring between the two door hinges has to be okay. That leaves just the two switches. Now you have to remember that for purposes of this story, current flow starts at the driver's switch. We'll assume the first switch is activated, so connecting it to the 12 volt feed contact causes current to flow through that contact, over to the passenger switch, through the first switch, (which is relaxed), through the motor, through the second switch in the passenger's switch, (which is also relaxed), back over to the driver's switch, then through that second switch which is also relaxed. That means the current flowed through four switch contacts. Any one of those can have a burned or arced contact that stops current from flowing. It could be a burned ground contact that is relaxed and causing the dead motor, or it could be a 12 volt contact that doesn't make a good connection when that switch is pressed.
The clue to a burned 12 volt feed contact is it only affects that switch. If the window doesn't work from the passenger switch, it will from the driver's switch. A bad ground contact will cause the motor to be dead from both switches.
That's the theory of the circuit operation. When it comes to troubleshooting it, we can figure out which switch contact is causing the break in the circuit by taking a series of voltage readings, but when we have to charge by the hour, we have to look for a faster way. Notice I didn't recommend using resistance readings because an arced or burned contact could still have very little resistance when no current is flowing through it, but it could be unable to pass the relatively high current the motor draws. Think of a bunch of broken strands in a starter cable. It only takes one tiny strand to read 0.0 ohms, which is good, but there's no way you're going to get hundreds of amps through that one strand. Voltage readings are much more valuable because those will let us see the RESULTS of the burned contacts.
Regardless, we need the fastest way to diagnose this so we can save our customer money. In this case it is to plug in a different switch. Normally we don't swap in different parts because we have to buy them, then put them in stock if it doesn't solve the problem. But in this case, you have two other passenger switches in the car. The bezels might be different between the front and rear switches, but they usually will all plug into the same connectors. Grab a rear switch and plug it into the front connector. If the window works both ways now, you know which switch assembly needs to be replaced. If the motor is still dead one way, it's almost certain the driver's switch assembly is defective. To try a different one for that you'll have to buy one from the dealer or find one at a salvage yard.
When you do these tests, don't overlook the connector terminals. You're looking for one that's blackened and overheated or the connector body is melted. You won't find that very often because current doesn't flow long enough to overheat them. Switch contacts can overheat much quicker.
Saturday, June 11th, 2016 AT 8:32 PM