No matter the power window design, it still has to open and close the glass.
But that's not the only complaint you may face. Other common faults are windows that only move partially through their travel before grinding to a halt, work in one direction only or no longer respond to their auto up/down feature. A few simple checks, however, can often quickly and successfully locate the cause of these common problems.
Basic FunctionPower window systems use an electric motor to move the glass via a regulator to which the glass is attached. The motor is a DC (direct current) design, using two brushes riding on an armature, just like a starter motor. The direction of rotation is determined by the direction of current flow applied to the motor. The amount of current flow is influenced primarily by the amount of resistance to movement the motor has to overcome. This is true of any electric motor, whether it be a window motor, starter motor or even a fuel pump.
Some regulators are a mechanical design, much like those used on manual window systems. More and more, though, cable type regulators that use a cable and spool arrangement to transfer the motor's rotation to the window glass are being used. While they certainly offer lighter weight, the cables are prone to binding in the spool at the first sign of any kink in the cable, and often break away completely from the window plate. This usually results in a window that can be freely moved up and down, and you can hear the motor itself running with no effect on the window position.Don't rely on this quick check alone. The plastic gearing, internal to the motor or where the motor meets the cable spool, can strip and result in similar symptoms.
Checking OperationThe first step is to access the window components, and this involves removing the interior trim panel of the door with the bad window. Two basic designs are used to hold the interior trim in place. Some panels are held in place with snap fasteners, while others are secured by bolts or screws and then lift up and out of the door sheet metal. A few designs, mostly European makes, incorporate all of the components in one assembly, making access more challenging. Be sure to look up the proper removal procedure to avoid breaking something expensive.
Once you can access the window motor connector, you can perform a quick check of the electrical side of the system by plugging a test light in place of the motor. Careful, though! If the motor connector has more than two wires in it, it is a good bet that the motor contains some form of position sensor. You'll have to check the schematic to determine which wires actually supply power and ground to the motor before connecting your test light.With the test light connected, turn the key on and operate the switch in both directions. A good, bright light is a pretty good indication that the circuit to the motor is OK. If the light only lights in one direction, check the schematic to see if the switch actually supplies the power and ground to the motor. If so, the switch is usually the culprit.
Keep in mind that the passenger windows are also controlled by the driver, and the two switches are often wired in series, making both switches suspects. Modern systems also use a lockout switch that prevents the passenger switches from working, and I've seen more than one instance where this lockout switch was inadvertently turned on by the owner. If the switch is a control module input, use the schematic to test the operation of the control module and its inputs/outputs to isolate the problem.Want to practice your voltage drop skills? Use a Digital Multimeter (DMM) and backprobe the connector, checking for power and ground with the motor connected, key on and the switch operated. Remember, the roles are reversed when the switch position is reversed, so each connection should show full system voltage in one direction and a good ground in the other.
Visually inspect the condition of the regulator mechanism. Mechanical regulators often break the guide wheels that allow the regulator to move in its tracks. Broken guide wheels can cause the glass to bind and even break. Cable regulator failures are easy to spot by visually looking for kinked cable, or cables that have become detached from the plate that guides the glass. If the motor is working and there is no apparent visual damage to the regulator, take a closer look at the motor to regulator gearing. Many designs use plastic gearing that can break or strip.
With the glass disconnected from the regulator, move it up and down in the window channel, keeping it square to the direction of travel. It should move smoothly, with no binding. If not, check the condition of the rubber channel. Age can harden the rubber and dirt and debris can restrict glass movement. These restrictions require more current to overcome and might be enough to trip the current limiters or circuit breakers, causing the window to work for a short time and then quit until the breakers cool.Lubricating the window run can help the situation, but it has been my experience that it's a short-term fix. Replace the runs if you find a problem for a more permanent cure.
Many systems include express, or automatic, up/down features and obstruction detection logic, so be sure to check for any Technical Service Bulletins (TSBs). Updated software and/or components often are the answer. Smooth, free movement of the glass is essential, so check the window channels and adjustments to make sure the glass isn't binding. Also check the service information for relearn procedures to restore these functions.
Malfunctioning power windows are common occurances, and your ability to quickly diagnose the underlying causes of failure could make your shop the shop of choice for these repairs.