Q: How Does the Auto-Reverse Safety Feature Work on Car Windows?

Jam protection

Jam protection is the term used by the manufacturers when discussing this safety feature. All power windows have a few parts in common.

• Window motor
  
• Window regulator
  
• Window switch
  
• Window glass
  
• Up relay
  
• Down relay
  

Others, like one that has the auto-reverse feature, will have a few others.

• Window module (Usually mounted in the door)
  
• Window motor pulse switch
  
• BCM (Body Control Module)
  

Window motor

The window motor is the workhorse of the system and provides the motion that roles your window up and down. It consists of a worm gear and a reduction gear that only allows the motor to move the window. A worm gear is a long shaft like gear that drives a much larger diameter round gear similar to what you would see on a bike, the reduction gear. This gear system cannot be moved by forcing the window up or down. Only the motor, which turns the worm gear, can move the window. Any force placed on the window simply binds this gear system more and more as you apply more force.

Window regulator

It is not unusual for the regulator and motor to be replaced as an assembly. The motor and the regulator can be replaced independent of each other as well. The regulator is the part that holds the fragile glass in the door. The motor is attached at a central part of the regulator and when the motor turns, the regulator swings a single arm or multiple arms that move the glass up and down.

Window module and BCM

Not all power window systems utilize a module in the door. Some will use only the BCM. Either way the programming methods used to determine the position of the window and whether or not there is an obstruction are the same. Beyond a computer, there is one essential component that allows the auto-reverse function to exist, that is the pulse switch.

Window motor pulse switch

The pulse switch is more of a sensor than a switch. It is a device that creates a small voltage when it passes through a magnetic field. This property of electronics is know as induction. Mounted on the worm gear in your window motor is a magnet. As the magnet spins on the worm gear, it passes by the pulse switch inducing a voltage. Each time a voltage is created, the window module receives this voltage signal that is counted as one pulse. Each time the window module receives a pulse, it measures the amount of time it takes for the next pulse to occur. The longer the time between each pulse, the slower the motor is turning. This method of measuring a rotational speed is utilizing a method referred to as a pulse width.

Let’s break down what a pulse width is by defining each word. A pulse is a signal created by the pulse switch that a computer can count. The width of the pulse is the amount of time that passes before the next pulse. The term width is used to describe the pattern created when the voltage signals are graphed.

Terminology aside, the only important thing to understand is the window module/computer is simply measuring the amount of time that passes between each pulse. Each pulse can represent one rotation or the 360° of a rotation can be divided into smaller pieces. In the end, all that is happening is the comparison of the time between pulses.

So what does this all mean?

Ok great, you’re probably wondering what this has to do with an auto-reverse function. As the window motor rotates, the window module has learned what the normal speed is by measuring the amount of time between pulses. If the amount of time between pulses increases indicating the rotation of the motor has slowed, the window module is programmed to reverse its rotation.

Window switch

Your window switch simply allows a voltage signal to pass through to the BCM and or window module. The module then powers the up or down relays depending on the direction you requested by depressing the window switch.

Up and down relays

The up and down relays convert low voltage signals from the window modules into the twelve volts that the battery supplies. The window modules receive signals from the window switches that direct it to power the up or down relay accordingly.

If you run into any problems with your windows and need servicing, feel free to reach out to a certified mechanic who can diagnose and ensure the proper function of all of your windows components.