Tech In Plain Sight: Car Doors

There are a lot of common phrases that no longer mean what they used to. For example, you may have used the term “turn on the lights.” What are you actually turning? Where does this come from? Old gas lights had a valve that you did physically turn, and the phrase simply stuck around. Kids of the 90s have no idea why they “dial” a phone number. What about “roll up the car window”?  You don’t often encounter old-fashioned car doors with manual locks or a crank to roll up the window. These days it is all electronic. But have you ever wondered what’s going on inside there?

Let’s take a look at car doors, how they keep you safe, and how that sheet of glass slides into place, sealing against wind, rain, and noise. Of course, there are fancy car doors like suicide doors or sexy-but-impractical gull wing doors. At least one concept car even has a door that disappears under the vehicle when it opens; check out the video below. But even garden-variety doors are marvels of mechanical engineering. A compact structure that is secure and — mostly — reliable. Let’s look at how they do that.


When you think about it, a great many things we build convert something to something else. Energy to motion, for example. Or one type of motion to another. Car door mechanisms are no exception. Of course, the lock actuators and the window actuators both convert an electrical command to some kind of physical motion. But there is also the problem of getting the right physical motion in a confined space.

One of the interesting things about most car doors is that the locks have to be secure but offer many ways to open them. Electronic control alone is usually not sufficient: there is often a way to unlock the car manually. And you certainly have to be able to operate the door from the inside even with no power, to avoid trapping occupants. Some cars, like the Boxter or some Lotuses, have no obvious way to unlock the car without power. These cars usually have a hidden emergency keyhole or a set of hidden terminals that allow you to temporarily power up the system to unlock the vehicle.

Lock Up

The motor that controls the power locks in a car is known as an actuator. You can get universal actuators that can be made to fit most cars, even those that don’t normally have power locks. Because the door latch must be workable from multiple sources, the actuator usually drives a rod, and that rod often appears near the window so you can manually operate it as well. The rod engages and disengages the latching mechanism, and when disengaged you can’t open the door. In fact, often if the rod is disengaged and the latch is closed, you won’t be able to close the door if it is open.

Door lock mechanism from patent US4793640A

A typical actuator is a simple electric motor that rotates. Of course, we need a different motion, so gears not only reduce the motor speed and increase the torque but a rack and pinion gear converts the rotation to a linear motion. Because you usually don’t want the motor and the occupant fighting for control, there is often a clutch that does not transmit manual motion back to the motor. The motor only runs momentarily. Some cars have feedback and will try a few times if something goes wrong.

Older cars used vacuum pumps to suck or blow air to operate doors because electric motors were noisy and prone to failure. Mercedes, for example, used air-operated door locks until around 2003.


Windows are a bit easier because you don’t normally need manual control. There are some special concerns, though. For example, have you ever noticed that your window stays perfectly level? In addition, it should be almost impossible to force the window down for security reasons. This is often handled by using a worm gear. Because of the angle of contact between the worm gear and the spur gear, the worm gear can turn the spur, but not the other way around because of friction between the gear teeth.

Since the window has to go up and down, there is wiring similar to an H-bridge that can run the motor in either direction. For a car with a sophisticated controller, it probably does use an H-bridge and your window buttons are just inputs to the computer. But basic cars may simply have wiring that allows either the driver or passenger to rotate the window motor in either direction.

Very old cars actually used hydraulic systems to operate the windows. This had the advantage of allowing the window to roll down with no power, but you did need power to operate the pump to raise it again. However, hydraulic lifts fell out of favor long ago in favor of electric motors.

Window linkage from patent US6502352B1.

The actual mechanical part can be in several different configurations. With direct drive, there will be either a scissor-like mechanism or a single arm with teeth. Both of these configurations often use a spring to maintain tension.

Many modern cars use a thin steel cable on a reel. By playing out the cable or taking it up, a roller along the bottom of the window can move up or down. A thick plastic tape with sprocket holes serves a similar function in some systems. Altogether, these are known as a regulator. Like the power locks, you can find universal regulators that will repair any power window system or add power windows where you previously didn’t have any. However, as you might expect, these are more complicated and expensive than the comparatively simple door locks.

So What?

This is one of those things where you probably use your car door hundreds of times a year, but you don’t think much about what is in there until it breaks. But even if you never need to repair a door, consider this: universal power lock actuators make excellent linear actuators for all sorts of projects. If you are doing a one-off, you might even be able to harvest one from a junkyard for nearly free. Even if you buy them new, you can find them for $5 or $10 online.

Power window mechanisms cost more. While there are also universal windows available, they are probably harder to find a use for unless you really need something to pop up and down like a car window. If you do a project with either of these, don’t forget to let us know so we can share it with everyone.

Of course, these car parts are pretty beefy. If you need something more modest, you have options. You can even convert a servo.

[Banner image: “Car door sides” by David Rosen, CC BY 2.0. Thumbnail image: “Muscle Car Door 47” by Steve Snodgrass, CC BY 2.0.]


54 thoughts on “Tech In Plain Sight: Car Doors

      1. As much as love tech and gadgetry, I don’t want it controlling my car!
        All those actuators, relays, buttons and wiring weigh more than manual units and introduce multiple points of failure.

          1. Gronk’s club is more likely to break than the rock.

            It also isn’t going to trap him in a small space when it breaks or cost thousands of dollars to fix when it does.

        1. Just yesterday in my country, a burnt out car was found pulled over on the side of the road in such a way it looked like they had some emergency. Occupants still inside.

          It reminds me a place I live next door to. Dead locks on the doors (they’re called that for a reason) solid metal bars over all the windows. I though if they ever have a fire they will never get out alive.

    1. We used to have a 1994 Isuzu Trooper. One day the driver’s window wouldn’t roll up. Tore into it and found that the power window switches were glorified DPDT switches. The center pins went to the window, then power supply + and – were swapped on either end so it would change which polarity was sent to the window motor. The driver door window switch was messed up, but I was able to short the wires together to get the window rolled up to work on it later. In the end, I swapped the driver and passenger door wiring so the switch that should have run the passenger door ran the driver’s door and vice versa. You could still roll down the passenger window, you just had to lean over and use its button to roll it back up. Worked great, but always messed with anyone else who tried to use it. :-)

      1. This was almost universal for a long time I think – and why not, it’s very simple and it works.

        Certainly almsot everything Land Rover (and likely by extension British Leyland / Rover, Jaguar, etc.) used that for a long time.

      1. Mercedes W220 electric windows switch units communicate over CAN Bus. I heard, but cant confirm, that US and Euro switch units have reversed messages, resulting in US part controlling opposite window when mounted in Euro car.

    1. Modern dimmable lamps use knobs for power/dimming, not only because it was traditionally the way gas lamps worked, but also because in transitions well to a potentiometer in a triac dimmer circuit

      1. potentiometers have always been available in both rotary and linear formats. It is also quite intuitive to push the knob up to turn on the light and down to turn it off, the motion and effect are the same as raising and lowering a blind, also a common device in that time period.

        The real answer as to why dimmers use rotary potentiometers instead of linear ones, it is because they are cheaper. This stuff was fabulously expensive for the time and any way to drive down the cost was welcome. My dad was a potentioemeter salesman in the defence business and he had all kinds of nifty military grade potentiometers for me to play with when I was a kid.

  1. The BMW Z1 and at least one production Japanese car (only released in Japan) had car doors that retracted downwards into the car, underneath the driver, and there was a 1950’s American full-sized car (maybe the Muntz Jet?) that had doors that slid in tracks backwards into the rest of the car body. There have been a lot of interesting solutions to doors. This was a bigger deal in the 1950’s and 1960’s where there were a lot of sporty cars with rear seats, but to be a sporty car they were designed with only one door on each side so they were REALLY LONG doors. My brother had a 1971 Pontiac SJ whose door on each side was as long as the front and rear doors on my current car. It was really difficult in a parking lot because you couldn’t open the door enough. I’m really interested in the double-articulated gullwing doors that can open in a zero side clearance situation.

    My older car has a mixer cam where the indoor handle, the outside handle, the locking mechanism, and the door latching mechanism all connect. It has a primitive mechanical logic mechanism, where it implements (indoor handle or outside handle) nand (lock) before activating the latch release so the door can open. It broke one time (because it’s cheap British Leyland plastic) and I had to machine a replacement out of aluminum, so I got to learn a lot about door guts.

    1. The car with the sliding doors was the Kaiser Darrin, a lovely design. The doors slid forward into the fenders behind the front wheels.
      No windows, so it was a phaeton that had limited utility, but great-looking by the standards of the day. It, along with the corvette and the thunderbird, were early attempts by the US to compete with European sportscars.

      1. A Phaeton is a four door convertible, originally one without roll up windows. Essentially a four door roadster. Roadsters have two doors and don’t have roll up windows.

        The VW Phaeton was NOT actually a phaeton, not even a convertible sedan. It was a sales dud because not many wanted to pay $96,000 to $126,000+ for a *Volkswagen* in the early 2000’s. Terrifically nice cars though and now can be bought dirt cheap – but like most technologically advanced German cars, they can be bloody expensive to fix when their fancy stuff inevitably starts to fail.

        Then there are all the vehicle models called “coupes” that are in no way an actual coupe body style. One of the worst offenders was the 1980’s Audi Coupe, which had COUPE molded in bold black letters in the giant reflector panel between its tail lights.

        Despite all that, it was really a three door fastback/hatchback.

        Ford hasn’t made a Mustang coupe since 1994. Since then they’ve all been semi-fastbacks or convertibles. GM kept calling the C6 Corvette a coupe. Nope. Semi-fastback. Put the hard roof on the convertible C6 and *that* was a coupe. The C5 I’d say qualified as a coupe despite its fairly steep rear window angle because of how far the bottom of it was from the rear of the car. Call it edging the line between coupe and semi-fastback.

        A coupe should have two doors, no more. Not even a pair of half doors that can’t open before the front doors. It should have a sharp break between the rear window and the trunk/boot lid. Ideally the rear wind should have a slope of no more than 45 degrees.

        Someone should make a T-Shirt with side view outlines of all kinds of non-coupe body vehicles, with lines outlining the doors. Call it “Vehicle Body Style Identification Guide for Automotive Journalists” and label every one of them Coupe, except for in the middle have a Mustang coupe, any year 1964.5 to 1994, with ????? under it.

        A body style name that’s fallen out of use despite the type still being rather abundant is Brougham. What’s a brougham? A four door fastback. They were common in the 1940’s and 1950’s, with either no break in the line at the rear window bottom or only a slight break. They were called broughams and the car buying public new what the term meant.

        1. Me when I was a kid: ‘You’ll know I’m old when my car doesn’t have more cylinders than doors.’

          Checking the collection…I’m old. But only because I drive the econo car most. Only car that fails the lame door to cylinder ratio test. Even there It’s equal. The 4×4, half race and classic are all healthy V8 2 doors.

          Your main argument is as pedantic as ‘sports cars must have only 2 seats’! Technically correct…nobody cares.

  2. Don’t know about hydraulics, but I know a lot of old German luxury cars had a whole lotta things controlled pneumatically.

    I had a very early model vw Golf 3, some 6 or so years ago that even had pneumatic central locking system, with a valve block, vacuum reservoir and pump in the right side of the trunk.
    It worked, mostly, apart from the trunk hatch which had been disconnected in the previous owners time.

    1. I *love* troubleshooting these. . Think VW used them as the actuator was cheaper and more robust than a motor, mercedes definately used them because they were quiet and slow. And yeah the VW fuel door lock actuator is almost always the cause of trouble as the diaphragm tears.

      1. I have helped a friend debug the pneumatic rear trunk lock on his old Mercedes. “Find the leak” in hoses that run from inside the engine compartment, through the body panels, under the floor, and then to the trunk hatch. Fun times.

        Electricity is easier.

        1. I helped a bloke in a Volvo that wouldn’t start. Loose battery terminals.

          The battery was in the boot (trunk in American) under the spare wheel with larger than normal leads to the starter motor (because if the distance) under the bonnet. A copper mine right there!

          At first the boot wouldn’t open because of the loose battery terminal so it’s a jump to an external battery just to open the boot.

    2. Many years ago I hand an A2 Golf with that system. You could open the door by drilling a hole in a tennis ball, placing the hole over the key slot and then giving the ball a quick push from behind. This would force air into the system, thus create overpressure in the lock and it would open.

      Yet, the one time my car was broken into they just stuck a screwdriver in the lock and forced it open.

    3. Early bugs ran the windshield wipers off the air pressure in the spare tire.

      The Germans do make some inexplicable engineering decisions. Then pat themselves on the back for it.
      But at least they never shipped Tesla hiding door handles (guaranteed to break). Benz will see the repair revenue (‘brilliant’) and license the tech as soon as they can get them to outlast the warranty period.

  3. A couple of years ago, I was about an hour into tearing into my wife’s passenger side car door trying to fix a non working power window when I found a child lockout switch on the driver’s door.

    1. Everyone was a kid at some point in time, no?

      But there definitely is a time point at which people growing up won’t have exposure to things like dial phones, and I’d bet that’s around the 90s?

      Icons for “telephone” still evokes that same device, which my son has only seen b/c I think they have an awesome mechanism inside. None of his friends have seen the device that’s on signs everywhere as “phone”. Kids of the 20s!

  4. Some power window mechanisms can be found incredibly cheap… most commonly on cars that have the original ones fail with high frequency. When the window regulator in my E46 (97-06 BMW 3 series) crapped out, a new replacement mechanism was $14. Super common issue on those cars.

  5. There are all sorts of useful electrical components in vehicles for the knowledgeable hacker/tinkerer.

    Throttle Position Sensors are nothing more than variable resistors with rotational actuation. The resistive values usually vary with make and model, so some trial and error is needed. They have the added features of being highly tolerant of vibration and high heat environments, and have a waterproof connector.

    If you need something along those lines, but has a more linear travel to it, look at Suspension Level Sensors, particularly early 00’s GM passenger cars.

    Need to sense when a metal object passes within mm’s within a dirty environment? Look at Crankshaft and Camshaft Position Sensors to find Hall Effect Switches encased in plastic with a waterproof connector. Alternatively, look at ABS Wheel Sensors. Don’t forget to buy the other end of the connector, unless you’re harvesting from a junkyard.

    From the late 00’s to today most vehicle manufacturers have added inductive current sensors on the positive battery cable. Obviously meant for 12vDC, but capable of sensing 200a.

    Need to turn on a pump or fan when the liquid reaches boiling temperature or near it? Look at Coolant Temp Switches for simple on/off, or Sensors for variable readings.

    Keep in mind, unless you find a parts person with older paper catalogs, you will need to find most of the sensor values through testing. The newer catalogs have become fairly useless for the hacker, not listing sensor values, relay amperage ratings and schematics, and in some cases switch types.

  6. That little spring in the door lock assembly is far too small in some models and it metal fatigues right where the last 3/4 look of bent at right angles to make the hook. Then it breaks the door handle (inside or outside) can’t engaged and so it moves but doesn’t open the door. It’s a right pain to fix as you have to open the door to get the inner panel off on many models.

    You can just bend another hook from the remaining spring and it works fine. Some hardware stores have suitable replacements that are a little firmer. When one breaks the others are not far away so do them all at once.

  7. So I am wondering about welded door hinges versus bolted door hinges. I used to think that welded hinges were only for cheap cars and now I am seeing them on much more upscale vehicles. One would think that the bolted hinge would be a tremendous advantage because it’s adjustable but then again that could be a liability. What is the deal?

    1. Planned obsolescence.

      It is currently impossible to economically keep a newer German alive much past 10 years. Soon it will be completely impossible.

      Who expected the Germans to perfect the warranty timer? Especially after all the decades of effort GM put into it.

      But here we are, with German car manufactures telling you to ‘never change your trans fluid’. It’s good advice, if your transmission isn’t broken you might be tempted to start pouring money into a ‘never ending money pit’. Everything breaking at once would be a great service to the chumps.

  8. Its worth learning about the mechanical window switch mechanism and how you can replicate it with relays

    I recall Quinn Dunki claiming she could not use two single I/O and get a reversing polarity with 2 standard single pole double throw relays. Instead of the Forward and Reverse she wanted, she went with one output to switch the direction, and one for Activate

    Once you realize that the switch constantly grounds both motor leads, and pressing forward or reverse merely switches one of the leads to +12-14v (15v if you are Ford and enjoy boiling your battery, a different story) you can then understand how the wiring passes through the master switch and each door switch yet both work.

    On a separate note, a bit flipped in my friends 2008 GTi one day, now one of the window switches works the opposite direction. Obviously because the Body Control computer reads the networked switch before it will activate the relay or drive transistor to work the window motor.

    On a separate story all together, BMW radios around 2004 unlock themselves automatically if they see valid data on the body control network, there always being such data available you never notice until such time as the network level shifter dies and the radio refuses to unlock XD XD IBus chip ELMOS 10020B

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