Topics:
Door lock:
Door locks are mounted in the doors. Their purpose is to enable the doors to be opened and closed. They also serve safety purposes, so that in the event of an accident the door cannot simply spring open. After an accident, the doors must be easy for emergency services to open.
Door locks can be locked and unlocked mechanically with a key or electronically by a control unit. The electronic control is often part of the central locking system, which is discussed in the next paragraph.
Door locks can only be removed once the interior trim has been taken off. On older cars the lock can be unscrewed from the outside, but that is too vulnerable to crime. That is why disassembly is made as difficult as possible. Sometimes the entire window mechanism, including the window, must even be removed.
The image below shows a door lock module from two sides. This lock has a built-in electronic control unit which, on command, locks and unlocks the lock. At the top we see a locking clip for the Bowden cable of the interior door handle. The end of this cable is connected to the Bowden cable mount. When the interior handle is operated, the release lever tilts to open the latch hook. The release lever of the exterior handle is slightly less visible, but works in the same way. In addition, we can see the wiring of the internal microswitch that registers the position of the latch hook, so that the interior lighting, indicator LEDs and the alarm respond to the opening of the lock. Finally, we see the key operation; when the key blade is inserted into the lock cylinder, the end of the key fits into this slot. When it is possible to turn the key (the lock cylinder rotates), this component in the lock module will rotate with it to mechanically unlock or lock.
Central locking:
In recent years, individual door locking has increasingly been replaced by central locking. By manually operating the lock cylinder in the driver’s door, the locks of the other doors and the tailgate are also actuated, because they receive the command to “open” or “close”. Nowadays, almost all cars are equipped with remote control. An extension of this is automatic control with a remote control or comfort access; in this case, no physical contact is required between the key and the vehicle to operate the central locking.
The central locking system causes a locking pawl in the door lock to be moved. This can be done in different ways:
- pneumatically;
- electromagnetically;
- by an electric motor.
Pneumatic:
Until the late 1980s and early 1990s there were manufacturers that used vacuum-controlled central locking. By means of vacuum, the lock actuators were switched between positions (opened or locked). A diaphragm pump makes it possible to alternately pump overpressure or vacuum. In each door lock there is a diaphragm chamber in which a locking pawl is locked or unlocked, depending on whether there is overpressure or underpressure in the chamber. This system is sensitive to leaks. Frequently opening and closing the doors, and thus bending the vacuum hoses, can eventually lead to breakage. The sealing rubbers in the air chambers of the locks can also seal less effectively as they age. In the event of a leak, you can check the vacuum with a small vacuum pump by successively closing off one line at a time, or check the system with a smoke machine by tracing visible smoke.
The image below shows how many vacuum hoses were present in a Mercedes to control the lock operation of the front doors and the tailgate pneumatically.
Electromagnetic:
With electromagnetic central locking, door locks are fitted with double magnets. One magnet provides the upward movement and the other the downward movement to lock or unlock the lock.
By energising the correct electromagnet (5 or 6), the locking pawl is pushed into the correct position: lock or unlock. Number 7 indicates the magnet core.
This type of central locking, like the pneumatic variant, is no longer used. The undersigned has not been able to find wiring diagrams of a vehicle that uses this system.
Electric motors:
The central locking of modern vehicles is equipped with electric motors. The rotary motion of the motor is converted into a linear motion by a gear mechanism and plastic linkage. Usually these electric motors do not have an end stop and the mechanism runs against the stop and blocks the electric motor.
To make the electric motor lock or unlock, the current direction, and thus the direction of rotation of the electric motor, is reversed. The wiring diagram below shows the components in the central locking system of a modern car.
Legend:
- 30: battery positive
- 31: ground
- ACC: switched positive (terminal 15)
- P23: on-board power supply control unit
- M55r: tailgate lock unit
- M31: driver’s door lock unit
- M42: left rear door lock unit
- S23L: central locking switch
- S31: left front door contact switch
- A28r: central locking receiver
In this diagram, the lock unit of the driver’s door (M31) and the left rear door (M42) is shown. In reality, there are two more of these lock units (right front and right rear) in the diagram, but they have been omitted due to the size. Furthermore, we see the lock unit of the tailgate (M55r) and the central locking switch (S23L), which is integrated in the dashboard, centre console or door trim. Switch S31 is mounted separately in the B-pillar, but in some cars this switch is integrated into the lock unit. Finally, we see A28r: the radio receiver for the central locking, which receives the signal from the remote control. Control unit P23 processes the signals from the switches to control the locks and, if applicable, the lighting and indicator LEDs.
The two images below show a door lock of a Volkswagen Golf mk5, one in the unlocked position and one in the locked position. In the locked position this type of lock can also be double locked, the so-called “safe lock”. This double locking is explained in the next paragraph.
When you press lock on the remote control, the double locking is activated. In the image above, the small mechanism with the light blue plastic part rotates clockwise to the left. The lock can no longer be unlocked if the double locking has been activated.
The double locking prevents the lock from unlocking when the interior handle is operated. This prevents someone who smashes a window from opening the door from the inside. How the double locking is activated can differ between cars:
- Pressing the lock button on the remote control once locks the door “normally”. Only when the lock button is pressed twice is the door double locked;
- At the first press of the lock button, the door is already double locked.
The following diagram shows the door lock module (below) of a VW Golf mk5 with the control unit (A32m) above it, which controls the lock.
The two oscilloscope images below were recorded during locking and unlocking of the door lock of this VW Golf.
- Channel A (blue) is connected to pin 9, the white wire on M34: this is the double locking motor;
- Channel B (red) is connected to pin 2, the yellow wire on M32: this is the locking motor.
The first scope image was recorded when locking with the interior switch and the second with the remote control. At time t = 1.0 s in both scope images, the lock is locked and at t = 3 it is unlocked. In reality, the time is multiplied by the number of ms/div; in this case 500. When locking with the interior switch, we see that the locking motor (red signal) provides the locking of the lock, but that the double locking motor (blue signal) is not actuated. We do see this in the second scope image; here the double locking motor is actuated. In this case, the lock is locked and unlocked with the remote control. With the double locking activated, the doors cannot be opened from the inside.
Child lock:
The rear doors contain a switch with which the child lock can be activated. When the child lock is activated, the rear passengers cannot open the door from the inside. When the handle is operated, there is no noticeable resistance and nothing happens. Children cannot open the door themselves. The child lock works independently of the door locks; it therefore does not matter whether the locks are locked by means of the central locking system.
The type of switch depends on the manufacturer: sometimes it is a small lever that can be slid with the fingers, other times the key must be inserted into a slot in order to turn the switch.
