I decided to begin work on my next project; adding a central locking switch.
In my car the central locking works from the key in either of the front doors or the trunk.
When you’re in the car you can lock the whole car by pushing down the lock in either of the front doors. To unlock from the inside, you have to physically open one of the front doors. That’s where the problem comes in. It’s slightly inconvenient to open the door every time I want to unlock it.
My car is European spec and the door locks are smooth-there are no mushroom heads from which to pull the lock open. So the only way to unlock is to open the door.
I always though that it was less than ideal. It would be nice to have a dedicated switch to operate the central locking.
The later 124s with infrared remote locking have a central locking switch which is located where the fader went in the earlier cars. I grabbed one of these switches in the JY along with its socket and some wire. I bought a center console wood with an extra switch hole and it will look like a completely factory stock installation.
I thought that I could just splice into one of the control wires for either of the front doors or the trunk. Then I could use the switch to either send a ground signal or a 12v (+) signal to fool the pump into thinking that the door has been unlocked/locked and thus trigger the central locking pump to run and voila- a central locking switch.
In my testing, it turns out to not be so simple…
It won’t work that way.
The reason is, the door lock, when down, sends a constant ground signal, and when up sends a constant (+) signal. (or is it vice versa)
So if I use my switch to send a (+) signal when the lock is still down- it will be a short circuit! And if I use my switch to send a (-) signal when the lock is still up, that will be a short too! - Not good. That also explains why each lock has a separate circuit to the pump independent of the other 2.
It’s a lucky thing that the pump has internal protection circuitry, otherwise I could have damaged it doing my testing.
The only way to make my switch work is to use two single pole double throw (SPDT) relays.
The two relays are in series with one of the door lock signal wires. When the relays are both unexcited, it allows the signal from the lock to pass through uninterrupted. So the door locks will continue to work as original.
If my switch is activated, it excites one or other of the relays to break the circuit with the door lock and allow my switch signal to go in its place. One relay for the (+) signal, and the other relay for the (-) signal.
The relays also need to have a timer function so that they stay activated for about 3-4 seconds after they are pulsed-to give the central locking pump time to finish locking the doors.
I found this diagram on the 500E board:
![Image]()
Now that I look at the switch, it also needs modification before it can work.
It’s a normally open (NO) single pole triple throw SPTT,CO (change over) switch.
The problem is that the common is floating while one of the throws is sharing the ground with the switch’s illumination. I need the ground for the illumination to be connected to the common for it to work. Then I permanently ground the common. I need one throw to ground one of the relays and the other throw to ground the other relay. The way the switch is now, it would be constantly grounding one of the throws.
Another thing this made me think of is I could also add a switch which is taking its signal from the speed sensor in the speedometer, so that the doors can lock themselves automatically above a certain speed like in my BMW. That would be a nice thing to have and very easy to add, once the above relays are wired up.
In my car the central locking works from the key in either of the front doors or the trunk.
When you’re in the car you can lock the whole car by pushing down the lock in either of the front doors. To unlock from the inside, you have to physically open one of the front doors. That’s where the problem comes in. It’s slightly inconvenient to open the door every time I want to unlock it.
My car is European spec and the door locks are smooth-there are no mushroom heads from which to pull the lock open. So the only way to unlock is to open the door.
I always though that it was less than ideal. It would be nice to have a dedicated switch to operate the central locking.
The later 124s with infrared remote locking have a central locking switch which is located where the fader went in the earlier cars. I grabbed one of these switches in the JY along with its socket and some wire. I bought a center console wood with an extra switch hole and it will look like a completely factory stock installation.
I thought that I could just splice into one of the control wires for either of the front doors or the trunk. Then I could use the switch to either send a ground signal or a 12v (+) signal to fool the pump into thinking that the door has been unlocked/locked and thus trigger the central locking pump to run and voila- a central locking switch.
In my testing, it turns out to not be so simple…
It won’t work that way.
The reason is, the door lock, when down, sends a constant ground signal, and when up sends a constant (+) signal. (or is it vice versa)
So if I use my switch to send a (+) signal when the lock is still down- it will be a short circuit! And if I use my switch to send a (-) signal when the lock is still up, that will be a short too! - Not good. That also explains why each lock has a separate circuit to the pump independent of the other 2.
It’s a lucky thing that the pump has internal protection circuitry, otherwise I could have damaged it doing my testing.
The only way to make my switch work is to use two single pole double throw (SPDT) relays.
The two relays are in series with one of the door lock signal wires. When the relays are both unexcited, it allows the signal from the lock to pass through uninterrupted. So the door locks will continue to work as original.
If my switch is activated, it excites one or other of the relays to break the circuit with the door lock and allow my switch signal to go in its place. One relay for the (+) signal, and the other relay for the (-) signal.
The relays also need to have a timer function so that they stay activated for about 3-4 seconds after they are pulsed-to give the central locking pump time to finish locking the doors.
I found this diagram on the 500E board:
Now that I look at the switch, it also needs modification before it can work.
It’s a normally open (NO) single pole triple throw SPTT,CO (change over) switch.
The problem is that the common is floating while one of the throws is sharing the ground with the switch’s illumination. I need the ground for the illumination to be connected to the common for it to work. Then I permanently ground the common. I need one throw to ground one of the relays and the other throw to ground the other relay. The way the switch is now, it would be constantly grounding one of the throws.
Another thing this made me think of is I could also add a switch which is taking its signal from the speed sensor in the speedometer, so that the doors can lock themselves automatically above a certain speed like in my BMW. That would be a nice thing to have and very easy to add, once the above relays are wired up.
