W(c)203 – Cheap central locking RF fix

It is common that the rear antenna amplifier fails on the C203. Its a window out job, and I am trying cheap fixes on this car.

The IR door locking works now, (see other post), but it is still a pain. On to plan b.

I purchased a cheap car remote control lock from ebay China (£7.99!!!) Link, Its basically a remote control relay with “rolling code functionality”. Its likely very easy to bypass etc, but I do not leave anything of value inside the car.

Its wiring diagram looks like this:


Its basically 2 SPDT relays connected actuated by a remote control (with some other wires that flash lights and put windows up, if required).

Early Mercedes have an odd quirk in that their interior unlock buttons work all the time (key off, car locked or unlocked). The idea here is to wire the output of this unit to the interior lock/unlock button so that the car can be locked/unlocked remotely.

To start, we must remove the upper control panel:

Then remove the silver screws:

Then remove the buttons (the unclip as one unit) and remove the circuit board:

After some nice probing with a mutltimeter it was decided that the buttons shared a negative ground, and had separate switched outputs.

The idea then, is to tie together the COM outputs on the RF module, and connect them to this shared ground, and then the lock/unlock wires to the other side of each of the switches.

To do this, we solder some wires to the relevant pads:


And hot glue them in place:

Note down which wire is which, then re-assemble:

Off to the CAR!!!

The control box requires constant 12V and Ground, easily obtained on the radio loom:



The other cables connect to the cables we soldered to the buttons, COM to Blue cable, then lock/unlock to the red/black cables.

The shove the whole lot behind the radio in the foam:

Put all the fascia panels back in etc, and you now have a functioning remote control for £7.99.



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