(This is a re-post of an earlier article. This one fixes a wiring error on the schematic, and offers some tips on the W124 model install as well.)
Here is how to install a keyless lock/unlock system for around $25 and in about one hour. I hunted through forum to find detailed instructions on how to do this on my model car, and only found bits and pieces, so here goes. This will only work on those cars that will lock and unlock all doors from the trunk (W124's later SD's, from what I understand). These cars have a vacuum pump either in the trunk (W126) or under the rear seat (W124) dedicated to the door lock system, as opposed to the W123's and older that use residual vacuum from the engine bay to lock and unlock doors. This makes this so easy because everything you need to do is at the air pump (no messing with the doors or actuators). Also, no messing with relays - some of the threads I read while doing research used relays and I found them to be unecessary with the unit I installed.
1) Purchase cheap unit from eBay. Mine came from China and was about $25. It is imperative that the unit have a signal delay for pneumatic operation. Search listing for work pneumatic, and if not there, don't buy. Do a search with "remote keyless pneumatic" as the words, and check off "include title and description" when doing your search. You will find some for as low as $15! Do NOT include the word Mercedes because these are generic units and will not list any compatibilities with any particular models. These are all Chinese units and the wiring appears to be the same on almost all of them.
2) Locate air pump in trunk for the W126. It sets next to spare tire and is easily accessible by removing a cover that is held in place with a couple of screws. On the W124, you will find the unit under the rear bench seat.
3) Remove the jumper on the wireless module for the pneumatic delay option to allow the 3.5 second hold that the pump will be looking for.
4) Unplug power from pump just to eliminate any chance of shorting something (it is the smaller plug with red and brown wires). You will need to splice the keyless unit into one of the wires (I did the green one which goes to the passenger door). You will also need to pick up power and ground signals which can come from the red (12V) and brown (ground) wires that feed power to the pump. Follow the schematic I show in the pic. This took me the longest as I could not locate a schematic anywhere that was a perfect match for what I needed.
Special note on the W124: You may find there are two wires of each color, so there will be two green wires. You will use one of the green wires, but only one works properly. You have a 50/50 chance of picking the correct one, so if things don't work, you will need to splice into the other one.
5) On the W126, I placed the keyless unit right on top of the rubberized pump holder, and underneath the trunk spare tire cover. It fits nicely in there, and you can secure it using some duct tape (or something more elegant). You can trim back all the wires in the unit that aren't needed and it makes for a pretty neat install. On the W124, there is room to place the module right next to the air pump.
6) Mine came with a blue flashing LED that flashes when the car is locked. I ran that wire up into my taillight, on the clear lens, and it looks really cool at night, and it lets you know things are locked up. Takes about 2 minutes. (I only did this on the W126 because I was already in the trunk.)
That's it. Of course, there are several wire connections you need to make using your favorite methods, but it is a pretty clean install.
Some tips:
-I tried to use some scotch connectors from Harbor Freight for the splicing. The concept is cool, but in practice I was not getting very good connections, and had to crimp down pretty hard on these to finally get them to make good contact and conduct properly. Soldering and heat shrink would probably be better.
-If you find things don't work, it might be a bad connection, and you just have to use a multimeter and start looking for continuity and proper ground and power voltage at the proper location on connectors. Took a little while to resolve my bad connections. Do it right the first time and it will save you time.
-Both units I rec'd had faulty wiring on the power wire and I had to cut and re-solder a wire at the fuse. This was a very strange coincidence to me. It was easily resolved, but points to the nuisance quality issues with a $15-$25 'made in china' unit. This is the only problem I had and once installed, things work great.
-The top three wires in the schematic are used for unlocking. The bottom three are for locking. The system requires a ground signal to lock, and 12V to unlock. The three wires merely route either a 12V signal or a ground signal to cause the pump to have pressure or suction.
-In theory, you could probably splice into the yellow wire (goes to trunk lock switch), or blue wire (goes to driver's door), but I used the green one.
-When you are done, you should still be able to use the key to lock/unlock all the doors, as long as that worked before installing the unit.
-On the W124, it is much tighter in that compartment under the seat, and is more difficult because of this.
Hope this helps someone out there, it would have saved me a couple of hours if I knew before what is in these instructions. Thanks to member locry for the idea to do this from this thread (
http://www.peachparts.com/shopforum/...d.php?t=279028) and thanks to member ah-kay for assistance as well.