DIY Keyless Entry Door Lock - W126 and W124
 

(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.

Sounds great.

I designed a relay board for mine that uses a Ford remote keyless entry module from an Explorer. That output has to pull down (or up if unlocking) that line for a couple of seconds for the locks to operate properly.

Installed mine last night. With the directions it took less than an hour. Thanks!

Does this work with or around the factory security system?

Sixto
87 300D

I weent to Ebay and searched on "remote keyless pneumatic" and could not find what is needed. Is there a particular vendor that has these divices? I would love to do this my 84 300SD.

Make sure the answer is YES to either of the following questions. I meant lock/unlock everything. Otherwise you will need an actuator inside the driver's door, that would be more work.

1) Can you central lock/unlock from the trunk with key?
2) Can you central lock/unlock from the passenger door with key?

3) Buy a keyless remote from eBay which supports pneumatic operation. Most do. Ask the seller. Most seller put the 'pneumatic' keyword in the description, not in the heading. Try this.

e.g.

http://cgi.ebay.com/2-FOB-Universal-Car-Keyless-Entry-Remote-System-Kit-/250597424095?pt=LH_DefaultDomain_0&hash=item3a58c53bdf

This install is keyless entry. The existing MBZ alarm function is NOT affected.

Hello Ah-Kay,
On my 1984 300SD the drivers door handle has a wire going to the key mechanism, but the trunk lock and the passengers door lock do not. By this I mean if I put the key into the drivers door and unlock it Everything unlocks, all the doors, trunk, and fuel filler. If I unlock the passengers door or the trunk with the key nothing else unlocks.

Will this keyless system work with my vehicle? The vacume/pressure pump is in the spare tire well.

If you cannot lock/unlock from trunk or passenger side door then you are out of luck. You do not have vacuum line goes to the driver door. You need an EXTRA actuator. It will be much more difficult to install. It is PITA to install unless you are very handy. You need to install an actuator to actuate the door plunger to make it do up/down, which in turn makes the wiring contact.

The unit from eBay will work but you do not wire it up in pneumatic configuration. It will work only after you install the actuator, unfortunately.

Good luck.

You can do this faster and cheaper on a w124. I bought a 2-channel momentary output remote pair and installed the receiver in my trunk. Tapped into the three wires going to the trunk latch, to get + and - for the receiver power, and one line controls lock and unlock. Grounding the wire (via output relay) locks, and providing a +12 to the wire (via the other output relay) unlocks. (I may have confused which is which.. but you get the idea).

Key still works everywhere as it should.

Don't care about the alarm since I've disconnected that, but I imagine this mod would work just fine. It emulates un/locking the whole vehicle from the trunk using a key. So if this key action would normally un/arm the factory alarm, then this model probably does the same using remote.

I haven't used a key in the door tumbler since...

The remote even has a lock / unlock symbols and is keychain style, and get this.. has metal and a slider cover... it's meant for vehicle application you can tell.

ebay... $10 GO CHINA!

working strong 2 years later

i wouldn't go the cheap method. use an mb keyless entry system meant for our cars:

http://www.mbkeylessentry.com/1.html

i've been running with it for two years now and it works perfectly (of course the guy who owns the website installed it himself on my car)

You're welcome to spend eleven times the money for essentially the same function! Just pointing out the economical option for those so inclined.

Toodles!

Very interesting indeed.:cool:
Might order one for the 300 SDL 1986.
Thanks SEV

On the system I installed I needed to tie into the wire that goes to the truck. Works great.

You're welcome! Glad to hear of it.

help! I screwed something up! I bought the exact same unit you show in the pictures. No problem getting power and having the module click on and off. Once I tried to get the lock and unlock hooked up (I have a W124 under the seat with one green, one blue, and two yellow wires) the module stopped functioning all together (pump and locks still work fine). Before I tried lock and unlock wires, it was clicking on and off with the blue light flashing then on one attempt it just stopped working. Should I have spliced all three top wires together or just one (which is what I did) and bottoms? Can the unit be reset? There is a button on it but no directions on how. Fuse did not blow so I'm thinking I shorted something out internally:(

Steve