Door Locks, Vacuum, and Ignorance, 300SD - PeachParts Mercedes-Benz Forum

treiberg · #1 06-10-2010, 01:01 AM

On my '85 300SD, the door locks work great...except for one thing. When locking (or unlocking) the driver's door, the other doors do not automatically lock (or unlock). I can go to the trunk or the passenger door and unlock those, and all the other doors will follow suit, locking or unlocking. But the driver's door does not activate the car's main system to lock or unlock the other doors.

I know the driver's door actuator is functional, as it will lock and unlock when you do it from the trunk or passenger door.

Took the door panel off to look around. I figured there must be an electrical switch that is not making contact to let the vacuum system know that it needs to lock/unlock. But it almost looks to me that this fabulous vacuum system is somehow entirely electronic free...it somehow activates itself with vacuum, besides using vacuum as a force to actuate locks. Wow, I really want to understand how this works so I can troubleshoot it.

Can some experts shed some light on how this works? What and how should I test to fix this pesky issue?

cirrusman · #2 06-10-2010, 01:45 AM

I'm not by any means an expert but I do know that the doors have an electrical switch that opens or closes the electrical circuit that goes to the vacuum pump in your trunk, sucking or blowing air to lock or unlock your doors. My switch is somehow smashed to smithereens, but it's there (obviously it doesn't work and needs replacement). Locate that switch and replace it as needed.

treiberg · #3 06-10-2010, 02:53 PM

OK, so there is an electrical switch that is used in these doors to activate lock/unlock? I do see one wire that goes up close by the lock mechanism, so maybe that will lead me to the switch. Hopefully, just switch can be replaced.

The reason I didn't think there is an electrical switch is that at the trunk lock I saw no electrical wires, only the vacuum line. And when I lock at the trunk, it locks all other doors just fine. So I am baffled at how that works if there is not an electrical switch. I think the older models used a three-hose vaccum switch, but maybe my 85 300SD does not.

cirrusman · #4 06-10-2010, 03:47 PM

There is a connection wire that goes to the trunk electrical switch from your driver side tail lights. The wire is really well hidden inside the trunk wall and it's not obvious at first sight, but it's there.
And by the sounds of it your system works quite well, so I'd say that your woes will be gone once you replace the drivers side switch, which shouldn't be too hard to do.

treiberg · #5 06-10-2010, 10:48 PM

It looks like the switch is maybe built into the actuator, because I see some wires coming out of the side of the actuator. Can a person just replace the switch part, or do I need to replace whole actuator? Does anyone know of a good DIY article on W126 actuator or switch removal?

cirrusman · #6 06-10-2010, 11:41 PM

I'd replace the whole thing. As long as you're at it, why not? It would take longer to dismantle the whole actuator to replace only the electrical component. Wouldn't make sense to me.

locry · #7 06-11-2010, 12:16 AM

OR... do what i did, i bypassed that switch altogether. i installed an alarm ecu and wired it directly to the vac pump. don't need to fiddle with keys anymore.

reason why i did this is because the keyhole on the driver's door was starting to freeze up. and i always wanted a keyless system.

just wire the lock/unlock pulse to a couple of relays to supply the pump with the required lock/unlock current. (=/-) IIRC

treiberg · #8 06-13-2010, 10:28 PM

I tested the switch on the actuator and it operates fine. The problem seems to be in the wiring. I unplugged the wiring at the door switch and back at the pump and I cannot get any continuity back to that plug. The passenger door is fine - when I lock and unlock it, I go from open to closed circuit on the wiring to the pump (or maybe vice versa). But no response from the driver's door wiring. Possibly have a cut wire someplace, but what is the best way to track it down? Looks like it goes back to the fuse box, so maybe I can do some checks there. But since the wiring goes through the door boot and then up and under the dash, and then through the firewall and to the fuse box, seems pretty tough to trace it out. Of course, there is a spider web of wires under that fuse box, so that is pretty intimidating (maybe it shouldn't be). Any advice?

It's funny how much time a guy will put in just to have some silly lock feature functional again. it doesn't make sense, but it does feel good when the problem is resolved, so I guess that's why we do it.

The keyless system might be worth considering if I cannot figure this out soon. How much do they cost?