door locks in w124

[oso]

All of a sudden my doors unlock themselves after a few seconds. I can lock the car just fine, but less than 15 seconds later up they go... Any idea?

[compress ignite]

1. If your let us know which w124 you have,in the future,advice will be more
timely and accurate.

There's an electric switch in the drivers door(and passenger too), that signals
the Electric Vacuum Pump (under the passenger side rear seat) to lock+unlock
the doors,fuel door,trunk.

There also may be,depending on your model, a convenience relay circuit that
allows closing all the windows and the sunroof by holding the key in the locking
position in the:driver's door,passenger's door and the trunk.

I've spun the Mercedes Wheel of Obfuscation in Diagnostic Frustration and it
likes the Electric Switch in the Driver's Door for this Offense to the Owner.

EDIT: for clarity .WE are talking about locking the doors with the Key?
OR are we talking about pushing the Driver's door "Button" down once you're
seated with the door closed?

[Jeremy5848]

Could this be a vacuum problem? The 124's use of electric solenoids operated by switches in the door and trunk/tailgate lock cylinders and controlling vacuum to the actuators is both complex and somewhat confusing. It took me several hours to get my '87 fixed (the PO had boogered the connections badly while installing an after-market alarm).

[compress ignite]

Yes,The Last and Greatest Offense
(And the most insidious), is the P.O. DIY "FIX".

I still cannot believe that the retro-fit of the aftermarket AutoMate keyless
entry system to the 124.128 ,by P.O.,has not resulted in some of that
maddening foolishness.So Far,So Good
(Fingers always crossed,when I push the remote button)

And Yes, It is "A" vacuum problem(the vacuum pump responding to some
electrical signal) ,BUT from which switch/relay in which system?
Convenience or Locking?

[No chance of My putting a DMM to any of this W124's electrical system,
So prognosticating that the Driver's door key switch got the most action
It MIGHT be the most failure prone,just due to more usage than the others.]

Right now we're kinda like ER room Physicians...The ambulance has just
radioed in that they are transporting a patient...And the Ambulance's
radio failed at that point.We know (We hope)it's a W124,whose locks are
acting up.

[Jeremy5848]

Remote-control diagnosis is always frustrating and potentially dangerous. We do need "Bear" to tell us which specific 124 car he has and also whether he is locking the car from the inside or the outside when this self-unlocking happens. I would guess "outside" but confirmation is needed.

Since (a) the doors lock/unlock by vacuum/pressure and (b) the 124 lacks the 123's vacuum reservoir, something is timing out after 15 seconds and turning the vacuum pump on in "pressure" (unlock) mode. From my examination of the electrical/vacuum schematic diagram for this subsystem, I see that there are connections between the so-called "central locking system" and the "anti-theft system." It could be that the wiring has somehow been disturbed such that when the alarm turns itself on 15 seconds after the doors are locked (if that's what it is supposed to do), it also causes the pump to unlock the doors.

Beyond that I cannot guess without more information from the OP.

Jeremy

[oso]

Sorry, guys... 1987 300TDT. Door is locked by the key on the driver's side. And yes, after the locks go up "by themselves" in just a few seconds and you try to open the door- the alarm goes off ( what better way to start a Sunday morning...).

[Jeremy5848]

OK, that's good information. Let me pray over the schematics for awhile and, in the meantime, Oso, tell us --

[I'm editing on the fly as ideas occur to me -- forgive the mess and length]

Try this experiment first -- find the vacuum pump (under rear seat, right side). Find connector X2 (see schematic, below, a round connector with three pins). Unplug it. Now lock the car -- lock all the doors from the inside except the driver's door, then get out and lock the driver's door. Does it stay locked? (This will not lock the fuel filler flap) If so, the pump is being told to unlock by one of the front doors or the tailgate.

Next, try removing one pin at a time from connector X2 and plugging the connector back in. That will disable one of the door or tailgate switches. Try locking the car again but now you can use one of the door locks from the outside and everything should lock. Go through the pins, one at a time, testing the locks each time. If my theory is right, one of the pins will be connected to a stuck switch or shorted wire. When that pin is removed, the self-unlocking problem will go away.

The rest of this was written before the above so you can ignore it if you wish.

(a) when the doors unlock, can you re-lock them with the key? and do they unlock again after a few seconds, or does something else happen?

(b) does it matter which door you lock with the key (driver, passenger, tailgate)?

(c) what is the build date of your car (it's on one of the data labels on the driver's door sill)? There was a change in the cars so that Mercedes labels all of its diagrams "May 1986" or "as of May 1987." I suspect your car is the latter but tell us anyway.

There are four pages of schematics for the anti-theft alarm system and one more (image attached) for the central locking system.

I wonder . . . could you have a stuck switch? The door locks (driver, passenger, tailgate) all have single-pole, double-throw (SPDT) switches attached to them. If one of the switches were stuck in the "unlock" position, what would happen? Might the stuck switch unlock the doors after you locked them (leaving the alarm activated)? Question (b) above is very important!

Something is telling the vacuum pump, which Mercedes calls "M14/2 Supply Pump," to "blow" (unlock) instead of "suck" (lock). According to the schematic, the only things that can do that are the three door lock switches or something broken in the pump itself. . . thinking . . . this is a W124 car, after all . . . how about broken/shorted wires in between one of the doors and its door pillar? That is usually a rear door problem (the rear doors play no part here, IMHO) but could also affect the front doors or the tailgate. A shorted wire could act like a stuck switch. The driver's door gets used more, might be the prime suspect.

If it is a stuck switch or shorted wire, then removing one pin at a time from connector X2 where it plugs into the pump would be a test to isolate the problem to one door or the tailgate. If you simply unplug X2, then there is nothing to tell the pump to run. You would have to lock the doors manually (how crude!) but they would not unlock by themselves.

All of this assumes that your wiring is virgin -- no after-market wireless locks, no after-market alarms, no after-market stereo boogered-wiring installation jobs. (In which case, all bets are off.)

Jeremy

[oso]

Thank you Jeremy5848- I have my homework assigned and I'll get to it after work. One problem I know I will have is my inability to get the seat cushion to lift and give access to the pump. Tried to get to it few times but would not budge.

[Jeremy5848]

I don't know if the T-sedan's rear seat is different -- some of them fold up to make a bigger cargo area, right? Even the sedan's rear seat is sometimes hard to remove, the release clip seems determined not to release. I'm also not positive that the pump is under there in the T-sedan, even though that's where the parts locater says it is. We need help from other wagon owners.

[BrierS]

Read through quickly so excuse me if I missed something. I believe he has a 1987 300TD (estate wagon with turbo). If so, I'll check mine in the morning (once the sun is up) to see if the connection you are referring to is there or not. I don't recall seeing it over the many times I have had the rear seat (not the reversed seating position folding seat in the back) up in mine.

As to lifting the second row seat, are you pulling the two release handles on either side? If you have not yet located those, they have molded finger/hand reliefs in them and are found on the outer ends of the lower seats once the rear doors are opened.

Like I said, I may have missed some details as I skimmed.
Steve

[oso]

Jeremy5848 had it right- clip does not want to let go no matter how much I tried to work the release lever.... Something must have broken inside.However, after following your recommendations I have this to add: key will lock the doors properly from the passanger side door as well as the tailgate side. So, I gues I should take the panel off the driver's side and look for the wires?

Wes