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  #1  
Old 10-25-2017, 10:04 PM
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W124 Central Locking

Trying to troubleshoot a central locking issue I'm having, which is being made slightly difficult by the fact that I really have little idea how the system works, lol. This is what's going on:

Several weeks ago, I noticed that I couldn't activate central locking/unlocking from the passenger side door. I was a little annoyed, but not overly concerned, as this is after all a 25-year-old car. I kind of looked into it a little bit, but after a couple of days it started working again, so I forgot about it.

Then a few weeks later, the driver's door started to do the same thing. I kind thought WTH? but then it, too started working again - and then stopped again. Now it's been a few weeks where it hasn't worked, so I'm pretty sure it's gonna stay that way.

What's funny is sometimes I'll unlock the driver's door, and start driving around - and a few minutes later, all the doors will unlock, like they just realized the one door was unlocked. It seems to happen especially when I'm applying the brake. I thought about that for a little while - I know the brake booster is intimately involved in the vacuum system, and I wondered if maybe somehow by braking I was increasing the vacuum signal somehow. But then I realized that the central locking system has it's own vacuum pump, beneath the rear seat - so what gives? Are the two systems completely independent of each other? If so, how and why would braking affect the central locking function?

Ideally I would like to fix this issue, but I'm not really interested in ripping off all the door cards to troubleshoot everything. Part of me thinks it can't be an issue with the individual diaphragms in the doors, since they work sometimes. So maybe it's a VP problem? That would be preferable, because the interior VP is very easily accessible. Any pointers on where I should start?

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  #2  
Old 10-25-2017, 10:21 PM
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Quote:
Originally Posted by Bimmer-Bob View Post
Ideally I would like to fix this issue, but I'm not really interested in ripping off all the door cards to troubleshoot everything. Part of me thinks it can't be an issue with the individual diaphragms in the doors, since they work sometimes. So maybe it's a VP problem? That would be preferable, because the interior VP is very easily accessible. Any pointers on where I should start?
Perhaps a review of the FSM section on the central locking system?
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  #3  
Old 10-25-2017, 10:23 PM
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Does the W124 have an electric vacuum pump in the trunk like the W126 does? If so, you could have issues with the electric pump, it will control the locks not the main vacuum system off the engine.
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  #4  
Old 10-25-2017, 10:43 PM
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Yes, his car has an electric pump.
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  #5  
Old 10-25-2017, 10:54 PM
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Sounds to be an electrical problem locate the pump under the rear seat passengers side and start trouble shooting.
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  #6  
Old 10-25-2017, 11:06 PM
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Does the pump lock and unlock from the passenger door and trunk? Do the doors lock from the driver door? If so it sounds like the driver door central lock switch is bad.

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98 E320s sedan and wagon
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  #7  
Old 10-25-2017, 11:06 PM
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Thanks everyone for the tips. I will start with the electric pump. I'm just confused about why the unlocking mechanism is activating when the brake is depressed - any ideas?
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  #8  
Old 10-25-2017, 11:08 PM
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Originally Posted by sixto View Post
Does the pump lock and unlock from the passenger door and trunk? Do the doors lock from the driver door? If so it sounds like the driver door central lock switch is bad.
Oh, that's good info that I meant to include. The passenger-side stopped working, but now works fine again.

The driver-side door stopped working, started working, then stopped again (current state).

The trunk has always worked.
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  #9  
Old 10-25-2017, 11:21 PM
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When you say stops working, do you mean doesn't lock or unlock or only doesn't unlock?

IIRC there are two connectors at the pump. The round connector has a wire from each of the door switches. +12V from any wire tells the pump to unlock, ground tells the pump to lock. Although the signal is continuous, the pump runs until it detects the action is complete with a 30sec(?) hard time out. Test switch function at the round connector. Phantom unlocking could be braking motion moving loose contacts in the door switches.

Sixto
98 E320s sedan and wagon
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  #10  
Old 10-25-2017, 11:50 PM
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Quote:
Originally Posted by sixto View Post
When you say stops working, do you mean doesn't lock or unlock or only doesn't unlock?
Both doesn't lock or unlock.
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  #11  
Old 10-26-2017, 12:18 AM
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Hmm... the door switches get power from the same circuit as the pump itself. The trunk and passenger door and trunk are grounded locally, the driver door grounds through the key-in-switch warning circuit. I don't know how any of this is useful. Get a remote entry kit from ah-kay and be done with door switches

Sixto
98 E320s sedan and wagon
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  #12  
Old 10-26-2017, 06:22 AM
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All information is there in the link from Alec 300 did you check it out the link it is in blue .I would get a might vac tool , and check the vacuum pressure ..
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  #13  
Old 10-26-2017, 08:16 AM
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I had this problem with my 300TD also.

The problem is there are wire splices in the cable duct under the floorboard under the passenger front seat.

You'll need to remove the passenger seat, the door sill plate on the right rear door, the b-pillar trim, the right rear seat cushion, and the escutcheons around the seat cushion support pins. Now you can peel the carpet back and open the wire tray.

You will see a taped up wire harness area. Very carefully cut open the tape and you'll find several sets of wires tapped together in white capped connectors. If yours is like mine the wires had corroded internally and pulled out of the connectors. These wires provide power and ground to the door switches and the pump.

The door switches get a power line and a ground line, and the yellow/green/blue wires at the pump either get power or ground to lock or unlock the doors.

Before starting work remove fuse "C" to de-energize these wires, they are live all the time.

Solder and heat shrink solved the problem.

There is also a splice there for the rear dome light switch from the tailgate and the rear doors. Fixing this splice also fixed my rear dome lights (c- and d- pillar lights).

Credit to pwogaman who found this when fixing up his $450 salvage auction wagon.

Bonus: while you are in there, add keyless entry for cheap!

DIY Keyless Entry Door Lock - W126 and W124

Be sure to start at around post 59, there was some bad information at the beginning of the thread.

And one more tip: the 3 pin connector for the door control leads on early 124s has a nasty habit of crumbling. They used a plastic that degrades badly. The later 124s used a better plastic. The connectors can be salvaged from later 124s in the junk yard and spliced in.
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Last edited by jay_bob; 10-26-2017 at 08:28 AM.
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  #14  
Old 10-26-2017, 09:53 AM
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Quote:
Originally Posted by sixto View Post
Hmm... the door switches get power from the same circuit as the pump itself. The trunk and passenger door and trunk are grounded locally, the driver door grounds through the key-in-switch warning circuit. I don't know how any of this is useful. Get a remote entry kit from ah-kay and be done with door switches

Sixto
98 E320s sedan and wagon
I was just thinking that but I think he just sold his last one! For a little while anyway. Does his kit just piggyback onto the vacuum pump in the rear?
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  #15  
Old 10-26-2017, 09:54 AM
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Quote:
Originally Posted by Alec300SD View Post
Quote:
Originally Posted by optimusprime View Post
All information is there in the link from Alec 300 did you check it out the link it is in blue .I would get a might vac tool , and check the vacuum pressure ..
Oh yes, I meant to thank him for that. Looks like a very specific troubleshooting procedure, which is what I'm after. So thanks, Alec300SD!

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