W126 Vacuum Pump (door lock) problem
 

In my (new to me) 126 the door locks haven't worked so I took a look at them today. The 126 has a dedicated pump for the door locks that resides in the trunk. When I pulled it out it was unplugged, so I plugged it in. It ran for about three seconds then blew the fuse. I replaced the fuse, plugged it back in and it ran for about 20 seconds. While it was running I tried the door locks from the drivers door using the key. Nothing happened. By this time the pump had stopped and not blown the fuse. I then unplugged the pump and plugged it back in. In ran for a few seconds and stopped. I tried the key again and now, the locks still don't work but the pump runs for about a half second each time I turn the key. It is my understanding from searches, that this system uses vacuum to lock the locks and pressure to unlock. I plan to pull the vac. line off the pump and draw a vacuum on it with the mity vac and see if it will lock the doors. If it won't I know I have a leak to find. If it does, I think then I need to investigate the switch on the drivers door lock. Does this sound like a good plan? I will need help testing the switch as I have no idea how. I do have a meter and a little knowledge of how to use it. Any input is appreciated.

It could be that the pump had a problem that caused it to draw too much amperage, therefore; someone unplugged it.

Pulling a vacuum with your miteyvac, is a good place to start. See if you can pull a good vacuum on it. If you can, and it holds, that could indicate the pump. You can also test the pump to see if it pulls a vacuum, just be hooking the miteyvac directly too it, and plug it in. the gauge will indicate if it can pull a vacuum.

Thanks, Wodnek. I'll try that this afternoon. Any ideas on checking the switch in the door? From my reading, the drivers door has no vacuum lines. It is key operated and has a switch that operates the other locks.

High resistance in the electrical portion of the vacuum pump motor/circuitry is what blew the fuse. Sounds like a trip to the yard may be in order for another.

You may be right. It hasn't blown one since the first, but I don't think it had to pull a vacuum on anything either. I'll plug in the mity-vac and see what it can do. This may prompt another fuse blowing incident. In which case I'll head to the junkyard like you said. Thanks.

Well I have done a little more investigation and here is what I have found. I plugged the pump into my mity-vac to see what it could do. I plugged in the power, it came on and pulled and maintained a 14" vacuum, after 30 seconds or so the pump stopped on its own. I then drew a vacuum on the locks. The two right side doors locked at about 8", I continued to pump it up to 15" and left it. It maintained the vacuum at 15" for about 30 min. until I unplugged it. The drivers door didn't lock, but it has no vacuum mechanism it is key only operated and it is the switch for the other locks. The drivers side rear door didn't lock either but it is very difficult to operate by hand, apparently something is binding. Next I tried to operate the locks via the key. Nothing happened, the pump didn't come on, nothing locked or unlocked.

So from my investigation I have concluded (correct me if I am wrong) that the pump appears to be OK, at least for vacuum, the "switch" in the door lock seems to not work, the system held vacuum sufficient to work properly.

Remaining questions: I have not had the pump come on and provide pressure. It's my understanding that this is how it unlocks. How do I check the "switch" on the drivers door? I need to get to the mechanism on the rear drivers side door to see what makes that lock so stiff.

Any help on where I go from here would be appreciated.

It may be that you just need new actuators in your doors. In my 81 300SD the rear driver's side actuator got into a bind when a mechanic worked on the (forget what it is called) door return thing that connects to the B pillar. Seems like the actuator was pretty cheap - about $25 but it has been a couple of years. Very easy to replace. I went through all the motions with the Mityvac and ended up replacing my trunk-bound pump (because I took apart a perfectly good pump to see how it worked and could not get it to seal again) along with two door actuators. My actuators were the only problem all along. All locks work perfectly now.

Shouldn't something happen when I turn the key? The other day when I did it the pump would run for a half second. Now nothing. I guess the door panel needs to come off and take a look at what's there. At least I know the system holds vacuum, and I assume, pressure. The actuators in the to passenger doors are OK. I just need to figure out why it won't turn on and off.

There is electrical switch in the door that runs the motor, and the vacuum/pressure switches in the pump tell it when to turn off. The two wires, blue and yellow, to the pump come from the driver's door and one of them should have voltage to ground. Whenever the door is locked/unlocked the voltage switches between the two wires. You could have a bad actuator in the pump unit that isn't switching properly which might account for your symptoms.

With the mityvac hooked into the vacuum circuit use the key in the door to cycle the pump (i.e., lock the door). Does the mityvac register vacuum or pressure? Now unlock the door. Does the mityvac register the opposite now? What happens when you try to measure the voltage from the door switch at pump connector?

Also, the current through the fuse also travels through the door switch. It's possible the excessive current has burned the contacts and now the motor isn't getting the current it needs to run.

Thanks for the help Micalk. The only way I can get the pump to run is to unplug it, then plug it back in. When I do that it produces vacuum (14"). Turning the key won't turn it on. It's dark here now, so I'm done for today. I will try to "T" the mity-vac into the system tomorrow and see if I can get it to produce pressure. I'll also check voltage at the pump while turning the key both ways.

xnam, welcome to the forum, it looks like your posts are all over the board. which vehicle do you own? it seems you are knowledgeable on many fronts!

authorized people? $140/hour Mercedes Dealer authorized or $110/hour independent shop Authorized?
nahh. this site is dedicated to assisting mercedes owners repair their vehicles for FREE! (well, the written assistance is free anyway...)

if the pump pulled 14" and the doors locked at 8" why did the doors not lock when you had the pump hooked up?
the electrical portion of the switch in the driver's door could be ok, but the wires in the doorjam area may be broken, so when you get the panel off the door, test the wires themselves. use some jumpers to see if you can activate the pump, if not, it may be the bundle of wires in the jam causing an issue. are there any issues with seat controls or window movement in the driver's door?