Bought this car about two years ago. The auto door locks never worked to lock, only to open the door locks. No other issues.

Last autumn, I couldn't get the trunk open with the key until I fiddled with it for about two hours and finally got the key to turn and the trunk to pop open. I haven't locked the trunk since but recently found that the key doesn't turn at all so I couldn't lock it if I tried. All the workings inside have been hit pretty hard with wd-40/liquid wrench and that did nothing.

About two months ago, the door locks quit opening with the key. I figure this is a vacuum issue.

Recently, cleaning out the trunk, I found what I believe to be the trunk lock actuator (rectangular black plastic box with vacuum lines coming and going) sitting in the spare tire area completely submerged in dirty water (I was aware I had a leak, not that it was collecting in a sizable pool). Ok, I figure, maybe that will be the fix for the doors and trunk. Also, a few months back, it would just run and run and run and run unless I went and locked the driver door. So besides being submerged in water, it may also have just run itself into the ground (it's possible that I didn't hear it running, went inside, and it ran for hours).

Now, about three weeks ago, my radio and battery stop working. I find a busted fuse, see that it controls the door locks also, replace it, and it lasts for about two hours before it blows again.

So are all these related? Is the fix just getting a new actuator (which I'll need to do anyway)? Or is the fuse issue separate?

Many thanks.

The box in the trunk is a vacuum pump for the central locking system. The fuse is for that pump, which all w126s have (w123s do not). If you put an electric pump into water, it will blow its fuse. That blew the fuse.

If it was running continually when the doors were shut, before that, then you have a vacuum leak that prevented the locking system from working properly.

Your battery died. It's not really related to the fuse being blown.

If the pump ran for extended periods, it did not help matters but likely the battery was not long for this world anyway.

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Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22

First of all the door lock pump has a circuit board in it and circuit boards and water don't mix. You could try opening up the pump and washing every thing with electronic cleaner. Also these cars are notorious for having broken wires at the hinge area of the drivers door and then you need to splice in new wires.

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1983 300SD
200000miles

Also, make sure you have a master key for that car, the valet key won't even go in the lock, it's too wide, glovebox is the same deal.

The engine not shutting off is not connected to the door lock set up. Using a vacuum measuring tool, find out how much vac is being produced from the pump, you should get ~21inHg at one orifice with the other blocked off and less vac going to the VCV.

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83 SD

84 CD

Try looking at this post
I wrote up a description of the door locks at post # 9 and #13
Door locks 1982 300SD W126

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1983 300SD
200000miles

I could have been more clear with my timeline. But both times the fuse blew, the pump was out of the water and dry. As a matter of fact, it could have been in water for 4 or 5 months, during which time the fuse never blew. It wasn't until two or three weeks after I cleaned and dried the trunk that the fuse starting blowing. So I don't think it's as easy as blaming the water.

Also, I had written considerably more before editing this post. I talked about the car not starting but have isolated that to be either the neutral safety switch or shifter bushings. But that problem was most definitely not my battery.

@Phil: Instead of opening the pump up, I'll likely grab one at the junkyard. I see them there all the time. $5 and I'll hopefully be on my way with that issue. (returns accepted if it doesn't work). As for splicing in wires or anything where I have to take door panels off, that may have to wait until I undertake a much dreamed about interior restoration. The proper 126 has yet to come up at auction, but I've come close. I've got regulator work to do that is also waiting for an interior renovation to happen.

@toomany: All I have is a master key. That is not the issue here but thanks for the heads up. I have no problem with the engine not shutting off, so I'm not sure what you're referring to about that.

I see, I miss read the original post, sorry.

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83 SD

84 CD

The pump should not run forever with or without a proper vacuum source. Even disconnected from the central locking system, the pump will shut off after a minute or two. I suspect the pump or electrical parts of it are not working properly. The fuse blowing could be caused by that or a break/short in the wiring to the pump. These pumps are not meant to be submerged in water.

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1991 F250 super-cab 7.3 IDI. (rebuilt by me) Banks Sidewinder turbo, hydroboost brakes, new IP and injectors.
2003 S430 - 107K
1983 300SD - Tanoshii - mostly restored ~400K+.
1983 300SD - Good interior. Engine finally tamed ~250K.
Monark Nozzle Install Video - http://tinyurl.com/ptd2tge