1985 Mercedes Benz 500sec Battery is going dead over night

What I remember on that system
that it is a vacuum actuator
you have a vacuum pump and lines
so if its a locking system issue I would be looking at the vacuum pump and not the actuators as they are operated by vacuum and not by power from the battery
system pump is in the trunk by the spare tire well
check it out

Jack,
That would make sense perhaps. I'll instruct my mechanic to take a peek at the vacuum pump in the truck area to see if it is pulling amps. He might be able to disable the pump and continue the testing to see if an amp drain still exist afterwards. Part of the problem is that this has been an intermittent problem, however more regular now days. The car is useless in its current condition however. The drain must be found, or else.
LKL

This is a classic amperage draw that is being caused by a control unit not fully turning off after vehicle shut down. Here is how I have always attacked this issue on MB vehicles. First what is your static reading on your meter after the vehicle has been locked? Note that this measurement should be taken after 15-30 minutes of sitting locked. Most of the time amperage draws will be under 1 amp thus explaining why the vehicle goes dead over night or in a few days. Here's what I recommend as far as an order to diagnose this.
1. Role down all windows and lock the doors so that you can have access to the fuse panel under the drivers dash (this may require some acrobatics to get to with the door closed) Make sure everything is locked and off
2. Next open the hood and trunk and manually close the latches for both with a screw drvier (push the screw driver into the latch until it fully closes) Hood will have 2 latches trunk will have 1
3. Now with your meter connected in series between the negative battery post and cable terminal let the vehicle power down for 15-30 mins.
4. Once you have reached a static amperage reading go to your fuse boxes
5. Start pulling out fuses one at a time and after you pull each fuse check your meter.
6. What you are doing is trying to isolate the component circuits
7. Repeat this for every fuse in each fuse box one at a time (this is time consuming but it works)
8. What will eventually happen is when you pull the fuse that is for the component that is drawing your power the amperage draw you see on your meter will drop to appropriate levels. When this happens check what fuse you have pulled and the component it serves will be your faulty component that is drawing power. Replace the component serving this circuit and your draw should be gone.

Note that this often times will take hours to find but this is the only sure fire way to isolate an amperage draw. Also keep in mind only pull fuses not relays. Also when you plug some of the fuses back in you may reawaken the vehicle in which case you will have to reinstall the fuse and let the vehicle power down to static amperage.

Also let me know what has been replaced on the vehicle and any other repairs or circumstances that surround this issue. I would definately recommend not throwing parts at this though because most of the time this approach never works with these issues. An amperage draw is something that is measurable and any good tech will be able to tell you with absolute certainty if the problem is fixed or not because all you have to do is look at your meter to see if there is still a draw. Time and patience will be your best tool in this repair.

I appreciate the feedback. I'll have to run some of this by my mechanic. I'm pretty sure he has done a lot of the testing, exactly as you have laid out. I did as well. The 15-30 minute wait time is different however. Frankly, my multi-meter tester didn't have the ability to handle the loads. That why I had to throw in the towel and take the car to a trusted mechanic (well, plus - I had just about finished pulling ALL of my hair out and couldn't afford to loose any more). The only fuse panel I've ever seen in this car is the one under the hood on the driver's side. All of the assorted relays are in there as well. You are right about leaving the windows down while testing though. The door locks drop when you mess with the battery and/or fuses. With the windows up and your only keys inside, this can be a problem (ask me how I know this). Thanks again for the additional feedback information. I'll continue the search with my mechanic's hand. LKL

Well, all of your help appears to have come in just moments too late, although somewhat confirming. My mechanic worked on the 500SEC last night, finally determining that indeed, the Mercedes "central locking vacuum pump is bad". Unfortunately, it is related to so many operations through-out the car, it has to be replaced. I've been quoted $476 for the pump and another $100 labor (on top of the $350 labor it took to find the problem and pin it down to what it is). Thus far, I'm tapping $950 or more to get this thing put away, once and for all. My question at this point is, shall I seek out the part elsewhere, through my foreign car parts suppliers and perhaps install the silly thing myself? I mean, is it that tough to swap out the part now that we clearly know what part must be changed out? I've already got a replacement ignition switch and heater control module standing by, ready to swap in once I get the car back. Why not the 'central locking vacuum pump' while I'm at it? What does swapping the pump itself out really involve, do you know? Lastly, with all that I need to do (three parts to be replaced), am I causing myself undue trouble attempting all this without the air of a Mercedes manual with exploded views, etc.? Heck, I'm not sure what I'm in for swapping out these three components (and I'm an ex-aircraft mechanic, scary, huh?). Thanks a bunch. LKL

OK, we've pinned down the source of the battery drain. It is indeed the Central Lock Vacuum Pump. I found one by Hella (new) for $189, however it has one additional single pin behind the three inline Molex type male pins, right next to another single platsic black pin. The pump I removed DOES NOT have this single, stand alone pin behind the three inline pins. Any clue as to whether or not the new pump WITH this pin will work fine or not?

LKL