i have two questions, if any one could give me some advise on what to look for and possible solution.

1. central locking sytem seems to be tempermental. sometimes it will lock everything at other times it won't.
what should i look for?

2. original door locks on car. The key works fine on the driver side, however will not turn completely on passenger to open and close. Any clues. thanks

Vinny,

What car do you have?

On the 78 450SEL I had until last week, the central locking would work for about half an hour after turning off the engine. The locks would remain locked after that, but the central locking and unlocking wouldn't work. I suspect there was a vacuum leak.

On my 91 300SE, I can lock and unlock the doors using the key on the driver side. I can only unlock the doors using the key on the passenger side, but I can lock the passenger door from the inside with the door open on the passenger side. Everyone tells me I shouldn't be able to do this.

Good luck,
Sixto
91 300SE

door locks
 

most likely a vacume loss problem, also a check valve could be going bad. If you want to have fun trying to solve a puzzle, buy a vacume gage. about $20 at any auto parts store. locate your vacume lines under the hood and start testing to see if the lines are holding a vacume. the problem of loosing vacume after shut off, may be the storage tank for the vacume. The MB manual or CD has an entire section for trouble shooting on this subject. You should be able to isolate the problem, most of the parts are inexpensive. The DIY is time consuming.

I don't think the actual vacuum reservoir would be the problem. They are built out of some might tough plastic, mines nearly 29 years old, still looks brand new. The vacuum line going to the reservoir or the check valve may be bad. Could also be the early sign of a door control unit failure, a small tear in the diaphragm that sometimes seals itself.

My door locks were intermittent for a while until the right rear upper diaphragm finally failed completely.

Remember you have three units (if a 4 door) in the doors (driver's door has the master valve), a unit for the trunk lid and one for the fuel door. That is on top of any climate control lines or engine shut off lines your particular vehicle may have.

I have both a hand vacuum pump and a simple vacuum gauge. The hand vacuum pump will be indispensable in the diagnosis of your vacuum trouble. Pull a vacuum on each separate control unit, let it sit and see whether it leaks down or not over a given period of time. A bad leak will bleed off in a matter of seconds. Don't forget to pull a vacuum on the check valve too (that was my final vacuum loss point, only intermittent as the valve would sometime check sometimes not).

What model car?