problem,central locking will unlock from door key once then I have to unplug the pump and plug it back in again and it works again once only.Thanks for help in advance.Wazza.

I don't know if this will help at all, but since the system is vacuum operated it need help. i have heard that once you shut the car off there is only enough vacuum to operate the central locking system 7-9 times. After that you would have to restart the car. Maybe you have a small leak somewhere or your pump is vacuum starting to fail and not creating as much or as strong a vacuum as you need

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Jason Priest
1999 E430
1995 E420 - retired
1986 420SEL - retired

The ability to lock and unlock only some number of times is only a factor on cars using the engine for vacuum and the engine is not running. The 126 chassis uses a vacuum pump and should lock and unlock till the battery runs dead.

The pump is designed to shut off after a certain pressure or vacuum is achieved each cycle. If the system has a leak and the pressures aren't achievable then the pump shuts off after some time period. I suspect something along this line to be your problem. To test locate the pump in the spare tire and watch what happens. Does the pump shut-off after similar intervals both locking and unlocking? If the time is long try removing the line and plugging it and see the time length.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Thanks for help Steve,central locking pump cycles ok on lock[ie positive pressure]but time cycles out on unlock[ie vacuum side].As there is only one line in and out how do I test the system to find problem?.I pulled off the drivers side door lining but the keylock only has wires to it but no vacuum/pressure tube although there is a short pipe at the base of the(blue colour)lock.Thanks again Wazza.