I've spent ages working on a central locking fault and am hoping someone who has access to a proper wiring diagram can help! The Haynes manual wiring diagram I have doesn't include the central locking circuit.

Q1. What is in the circuit, if anything, between the fuse (in engine bay fuse box) and the central locking pump 12V feed

Q2. Should the fuse be "live" with the ignition off? (fuse shared with electric window and aerial)

Q3. Can you confirm that the 12V feed for the pump and the 3 door lock switches is common?

Q4. What is the purpose of the second "door open" plunger switch in the front door's leading shut face (ie in addition to the courtesy light switch).

Symptom for those who like puzzles....
- The pump appears to be ok (it's the 12V feed that is the puzzle)
- No faults on actuators and lock switches
- No air leaks
- There is no 12V feed to the pump or door lock switches UNLESS the ignition is switched on AND either of the 2 front doors is physically open...in which case the central locking works fine!!

You'll notice electric windows work with ignition off, if the door is open. That's what the second plunger's for, and why there's live at the window fuse with the ignition off.


good luck

Russ

__________________
190E's:
2.5-16v 1990 90,000m Astral Silver
2.0E 8v 1986 107,000m Black 2nd owner
http://www.maylane.demon.co.uk/190esmall.jpghttp://www.maylane.demon.co.uk/190esmall2.jpg

Many thanks Pentoman, that's answered a couple of questions.

Given that the 2nd door plunger switches are to enable the windows to be operated with the ignition off, it is a puzzle why it is affecting the operation of the central locking also. However, they are both fed from Fuse 12 (which is always live). which may be a clue.

The answer to the puzzle lies in my first question I think and I'd be grateful for any ideas that anyone may have...

Q1. What is in the circuit between Fuse 12 (in engine bay fuse box) and the central locking pump 12V feed (under the back seat). I now suspect that there may be a relay somewhere but I'm just guessing.

Thanks, Pete

Problem fixed! The bizare symptoms fooled me into thinking it was obscure problem. It just proves that you you should check the simple things first. The fault?.....oxidised fuse contacts. Fuse cleaned, problem solved.

The tip that the 2nd door plunger switches are there to enable the windows to be operated with the ignition off was very helpful. The fact is that this wasn't the case on my car....got me thinking. Explanation...the central locking / electric windows circuit is served by fuse 12 with the ignition off, but has a direct 12V feed via the ignition switch when the ignition is switched on.
Thanks , Pete

This is a really old post but thank you. My 190E had exactly the same symptoms, and I'm so glad I found this thread.
The Mercedes mechanic had the car for two weeks and they could not isolate the problem. No one would have thought of the fuse contact!
You've saved my sanity, as it had been driving me nuts.
Cheers