W123 Central Locking Vacuum - Help please!

penybontboy

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Having read the various threads on fixing W123 central locking, I've decided to have a go at fixing the lack of vacuum in my Merc and fallen at the first hurdle. Can someone tell me if the pic below is what I should be testing?


Do I pull the tubes off to check the vacuum, and are they just a push fit back on?
The piston in the unit has no resistance at all whether the engine is running or not. Does this suggest it's faulty?

Any suggestions gratefully received!

DSC0260826.jpg
 

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Number_Cruncher

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If the system is vacuum tight, and the one way valve is working, the system should also hold vacuum - so that the central locking can also unlock the car when you come back to it.

I agree with TV. The first place to check is to measure the vacuum at the tank, and if the vacuum leaks away quickly, remove and blank off one line at a time until you find the culprit.

The fault finding won't be quick or easy, but, it shouldn't cost you much!

Have you got a good vacuum gauge, a short length of pipe and a tee connector?

The part in the piccie is just a switch, it shouldn't have any resistance as such, but, it should be vacuum tight.
 

Alex Crow

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hello p.b.b, yes you can pull the pipes off, no it should not have any significant resistance to movement and no it is not likely to be faulty.

you say the locking does not work - at all? if so it will be easier to trouble shoot than the usual complaint of locking ok but not unlocking the next day.

the w123 vacuum locking had one major design flaw over all the later systems, it used engine vacuum stored in a tank, needing to be entirly leak tight to work. even just a tiny air draw would exhaust the vac tank over night, meaning unlocking next day would not work. later systems, eg w124, used a vacuum/pressure pump that ran when the vacuum/pressure was needed, and so could cope with considerable leaks and still work.

the place to start with yours is under the bonnet. at the o/s bulkhead you will see various vacuum pipes. the two yellow ones are for the locking system. one is yellow with a black trace - this goes to a vacuum storage tank at the rear. the other pipe is plain yellow and this goes to the valve you have removed.

when the drivers door is locked or unlocked it moves the piston in the valve as you will have seen. you will note that the other two pipes connected to the valve are yellow/red and yellow/green. the red trace denontes the locking circuit and the green trace denotes the unlocking circuit.

from your drivers door the pipes then go to a junction point under your drivers foot mat, near the o/s of this area under a plastic duct. this is the easiest place to isolate seperate parts of the circuits for diagnostic purposes. you will note that each of the two pipes coming from the driver's door branch into three seperate pipes, one goes across the chassis to feed the n/s doors through a further junction in the passenger footwell, one goes to feed the o/s/r door, the other feeds the fuel flap and boot/tailgate. note there are two pipes to every lock mechanism, but on later models the filler cap has only a locking pipe.

this job is all about following the vacuum through the system, starting at the one way yellow valve and two pipes under the bonnet and working back. you would not usually remove the drivers door valve, rather testing the system and blanking off parts to trace leaks. it is best done with a vacuum gague/pump, such as a mytivac, but can be done with no special tools at all, just some golf tees to blank off the pipes.

usual causes are leaking diaphragms and boots on door cells, and now they are getting very old we see cracked pipes too. it may be as simple as the pipe at the front being disconnected or the one way valve being broken. the main vacuum feed is from a grey pipe connected to the intake manifold. this will also feed purple pipes for the head light level adjusting and possibly some other features - can't remeber, it's been a while!!

there you go, any more questions?

good luck!
 

Alex Crow

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the one way valve at the bulkhead isolates leaks from other systems, although headlight leaks may lower the available vacuum slightly, but should not be much.
 
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penybontboy

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Many thanks to you all for taking the time to reply and your kind words of welcome.
I'll have another crack at it this weekend.

In response to your questions:
NC - No I dont have the equipment but know a man who might have!
AP - Doors only work manually/individually
TV - Some movement in headlamps but not much.

What keeps cropping up and confusing me are leaky door cells and diaphrams. Where are these if not the bit in the photo?

I've now discovered that the rear electric windows have packed up (fuse is OK) and the master brake cylinder is leaking. Oh the joy of old cars!

I'll post how I get on - thanks again.
 

Number_Cruncher

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The part in your picture is the vacuum switch - the shaft moves when you lock or unlock the driver's door, and directs vacuum to either lock or unlock the rest of the car as required.

The diaphragms or elements are the units in all the other doors and the fuel flap and boot which respond to the vacuum signal and lock or unlock that door.
 

stever

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Does the vacuum work with the engine running? If it does, it suggests a leak at one component (or split pipe) rather than a fundamental failure.

I had a w123 with u/s central locking because a previous owner had disconnected the pipes in the engine compartment - I did them back up and hey presto!
 

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Does the vacuum work with the engine running? If it does, it suggests a leak at one component (or split pipe) rather than a fundamental failure.

!

A bad leak could affect the running as well,,max vacuum is at idle
 

100haggis

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Hi All,
My central locking has never worked at all which i put down to vacuum issues to be sorted "whenever". However my hand has been forced as now my engine (300 d) is running on when switched off! I recently removed, cleaned and refit my fuel tank which coincided with the problem starting but I can't see that I've disturbed anything? Can't find any obvious leaks either so any advice would be great.
Cheers
Matt
 

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The shut off is vacuum controlled on a diesel,is it a 124 or 210
 

100haggis

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Malcolm,
Sorry I should have been more specific. It's a w123 and if i suck on the pipe the engine shuts off so I assume the diaphram/shut off is working?
Also it shuts off with the pump stop lever so I don't think it's the pump either.
Matt
 

100haggis

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I will check tomorrow. Are those connected to the rear of the ignition switch? Also where can i get hold of that parts info you have? I asked my local MB dealer about a parts CD and have trawled the web but all to no avail!
Thanks
 

Alex Crow

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hello mr haggis, you may have left the pipe off the rear vacuum tank when working on the fuel tank. this would cause a big leak.
 

100haggis

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Thanks again Malcolm.That info is great. Alex Patient I think you may be onto something as the coincidence is too great. However the only vacuum pipe i came across when doing the tank went down vertically through the boot floor into the vac. tank and I had no need to disturb either. I've checked the pipe to tank connection and it looks OK? It's starting to annoy me!!
 

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I would start by blanking off all of the pipes that leave the tanks, with no vacuum in the tanks nothing can work,,any T pieces are known to be a leak source owing to the age and they are cheap to buy,, the 107 suffers from these T pieces leaking
 

Alex Crow

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if you connect the grey pipe directly to the brown pipe near the brake servo this should verify the ignition vacuum switch and pipework is ok.
 
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