1996 R129 Central Locking - Help with troubleshooting reqd

[SL Project]

This car and Mercedes in general are new to me. I only have a physical key. When unlocking the car from the drivers door, all other items unlock too, boot, petrol flap, passenger door etc.

Unfortunately, when I wish to lock the car, it is only the drivers door that locks, nothing else locks. so I have to do a little dance around the car locking each item manually.

Is there a good online resource / forum doc or two that someone can kindly point me at, so I can try to understand the vagaries of this vacuum system, the like of which, I have not encountered before.

Thanks

 

[LostKiwi]

Sounds like you may have a weak pump. Try measuring the vacuum and pressure on a line. Pressure should be at least 450 mBar (just under 7 psi) vacuum and pressure.

Depending on year the pumps and hose layouts are different. Mines a 93 so may not be the same as yours.

On the top of the pump on mine there are three connection areas: ZV, IZV and PU. Each perform different functions.
PU goes to the engine bay and connects to a manifold in the module coffin (passenger side in front of the windscreen). Common for these to crack and cause leaks but easily fixed with araldite.
ZV locks the doors, boot and fuel cap. IZV locks the glovebox and cubby boxes.
Normally the pump should run for a few seconds then shut off. If it detects a minor leak it will run, shut off and repeat upto 31 times. If it detects a major leak it will run for 30 seconds before shutting off.
I've attached a troubleshooting document.

 

[SL Project]

Thanks Kiwi, a useful place to start, much appreciated. I will report back once I have checked the vacuum pressure achieved.

One question though, if there is enough pressure to open everything, is there a greater pressure required to close everything?

Do the vacuum operated items in the doors and boot need dismantling, cleaning and lubricating?

Sorry, that is two questions...

 

[SL Project]

When unlocking I can feel the pump activate for a couple of seconds and everything unlocks, as mentioned.
When trying to lock the car there is no discernable activity at the pump, and nothing happens, so a problem on the negative side of the of the pump circuit. Might this just be a bad electrical connection, somewhere still to be determined?

 

[LostKiwi]

The pump locks use pressure and vacuum to lock/unlock (sorry don't recall which is lock - pressure or vacuum). Either way is +/- 450mbar to operate the locks.

Do your heater vents operate correctly? If there's a leak sometimes they will only run on demist irrespective of the vent selection.

 

[SL Project]

Hi Kiwi
I was pointing out that the pump gets no intruction to activate on lock command. As the pressure goes, it is enough to unlock everything. Just nothing happens at the pump at all, no buzz, no whir, no vibration, when trying to lock. So reading the doc you attached, it would seem that it is not going to a negative state when lock is requested.
So I would assume an electrical problem rather than a pump pressure problem.
I will get my multimeter out and see if the pump is receiving the lock instruction but not responding.

 

[LostKiwi]

Pull the connector off and look at the contacts. If the fuel filler pipe leaks it drops water directly onto the pump (mine failed from that cause).
Also carefully remove the base of the pump to expose the PCB. Look for green corrosion on the PCB and dry joints on the connector.
If it's suffered water damage it may require replacement.