Vacuum pressure - Irregular gear changes

[purdie]

On a 1983 300TD, 123 Series, does anyone know whether lack of vacuum pressure from the vacuum pump can affect at what speed the gears change on an automatic box.  At the moment the 1st gear goes in at 20mph, 2nd gear engages at 35mph and the 3rd gear wont engage until 65mph.  When the gears do engage it is very definite, they do not appear to slip, so I don't think that the gearbox is the problem.  As the car is old, albeit 'beautiful', it doesn't warrant replacing, first the gearbox (in case it's that) and then the vacuum pump (in case it's that) and so on - any ideas would be appreciated.  Thanks..

 

[Gordon]

Vacuum pressure

Yes vacuum pressure is significant but what vacuum pump do you refer to?
The speed at which the gears change is controlled by servos which tighten up bands on the appropriate gear.  These servos are controlled by the pressure from the governor which is affected by engine speed and by the manifold vacuum which increases as you put your foot down on the throttle.  
Also if you put your foot hard to the floor on the accelerator pedal there is an adjustable switch behind the pedal which will affect the vacuum control unit and delay the gear changes.
There are various adjustments that can be made but they are fairly sensitive and if you are not that experienced you ought to at least consider going to a Merc dealer or specialist but if you want to have a go, get a copy of the Haynes manual for the 123 series diesel which includes your 300TD and it will give you enough information.  If your local car bits shop hasn't got one get one from M-B Association shop on the internet.
Check the vacuum connections, fluid levels and things like that first.  Clean everything off before you make any adjustments - it just may be dirt rather than wear.  Good luck.

 

[Mark Norton]

Vacuum pressure

You probably have a leaking component in your vacuum system - if you have central locking, which I'm sure you have, (mines a 300td), then the first thing you need to do is isolate this off from the engine bay - get a bag of golf tees for this work, and plug the vacuum lines from the rubber distributing connector near the bulkhead and block off the line that goes to the front drivers door, should be a plain yellow pipe, and maybe block off the other one - that goes to the reservoir under the rear floor - then drive for about 2 miles, see how it performs then. If it goes back to normal, then you have a leak in the CL system.  It's a long process of elimination, mail me for further details - nortonian@yahoo.co.uk, or visit royfieldsmercedes.com and search the archives

GOOD LUCK

 

[Texas Diesel]

Vacuum pressure

I have a 1980 300 SD. My automatic transmission shifts somewhat inconsistently. I have noticed that my vacuum actuated door locks do not always work after the car has been parked for 4 hours or more. Once the car is running, the locks function just fine. Could these problems be interrelated? Thank you for your input!