w123 gearbox - rough changes!

[twill6464]

Hi,
I've got a 1983 280E Merc (UK spec). I've only had it for about a year but in that time the auto gearbox has developed a fault. it changes fine into 2nd gear but when it changes into third gear it will really rev (to maybe 3000 rpm) before it engages. Even if i'm not accelerating very hard. There's a bit of a problem going into fourth too but it's not as bad as 2-->3.
I've looked at the other topics in the forum about dodgy gear changes - is it to do with the vacuum? I recently had the central locking fixed because that was leaking and it now that works fine.
Would really appreciate your thoughts. I'm praying that i don't need a new 'box!!
Cheers,
Tom.

 

[Richard Moakes]

What you describe is termed 'flaring', whilst the vacuum modulator can be adjusted to compensate for this to a certain extent, this will make the change more harsh. Regrettably it sounds to me that yours has passed beyond the point where it will make much difference.

You have either got worn clutch packs inside the transmission, or one of the main pistons that actuates brake bands or clutch packs is slow in engaging. The choice is fairly simple...

1). If the fluid hasn't been changed for a long time, try a transmission fluid and filter change
2). If this doesn't work then a rebuild of the transmission or a replacement will be required.

A partial rebuild is likely to cost you around £700 if a recent rebuild of a transmission for a relative of mine was anything to go by.

Good Luck

 

[wireman]

The pneumatic locking system is totaly independent of the engine vacuum system.
The locks are blown on and sucked off so to speak by an electrical pump.

The transmission vacuum system comes from the inlet manifold (petrol) and is used as load sensing signal, i.e. lots of vac = light load so if the speed is sufficient change up,and low vac = heavy load so stay in low gear or change down.
A cable from the throttle is also connected to the valve block to give a demand signal to the box. In addition to these signals there is an electrical kick down solenoid valve that if engaged allows the transmission input speed to get to maximum permited RPM before up changing, this is operated by a switch/relay which closes when the throttle pedal is fully depressed,.
On diesel models a vacuum device is fitted to the fuel pump to simulate the petrol engine inlet manifold vacuum, presumably it gets its signal from the pump rack's physical position and translates this into a variable vacuum.

Before you bog your self down in striping any bits off ensure that the cable is correctly adjusted and that the vacuum pipe is connected and leak free at both the engine and gearbox ends.

From your comment that it jerks at moderate (light/medium load) throttle positions the vac parts could be dodgy, the rubber conector tubes which join the 4mm nylon vac tubes to the engine and box can get tired and leaky.

If your fault persists I would recomend taking the car an auto box specialist to diagnose the transmission before any major work is done to the transmission.

It may well be that Richard is correct and you have serious box wear trouble since the demand and load sensing systems work just the same in all gears and the fault you descibe does not occur in all gears.

Best of luck

 

[twill6464]

Thanks for the advice. I'm going to get it looked at I think because it sounds a bit beyond my capabilities! Was quite tempted to leave it for a bit but it is only getting worse and I don't want to make life any more difficult!
Many thanks once again!

 

[beemerjay]

Thanks for the advice. I'm going to get it looked at I think because it sounds a bit beyond my capabilities! Was quite tempted to leave it for a bit but it is only getting worse and I don't want to make life any more difficult!
Many thanks once again!

Just before you go Try checking your rear brakes
I've found over the years working on the old dears that if they're binding then the changes when everything is cold are incredibly harsh

 

[stever]

The pneumatic locking system is totaly independent of the engine vacuum system.
The locks are blown on and sucked off so to speak by an electrical pump.

There's no electric pump on the W123 - the central locking is purely mechanical vacuum.

 

[twill6464]

Just before you go Try checking your rear brakes
I've found over the years working on the old dears that if they're binding then the changes when everything is cold are incredibly harsh

I had the whole car checked over about 6months ago and the mechanic said the brakes were ok, i also had the transmission fluid changed then but it's made no difference to the problem ! It's just that it's the change from 2 --> 3 that's the bad one, all the others are fine (at the moment!). anyway, cheers for feedback!

 

[television]

You do have a vacuum for the gearbox its in the link

http://www.detali.ru/cat/oem_mb2.as...GA=722112&CT=GA&cat=09X&SID=27&SGR=001&SGN=01