722.4 shifting issues 3-4 and accumulator spring question

[Hutchinson123]

Dear all,


I own a 300 D NA (Euro) with 722.415 automatic transmission.
I bought the car with 190 k miles in 2018. Transmission was slipping when I bought the car (all gears). So first thing I have done is increasing modulating pressure which eliminated the slipping.
So far, so good.
Since about 2 and a half years I struggle to have the transmission shifting correctly.


Since the 2-3 shift was getting weaker in the first winter during my ownership I have installed the reinforced factory K1 spring kit.


What is left until today is sloppy 3 to 4 shift.
It is impossible to adjust the gearbox for good shifting quality from 3 to 4.


2-3 is nearly perfect. It's fast and without harsh kick, neither on half load nor on full load. In relation to the age and milage of the car it is perfect for me.


The K2 clutch seems to be much stronger. Nearly all shifts are accompanied with hard engagement and painful harsh kick. Even if I reduce modulator pressure so far that the 2-3 shifts starts slipping at light throttle again, most of the times I get a hard engagement from 3 to 4. But after that hard engagement, engine revolutions reduce slower than with higher modulator pressure. If I reduce more, 3-4 shift is feeling too slow and smooth.

It seems to be impossible to have a nice, firm and fast shifting without that neck-breaking kick (you really feel that differential and all the rest of the drivetrain suffer from this).


I have no vacuum issues and all the parts that can affect hard shifting outside the gearbox have been replaced (flex discs, rear axle mounting bearings...). So all this stuff is in good condition.



All vacuum stuff has been replaced during the last year:
- VCV
- Modulator
- Rubber hoses


Vacuum is great and my vacuum gauge confirms that the system is holding the vacuum.


I have tried "thousands" of different settings of bowden cable, modulator and so on. Nothing helps.


What makes me really wonder is that after a mid- or long distance highway trip (20 miles or more) the 3-4 shifts feel nearly perfect. Nice and quick and damped, without any noticeable harsh kick. So whenever the gearbox/oil is really hot, it works satisfactory.




Do you have any idea what's the issue within the gearbox here?
I want to get rid of this stupid problem and I want to have fun driving this car again. The current condition makes me absolutely unhappy and I'm sure that the next time I will go for a manual again.






Furthermore I would like to understand how the accumulator springs in the valve body are working.
For the K2 I have the piston and in the piston 2 springs. One long spring and another one which is short and has a smaller diameter.

What is the reason for the short small inner spring?


My thought/assumption:
If you compress the outer spring half the way, you start compressing the small spring as well. So there is a point where the whole thing is getting harder immediately.
Is that small spring the reason for the hard engaging of the gear?
What if I remove this small inner spring from the K2 accumulator? Could this lead to smoother shifting/eliminate the harsh kick without loss of rpm decreasing speed? I have the factory K2 kit from a W126 here. It has slightly firmer springs. If I only install the firmer outer spring without the inner spring, could this lead to smooth/damped shifts with fast decreasing rpm?


I mean the superior shift kit also does not use the inner spring.





Sorry for the long text. Hope you can follow my thoughts and help me to find a solution here.




Thanks a lot in advance.


Greetings from Germany


Tobias