722.4 shifting issues 3-4 and accumulator spring question - PeachParts Mercedes-Benz Forum

Hutchinson123 · #1 02-02-2021, 07:50 AM

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

tdoublenastywitit · #2 02-02-2021, 10:53 AM

I've never found anyone that can say for sure they completely understand the K2 spring. Nor is there much solid information on it.

Have you installed a new k2 spring kit yet?

As for your theory with removing the inner small spring I think you should go ahead and try it. Even if it doesn't work you will at least have more knowledge as to what it does.

I just scrapped a 722.4 that wouldn't even shift into 4th gear at all

Diseasel300 · #3 02-02-2021, 11:23 AM

Have you checked the fluid level to make sure it isn't low? When was the transmission last serviced and the filter changed? I notice that bit of information is glaringly lacking. Not only will changing the oil and filter get rid of the burnt oil from slipping, but it will get rid of the filings and dust in the oil and filter that can cause engagement issues or low fluid pressure.

Is the VCV properly adjusted? The correct setting on these engines is with the accelerator pedal floored, loosen the holding bolts on the VCV and turn it clockwise until you feel resistance, then tighten the bolts. The VCV is never adjusted again for shift quality issues.

Modulator pressure is adjusted for full throttle shift quality. You want a firm shift, but not barking the tires and not flaring/slipping under full throttle.

It's important that the bowden cable is adjusted for the correct shift points too. With the accelerator pedal held to the floor and holding the kickdown switch, you should be upshifting 2-3 and 3-4 at ~4600 RPM. If the bowden cable is too tight and these shifts are too late, all of your other shift points will be affected and rough shifting will occur.

The VCV, Modulator adjustment, and bowden cable adjustment all play into shift timing and quality. It's important that those items are adjusted properly before diving into mechanical issues in the transmission.

Hutchinson123 · #4 02-02-2021, 01:41 PM

Thanks for your replies.

The problem is:
Yes, everything is set properly and replaced with new MB parts.

Fluid level is fine and I have made 4 (!!!) full fluid and filter changes (including TC drain) since I have the car.
There no dirt or burned fluid - nothing

Engine power is fine. It reaches the factory performance values and shifts at around 4600 to 4700 rpm when I hold the kickdown switch during full acceleration.

That‘s the reason why I am so desperate. I have driven a scrappy 230 E that was not very well maintained during the last years. It had around 180 k miles and was shifting totally smooth without any kicks or slipping.

Is there a possibility that something in the valve body is broken/worn or just dirty?

Diseasel300 · #5 02-02-2021, 01:56 PM

The 722.4 transmissions aren't held in high regard for their durability or longevity. There could be issues in the valve body, or there could be wear to the transmission clutches/brakes. You might try a bottle of the yellow Trans-X additive if you can get it where you live. It's basically a heavy detergent that helps to clean gunk out of the transmission. My 300SDL originally shifted like Stevie Wonder was at the wheel and the Trans-X helped considerably. Kind of a hail-Mary at this point before shopping for a transmission.

dieselbenz1 · #6 02-02-2021, 02:08 PM

Recommended upgrades for thr 722.4 include k2 spring and b2 dog bone increase length +1 size from what is installed.

Sure sounds like k2 worth replacing and inspecting the piston and spring some have posted pictures of a broken spring.

Chevy's loose top gear first and soon afterward 3rd if that helps?

X2 for adding tranx as your last attempt.

Hutchinson123 · #7 02-02-2021, 03:39 PM

Diseasel:
Thanks for the recommendation but unfortunately the trans-x products are not available here in Germany. I already tried Lubegard a while ago.
That‘s a pity. Would like to give it a try...

dieselbenz1:
Sounds interesting but what do you mean with b2 dog bone increase length?
Can you explain what I shall do here?

K2 piston and springs are inspected already. They are in good condition. No broken parts.

dieselbenz1 · #8 02-02-2021, 04:54 PM

I recall reading it in the manual some 6 years ago. The piston is removed externally, that holds the dog bone ,(shaft) and replace it with a longer one.

Hutchinson123 · #9 02-03-2021, 08:34 AM

Thanks for the hint.
According to various google results the B2 piston issue is more related to non-engagement of gears.

I think next attempt will include removing the inner spring of the K2.
But still hoping to get some detailed information here before

Last option will be gearbox swap against another used one I bought same months ago.

dieselbenz1 · #10 02-06-2021, 01:09 PM

I came across this thread which looks interesting.

Pictures 722.xx broken B2 piston = no forward gears