350 trans keeps pushing fluid out the overflow

[s class]

This is my daily driver 1974 euro spec 350SE.

If I travel at speed (>100km/h), the transmission pushes fluid out of its breather until its about 0.5 litres below the minimum mark. As expected, due to being low on fluid, shift quality is then poor. If I refill the trans up to the max mark, shifting is great, and all is good until the next time I travel at speed.

This started about 6 months ago, after I removed, stripped and reassembled the valve body due to it having broken a spring. The only other change at the time was a new Meyle filter and fluid.

I am like 110% certain there are no mistakes in the valve body. The transmission otherwise behaves as it should, and all up and down shifts are correct.

I'm at a loss to explain why the trans is doing this - any insight appreciated.

 

[Helmet]

The only time I have seen fluid coming out of the breather in these transmissions is if they are over full and then run at high speed for a while. Have never seen them go under minimum level though. Is the oil cooler connected ?

 

[s class]

Yes, oil cooler is connected, and no reason to suspect a problem with it.

 

[sean sherry]

The Box is pressuring, and from experience if the problem only occurred after the V/ Body was striped that's where I would be looking. Maybe it is warped ?

 

[s class]

Hi Sean,

Thanks for the insight. What you say makes sense. I have to travel at speed for several minutes before the fluid starts coming out, so obviously the pressure is building up slowly. I guess there is a slow leak out of the valve body somewhere. Burning question obviously is how am I going to locate the problem. I suspect that even if I strip down the valve body again, the problem will probably not be obvious.

Thanks, Ryan

 

[Oversize]

Have you checked the pressures in the box? If it's bleeding off somewhere the pressure might be down. Why would it generate excess pressure? Pressure comes from the the two internal pumps and maybe the govenor or convertor. Perhaps a two way valve is jammed in one direction?

 

[sean sherry]

Perhaps you could tackle the problem by a process of elimination before more surgery.
Pressure in the Box may not be the result of high speed running , just when the symptom show up.
Two possible causes could be internal leakage and Air been drawing into the Pump causing foaming.
At higher road speed the Line Pressure is actually lower, as the load comes off the engine and the vacuum rises, pulling the Modulator back. To check internal leakage being the cause, run the engine in D, park brake on, with the vacuum line disconnected ( for max. Line pressure ) and feel if there is pressure build up using the back of your hand on the dipstick tube.
Our skin is very sensitive. Not being fimiliar with Mercedes per say, does the box have a breather either on top of the Box or the extension housing ?
If pressure is building up , it is back to the operating table !

As a matter of interest the only time in 40 years I had this problem was on a Ford Mustang I rebuilt, I had left one of 7 thrust washers out in the drive chain. Drove ok but lost oil at high speed. Self inflicted wounds !