Jason89s4

I have a 79 Auto that needs a new pressure modulator valve for the tranny and they are NLA through either Porsche or Mercedes, according to my searching. (I'm referring to the vacuum-operated modulator on RH side of tranny, NOT the oft-confused cable-driven Bowden modulator. ) Does anyone know of a source for new, used or rebuilt for a 78-79? All other years appear to be available new, there are even aftermarket versions.

In case you are wondering, the vacuum supplied by the engine opens a valve that allows ATF to flow and will also moderate that flow--which affects shifting. If the modulator does not hold any vacuum at all, such as mine, you get zero gears, as there is no ATF pumping at all. One way to tell is (assuming vacuum line, TT, convertor, etc are okay) when you start the car with wheels off the ground and row through the gears there is no change or drop to the properly filled reservor level at all--as you would normally observe. This is referred to as "Lubricating pressure" in the WSM.

Thanks, Jason

This modulator can also work okay but degrade over time. Some folks even suggest regular preventative replacement with ATF change. It is not terribly expensive.

Mrmerlin

does it have a good cap IE not cracked or missing?


Part #: 126-277-00-81-MBZ
from PP website

GregBBRD

You seem to be saying that if the vacuum line to the modulator falls off or if the O-ring which seals the cap fails, the car no longer functions?

This is news to me...not something I've experienced.

I've got several used modulators for the 722.1, maybe even new ones...I'll need to check.
I'm out of the shop, until Wednesday.

gazfish

If you consider that an engine at WOT pulls almost no tranny vacuum and is when the gears are engaged the firmest (highest pressure) then your description is the opposite of what happens. You likely have some other kind of issue if the fluid isn’t dropping and no engagement.