Weds

I've just changed the gearbox vacuum modulator valve on the 722.3 transmission, some of the changes were clunky (but not always) and there appeared to be some flair between 3rd and 4th when pushed hard. What i thought would be an easy change was made harder by the exhaust heat shield being in the way so the exhaust for that side came out along with the heat shield. Mercedes have produced an improved modulator valve for improved shifting as shown in this video


The question I have is having changed the modulator I've plugged in a pressure gauge to the port just below the modulator, the recommended pressure is around 42psi but on mine i get zero pressure until I rev the engine, then the pressure rises but soon drops again. Do i need to adjust the modulator valve to get the required pressure at tickover.

I driven the car as it is and the gear changes appear better than before but it was only a short test around the block.

I wanted to check the vacuum line to the modulator from the engine bay to check for leaks but sods law strikes and my vacuum pump has failed. A new one arrives this friday.

Cheers
Pete
1990 S4 Auto

andy-gts

is there a mercedes transmission forum somewhere, I only know a couple of people that could thoughtfully answer your questions, maybe Lizard, greg brown, cant think of any others off the top of my head....

Weds

That's rather stupid of me, my other car is a Mercedes, I'm a member of a couple of Mercedes forums but they tend to be 'check the codes' or 'Take it to your Indy' type forums..

Bertrand Daoust

Check the vacuum line first. If it's good, adjust the regulator.
If the line is good and you were loosing pressure at the modulator, maybe it's just the rubber cap that was not sealing properly. Just saying.
Never bad to change the modulator though.

Weds

The rubber cap was split, hence the change for a newer improved modulator.

andy-gts

i think catorce is a transmission guy . !!!!!!

GregBBRD

Modulator pressure is set with vacuum line off, going 60 mph.

Make sure you have ~18 inches of vacuum on the hose, at idle. Make sure that the vacuum goes away, with the engine off (I had to trace a "bad shifter" last week. Some genius had hooked up the vacuum to the modulator to the 4 way "cross" to the storage tank....so the modulator never saw a change in vacuum. Shifted very inconsistent, needless to say. Only took us about 3/4 of a day messing with TV cable adjustments and modulator changes (to no avail) before we figured it out. Finally had the vacuum gauge hooked up to the modulator hose with the engine shut off and noticed that it had 20 inches of vacuum. This was not a "quick" transmission shifting adjustment, but expensive hours of tracing of a problem, created by an idiot.)

I dislike the newer modulator. That 7/8" turn cap is a pain in the a$$ to get on and off, if you need to move the adjuster more than the "almost one turn" range.

They sell the rubber cap for the older modulators separate....I replace them on a routine basis....one of the things we check every service.

As far as the video is concerned....changes in the dimension of the hard plastic piece with temperature? Seriously? He actually said that without laughing!

Reduced mass I can understand....

Speedtoys

..all it needed was a new cap.

The new modulator isnt much of a total change..but that cracked cap is only $1, and is easily replacable.

Thats 99% of "modulator problems". Should be on every owners list of "If you didn't replace it yourself yet, do so"

jpitman2

Forum for MB is at mercedesshop.com....
jp 83 Euro S AT 57k

Weds

Thanks Greg

Unfortunately the hose on my pressure gauge is only about 3ft long, cant see the wife hanging out of the window at 60mph trying to read the gauge down by the door sill, time for a longer hose i think.
.
I agree the new caps are a pain. It looked so much better than the old style but I cant get the cap to turn even when its on.. I didn't see the old cap was split until I took the old modulator valve out. In hindsight I should have just got a new cap but i was sold on the 'better' shifting of the new style...

Speedtoys

Lots of info here on trans shifting issues, I mean, decades.

Just ask, if search finds nothing about an issue.

GregBBRD

In a Mercedes, I'm assuming that "better" probably means "softer, without flaring".

In a 928, I'm always looking to make shifts "crisper" without jarring. Most of my changes are in the valve body.

The reality is, I can't drive a car and tell what modulator is in it....so the difference is moot, to me.

Mrmerlin

If you smear some DC 111 on the rubber cap it will make it last forever and it will stay soft and seal properly,
same goes for the O rings

The Forgotten On

Stan, sometimes I wonder if you have one of these laying around with how much DC 111 you must go through https://www.ellsworth.com/products/b...y-3.6-kg-pail/

Anyways, I would stick to the old design until the unit itself fails, the caps are cheap and available, so why go to the hassle of installing a complete new part.