MB manual for 123 vacuum modulator replacement
 

I've got an '82 300D Turbo and have determined that the vacuum modulator unit isn't holding any vacuum. I tested this with a MityVac with the engine off. I'd like to replace the modulator, but read that I need to follow the MB instructions to prevent transmission damage. I've got the W123 Service Manual, but it has no section for Transmissions, and I can't find these instructions anywhere. I don't want to pull out the old modulator and swap with the new one if there's more that needs to be done. Does anyone have a copy of these instructions anywhere?

Exactly which manual do you have ?
It should be in one of the two chassis manuals for the 123....

I've got a soft-copy of the CD for the 123 chassis, but as I started looking through the files, they're all DISK2. I'm assuming that there's a DISK1 I don't have that would have the transmission info? If anyone has the vacuum modulator file, would you mind letting me borrow it?

Look up the appropriate section on the skinnerbox website here

disc one is exactly the same as disc two except its for gassers, unfortunately the cds dont have anything on transmissions bar basic maintainence

Yeah, I checked out the Skinner site, but section 27 - Automatic Transmission has no items with it.

So, I've got to get hold of the factory manual in Book Form in order to get Transmission info?

:) You sure its the modulator? It could be the cap on the modulator. It is made of rubber and if the cap is broken, your modulator will not hold vacuum. Take out the cap and examine it. In any case, I dont think they sell the cap alone so you are probably going to end up having to get a new modulator. Get the new modulator from Mercedes since they have a part upgrade which includes a new pin. You can get instructions on how to install the modulator at the bottom of this page:

http://transmission.articles.mbz.org/adjust/

Mike Senko recommends setting the modulator at 41psi.(ballpark) and then adjusting according to the ride. Follow his guide at:

http://www.wagoneers.com/DieselBenz/TECH/Trans-ADJUSTMENT.html

You will need to make a modulator pressure tester. Here are some instructions on how to make one:

http://www.wagoneers.com/DieselBenz/TECH/Trans-Pressure-Gauge.html

Hope this helps,
Adiel

Thanks for the links - I'll check them out. I never actually removed the modulator or got that good of a look at it. With my Mityvac, the line going to the modulator is fine, but when I plugged right into the modulator, it leaked like a seive. Here's hoping it's just a cracked cap :)

:) Hmm, there is something not right about this... If the line going to the modulator leaks but then when you connect the mighty-vac directly to the modulator it does not leak then I would understand that you have a leak in the line. But, in your case, you put the mighty-vac on the line going to the modulator and it did not leak (I am assuming you put around 15hg of vacuum for about 5 minutes and it did not leak)? If the line did not leak, then your modulator is not leaking. Even though you said you put the mighty-vac directly on the modulator and it did leak, this must have been a leak around the hose going from the mighty-vac to the modulator. My question now is, what is wrong with your transmission, why do you want to change the modulator?

Thanks,
Adiel

Um, it's the opposite actually. Sorry if I wasn't clear.

I disconnected the vacuum line from the v.m., plugged the v.m. end and put a vacuum on the line. It holds great, so no leaks in the line.

Next, I put a vacuum directly to the v.m. through the fitting the vacuum line ordinarily connects to, and it won't hold any vacuum at all; instant leakage.

My 1-2 shift is nice and firm, but I get flaring on the 2-3 and the 3-4 shifts. I'm managing to work around this with judicious modulation of the throttle, but it's one of those things that needs to get fixed. All of the downshifts are perfect, it's just the last two upshifts that aren't quite right.

You can buy the Transmission Modulator Cap separately and least that is so on the Green Colored Modulator used on 722.315 Transmissions.

In fact some parts places show the Modulator for sale with no Cap on it.