I've been trying to diagnose a perpetually slippy transmission in my 81 300td. after adjusting the nut on the VCV, the pressure at idle running to the modulator is 12" but goes UP to 15 when under full throttle. I took the linkage off the valve and when the engine RPMs stay at idle the vacuum goes down to zero as it should when I actuate the lever. Is this at all normal? It seems like quite a number of things in the vacuum system have been changed by previous owners, for instance, I can't find anything like a measured orifice as mentioned here in a variety of places.
Hmmmm... Possibly related: the modulator definitely has a leak and there is a leak at the shut off switch in the ignition but this VCV problem is there without either of those connected.
Any thoughts?
thanks!

Does not sound normal to me. What is the vacuum in the main line when at idle and at speed? I'm wondering if the VCV is not working and simply responding to overall vacuum levels in the system at different rpm's.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

vacuum in the line builds up to about 22" with the leaks plugged and then will rise a little with RPM's but only to about 25". I wish I had another car to poke around in right about now and see what I'm missing...

The 22 to 25 increase explains the difference between 12 and 15 at the VCV outpout at different rpm's.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Article on adjusting the transmision;

http://www.continentalimports.com/ser_ic20242.html

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84 300D, 82 Volvo 244Gl Diesel

In the below Pics the Red Arrow points at the Orifice that looks more like a simple connector.
The 2nd pic; inside of the Yellow Circle shows a Brown Plastic Piece that I believe is the Orifice.

It is hard to tell from the pic but in my case the Vacuum Line with the Orifice comes off of the Main Vacuum Line Nipple that is to the front of the Car.

Attached Thumbnails

   

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84 300D, 82 Volvo 244Gl Diesel

Great, thanks for those pics. I definitely haven't got an inline orifice. Is that setup from an '85? it looks a tad more complicated than mine. The 3" rise in vacuum makes sense without the VCV but since it seems to be functioning without the RPM's up (dropping all the way to zero) I can't figure out why this is happening. Maybe the volume of vacuum is way too much without the orifice installed? How does the vacuum volume (am I even using the right word?) relate to the amount of pressure?

I've wondered that same thing myself and don't have a clear answer, although when comparing a giant vacuum pump with a small leak and a small vacuum pump with a similar sized leak, it seems to me that the system with the small vacuum pump will lose vacuum more quickly and the system with the huge vacuum pump won't have much evidence of the loss. I'm assuming that circumstance is analogous to systems with and without orfices.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I should have been reading the "It's CRITICAL" thread before. It's definitely the orifice, apparently the volume is too high without it and the VCV can't bleed down effectively. I tested this with some vice grips around a vacuum hose. Now to find a few orifi or some very small drill bits....

Usually slipping and flaring aren't vacuum related. Usually it's internal to the trans. Vac problems cause hard shifting.

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Too much vacuum definitely caused flaring on my 116. Reduced the vacuum and flaring went away and shifts became tight.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

vacuum leaks do tend to cause hard shifting.

however, in my experience, a misadjusted vacuum modulator or VCV that doesn't bleed vacuum correctly can cause massive 'i think this transmission is toast' flaring.

to the OP - if you think the system has been tampered with a lot, think about resetting the vacuum modulator and sort of starting from scratch. i had to do this on my '82 a while back - http://www.peachparts.com/Wikka/Trans722VacMod

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car

At least 5 of the 6 possible choices were still available from MBNA late
last year. FWIW. I think they were around $6 each.

The orifice changes the "ramp" or of vacuum onset seen by the VCV, which can definitely tune the shift performance. At least that's my experience.

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--frankb

1982 300TD 617.912 <NLA> . 1975 2002 E10-EFI . 1976 914c6 . 1983 MG LMIII <NLA>. 1988 Montero / 616<NLA> . 2001 TLS . 2005 Saab 9-5 Wagon

I called mercedes and they don't have any of them anymore so I think I'm going to need to hit the salvage yard. The slippage was definitely the result of the modulator vacuum being too high at full throttle causing things to get very mushy. When the vacuum was disconnected the transmission banged around alot but didn't slip at all.
I made a temporary fix with a couple of fuses and a hose clamp around the vacuum line to restrict it. Seems to work fine for now but definitely not a permanent solution!

If you're close to a supplier of pattern making materials, they generally stock VERY small numbered bits for penetrating vacuum operated molds. #75 to
#90.

http://www.freemansupply.com/

HTH. I still have 3 or 4 of the colors, and can measure them, if that helps.

--f

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--frankb

1982 300TD 617.912 <NLA> . 1975 2002 E10-EFI . 1976 914c6 . 1983 MG LMIII <NLA>. 1988 Montero / 616<NLA> . 2001 TLS . 2005 Saab 9-5 Wagon