Originally Posted by Jeremy5848

I'll check the boost line for the appropriate pressure, thanks.

[edit] Boost is getting to the amplifier. With the engine at operating temperature, the vacuum level to the transmission drops to below 5" Hg at full throttle (the Mityvac is not calibrated below 5) so I think the system is working as it is supposed to.

Last night I re-read my collection of Mercedes manuals. In addition to reinforcing in my mind Mercedes' assumption (that readers of any one manual know about and have access to all of the other manuals), I finally figured out how and why the transmission vacuum system works. At least I think I did. Bear with me . . .

When we forum members use the generic term FSM (Factory Service Manual), we usually mean the "how to fix it" service manual for the engine (and I use it in that meaning here). As we become more experienced, we discover that there is also a maintenance manual, a body manual, a climate control manual, and others. One of the "others" that I have found most useful is the annual "Introduction into service" manual. It not only gives service information for models new in any particular year, but goes into more detail than the FSM.

Speaking specifically of the 1987 300D Turbo (W124) and the 86-87 300SDL Turbo (W126), which uses the same OM603 engine, the 1987 "Introduction into service" manual has almost nothing on the 603 engine. Why is this? Because the 603 was new in the 1986 model year, even though most cars sold in the US of A were imported in Fall 1986 or later and sold as 1987 models. Thus, to get the complete poop on the 603, you must read the 1986 "Introduction into service."

As regards the transmission's "temperature and boost-pressure-dependent vacuum control" (the subject of this whole thread), it gets worse: on page 179, the 1986 Introduction says that the vacuum control system "is the same as on engine 617.95 (model year 1985)."

So it is in the 1985 "Introduction into service" manual that one finally finds the how and why of this vacuum control system. This is covered in the section on the automatic transmission fitted to the 1985 300D (W123) and 300SD (W126) on pages 97 through 106.

The section on the "Temperature and boost pressure dependent vacuum control" includes the how and why and pictures of the components including the "Vacuum transducer" (we have been calling it an "amplifier"), the "Coolant temperature switch," the "Switchover valve," and the "Aneroid compensator (Alda). The heart of the whole thing is on page 102, the "Functional description" and the "Turbo boost pressure effect."

As simply as I can put it, the "transducer" uses vacuum from the vacuum pump, more vacuum from the VCV, and boost pressure from the intake manifold to create a vacuum signal that controls the "hydraulic modulating pressure" in the transmission. "More vacuum" gives softer shifts while "less vacuum" gives harder shifts. (This is why the transmission shifts hard when a vacuum line falls off.)

Below 50C coolant temperature, the "Coolant temperature switch" turns off the vacuum signal from the VCV at the "Switchover valve." Instead, the transducer gets (more) vacuum from the vacuum pump. This has the effect of increasing vacuum to the transmission, softening the shifts while the engine is cold. Once the coolant temperature rises above 50C, the switchover valve is turned on and the VCV controls the transducer. An increase in throttle produces less vacuum to the transmission, thus firming up the shifts.

Turbo boost pressure from the intake manifold also has the effect of reducing the vacuum produced by the modulator. Thus, as the engine develops more power, the shifts get firmer (to prevent slipping). All of these "signals" (throttle position, turbo boost, coolant temp, and vacuum from the pump) must be present and in the correct amounts for the transducer to do its job and control the transmission.

One final addition: sometime in 1987, the factory added a "no load" microswitch to the throttle linkage. This switch turns off the boost pressure signal at the transducer when you take your foot off of the accelerator. The resulting increase in vacuum to the modulator helps the transmission to not 'clunk' as it downshifts (such as when you are coming up to a stop).

Anyone struggling to fix this vacuum control system would be greatly helped by access to the information in the 1985 "Introduction into service" manual. I can scan pages 97-106 into a PDF if someone can help me get it into the forum.

Sorry to be so long winded but as Sam said at the beginning, "it's critical."

Jeremy