Jeremy,
As I expected, the vacuum diagram file you attached to your last POST was only 38K in size... thus the resolution was less than desireable. IF you look and see that the original file is larger than 38K, would you please send it to me by the direct email route at: MrSafety2@Verizon.Net !

This is a whole new dimension for me as I learn more and more about these Tranny Vacuum Control Systems

Sam Ross

Sam, the diagram is "in the mail." About 2 Mbytes.

OK, I tried Brian's suggestion to put the Mityvac on the transmichigan line, pump it down to 15" Hg and drive the car. Results -- butter smooth shifts! I wasn't driving with a lot of throttle, I suspect that had I floored the accelerator and made the car shift at 4500 RPM there might have been a little flaring. But it was great to be able to drive without having my neck almost snapped.

I also tried different 'amounts' of vacuum. Lower than 10" Hg caused harder shifts. This suggests that the VCV on my engine is dropping the transmission vacuum level too quickly in response to the throttle. I'll tee the Mityvac into the transmission line and take some more readings.

Procedure 07.1-1826 in the FSM is entitled "Testing and setting vacuum control valve." For the 603 engine, it specifies 385 +/- 25 millibars of vacuum in the line to the transmission with the engine at idle. This is equivalent to 18.45" Hg. When the engine is switched off and the accelerator is moved all the way to the full-load stop, the vacuum is supposed to drop to 0 mbar (atmospheric pressure).

In my car, the vacuum starts at about 16" Hg but drops only to about 7" Hg at full throttle. I'll try the adjustment procedure specified in the FSM (with accelerator at the full-load stop, loosen the VCV fastening screws and turn the VCV clockwise "until a resistance is felt," then tighten the screws.

Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

385 millibars is equivalent to 11.55" Hg. With your vacuum at 16" at idle, the system should have more than sufficient vacuum for very soft shifts. My 603's are all about 12" at idle and I've attempted to get them down to 10" for firmer shifts.

So, I suspect that the modulator has been adjusted to a much higher pressure than desired because very high vacuum is required to soften the shifts.

After adjusting the VCV per the FSM, I tee'd the Mityvac into the transmission line and took the car for a drive. The results were: much better (softer) shifts at low throttle positions, still very firm at or near full throttle. I suspect that is proper -- you wouldn't want the transmission to shift slowly with the engine at 4000 RPM.

The vacuum level drops from the idle position of 16" HG to about 5" Hg at full throttle, not the zero level that the FSM says it should be. I don't know if I have a problem with the vacuum amplifier, with some other part, or if the FSM section really doesn't apply to my car.

How does one adjust the modulator?

Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

The amplifier drops the vacuum from 5" toward 0" when it sees boost from the ALDA line. Check to be sure that this line is intact. There is a Y connection at the ALDA for the pressure supply to the amplifier.

If you've got acceptable shifts at all pedal positions, it's not critical that the vacuum level drop all the way down to 0".........if it works well........leave it alone.

The modulator is adjusted with a retractable T handle right on the valve. But, I wouldn't bother with it if you've cured the hard shifting problem.

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

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

Hello,

Sorry if this has been asked already....hate to beat a dead horse,but... I’ve skimmed thru the entire thread and can’t find a similar Q or A.

It’s always stated that the vacuum should drop to zero or near zero with the VCV at full throttle. How “near” zero is acceptable? I can only achieve 5” hg no matter what I manually restrict the starting vacuum to. I assumed a defective VCV and retrieved one from my donor vehicle and it yielded the same results. Two parts giving the same results seems odd to me.

Any thoughts?

My vacuum diagram is the same as post #42, page 3.

Thanks

If your transmission shifts OK I wouldn't worry about it. Mine goes down under 5" Hg at full throttle but I can't determine how much because the Mityvac's calibration ends at 5. I need a better gauge, I guess. It's possible that turning the boost up might bring the vacuum down but I wouldn't do that (adjust the boost) just to make the transmission vacuum signal lower. Your engine can't use much more than 11-13 psi of boost anyway.

Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

Thanks for the reply Jeremy.. Unfortunately my system doesn’t utilize a transducer…I’d probably be better off in the long run to find a system like yours at the bone yard and install it on mine, but until then I should (hopefully) be able find a happy medium with the current setup.

I am having a shifting problem and was starting with the VCV as a beginning point. I guess I am having some ‘wishful thinking’ that I can solve my problem at the VCV.

Chris

If it doesn't flare on full throttle shifts, you've got nothing to worry about.

It does......and part throttle shifts too.

Actually by removing the dashpot, the shifting has improved, but it's still not right.

How about a full description of the symptoms? The only thing you've provided to date is the fact that it won't go below 5" vacuum.

Provide exactly what it does on various pedal positions and various rpm points.

Have you measured vacuum to the modulator at idle?

Any monitoring of vacuum during various part throttle tests?

I’ve read the posts on other problems and didn’t want to beat the ‘dead horse’ here with all my symptoms, but since you asked….

Short shift on 1-2 at anything but WOT.
Flare on 2-3-4 any RPM range, any speed----not so much at WOT, but still present
No downshifts except when ‘kick down’ switch under pedal is depressed.

15” hg at modulator @ idle (tested--holds vacuum)

No drive/monitoring Hg tests yet—just starting diagnosis.

Throttle linkage is correct and I am getting full range of travel on the VCV. I also tested both VCV’s with the linkage disconnected and operating the valves manually with the engine idling. Both valves operated smoothly through the range and would only go down to 5” Hg. I used a pair of self-locking hose pinch pliers to restrict the supply vacuum to 8” Hg (as mentioned in your posts as a good starting point), still only 5” Hg at full travel. VCV's bench test good as per directions posted in thread.

I’ve had several vacuum issues with the car that I have been slowly working on and have now corrected and I am starting to work on the shift issues, before I cook the trans.

I am pretty sure I don’t have a major internal trans problem as through the course of fixing the vac. leaks, the trans would shift fine (or close enough) until I fixed another leak or two. As I tighten the vac system up, trans shifting became increasingly worse. I also drove to work once without any vac to the modulator. I squawked the tires on every shift,any rpm (no flare) and my neck is still sore.

I know it’s capable of shifting correctly; it’s just a matter of where I start to look and 5” hg at WOT didn’t seem correct to me from what I've read.

Thanks,
Chris

That vacuum level should be reduced to about 8-9". On the '84, there is a small domed cover on the side of the VCV. There is a tiny slotted screw that makes the adjustment under the cover. The movement of the screw will only be about five degrees...........don't go more. Adjust the level at idle down to 8" and see how it drives. You can use the Mityvac with the engine not running. The VCV will bleed all vacuum above the setpoint.

Thanks, I’ll do that tomorrow.

Right now with the dashpot removed from the system it is shifting correctly and firm at all RPM’s which is what I prefer. Only a barely detectable flare on occasion, but no downshifts still. I’ll see what I get after the VCV adjustment.

Thanks again,
Chris