The blue disk on the the front whel well (under the hood) is, I believe, a component of the vacuum system that assists in the shift timing of the tranny. Does this item take its readings, at least in some degree, from the same line that runs from the intake manifold to the ALDA?
Does there exist on this palnet a schematic of the vacuum systems for these cars? Haynes is ridiculously deficcient on this.
Thanks.

Yes, the MB manual has schematics for the vacuum systems.

Thanks, Ron. I've really put off getting the manuals. Any notion on the best place to get them?

From what I gather from the link below (in an earlier thread I couldn't find again...), the blue vacuum piece (see figure 7) is an amplifier installed on 1985 and later cars only.

Here's a snip from the web page:

The most important modification came with the 1985 model and is continued on all subsequent ones. It involved the addition of a vacuum amplifier to the system (see Fig. 7). The amplifier takes a large vacuum supply, the proportioning vacuum signal and a boost signal and creates the final signal to the transmission. This device did two things. First, it incorporated the input of boost to further tailor the load control of the modulator vacuum signal. The vacuum at the modulator now starts high - 12-17 in. - and is reduced to about five inches through straight throttle movement. As boost builds, the vacuum is further reduced to zero only at full boost.

The second thing this system did was reduce the sensitivity of the final signal to small changes in the proportioned signal. When this signal went straight to the tranny, a 10-20% fluctuation due to linkage or vacuum conditions (the orifices are very susceptible to diesel soot restriction in any of the orifices) caused great changes in shifting. With the amplifier, the proportioned signal is just one input and the output averages out the variations. Shifting in diesels was never better at this point.


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

I hope this helps.

Joel

__________________
Joel
1985 300td (sold)
1981 280SEL (4-speed)

From what I gather from the link below (in an earlier thread I couldn't find again...), the blue vacuum piece (see figure 7) is an amplifier installed on 1985 and later cars only.

Here's a snip from the web page:

The most important modification came with the 1985 model and is continued on all subsequent ones. It involved the addition of a vacuum amplifier to the system (see Fig. 7). The amplifier takes a large vacuum supply, the proportioning vacuum signal and a boost signal and creates the final signal to the transmission. This device did two things. First, it incorporated the input of boost to further tailor the load control of the modulator vacuum signal. The vacuum at the modulator now starts high - 12-17 in. - and is reduced to about five inches through straight throttle movement. As boost builds, the vacuum is further reduced to zero only at full boost.

The second thing this system did was reduce the sensitivity of the final signal to small changes in the proportioned signal. When this signal went straight to the tranny, a 10-20% fluctuation due to linkage or vacuum conditions (the orifices are very susceptible to diesel soot restriction in any of the orifices) caused great changes in shifting. With the amplifier, the proportioned signal is just one input and the output averages out the variations. Shifting in diesels was never better at this point.


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

I hope this helps.

Joel

__________________
Joel
1985 300td (sold)
1981 280SEL (4-speed)

Joel:

That link is a great piece of help. Finally makes sense of the matrix of influences on shift timing.

Thanks!

McCann:

You're quite welcome, though I can't take any credit. I wish I could remember who posted the link originally because it does a very good job of explaining the intricacies involved with this system. Also, I found the comments about the relative condition of the individual parts influencing the settings of other parts interesting.

As you may have noted, I found it so interesting that I posted it twice!

Cheers (and good luck),

Joel

__________________
Joel
1985 300td (sold)
1981 280SEL (4-speed)

MB is not producing the paper versions anymore--just a cd-rom version--any MB dealer or direct from MBUSA. I think Fastlane (on this website) will order them too--send an e-mail inquiry.

For the paper versions, best to look at e-bay or put out a wanted to buy on this web site. They show up fairly regularly.

Thanks for the guidance. By tweaking the ALDA, cleaning out the Overboost Protection Switch and adjusting the Bowden Cable, shift timing and power are back to normal. I was not looking forward to monkeying with the modulator on the tranny housing, but that may be required as this horse has been around the course more than a couple of times.