When I bought my '81 300D two years ago, someone had rerouted all the vacuum lines under the hood, and installed an electric vacuum pump for the transmission. I followed the diagram under the hood, but I have a couple of questions about the routing.

on the diagram (attached) there are 3 points that go nowhere. I have labeled them as 1,2,&3 on the pic. the way I am set up right now:
1 goes to the Break Booster (with another tap for the cabin controls).
2. is going to the vacuum modulator on the transmission.
3 goes no where.
Is this correct?

Next question:
I was doing some vacuum checks looking for leaks. The vacuum valve on the injection pump I did not get any vacuum to or from the valve at an idle. When I pulled the throttle lever the switch on the valve cover engage, then I got about 15" of vacuum. I gave it more throttle then it dropped to about 10" and stayed there. This doesn't seem right to me. Do I need a new vacuum valve?

In the pic i have labeled the connectors with yellow and orange arrows, the small green arrow is #3 above.

Attached Thumbnails

   

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MC Professor

1981 300D non-Turbo
with unknown 617. motor and 711.311 transmission

You want to at least plug 1 & 2 since you don't want to lose vaccum. 3 is supposed to be open. it normally runs into the cabin (thru a filter). The off-white device is the VCV, which serves to control transmission shifting. You can read much about it here, but basically it serves to simulate the intake manifold pressure vs throttle position relation one gets in the gas engines these trannys were designed for. The output of the VCV (runs to tranny, 1985 routes via blue saucer) should be full vacuum (~20" Hg) at idle and lose vacuum as the "fuel pedal" is depressed. Yours doesn't sound right, but it the vacuum comes on soon after you move the pedal, perhaps adjusting the linkage will work. An electric vacuum pump sounds smart to me, especially if you replace the engine pump w/ a cover (or gutted pump), as many here are working on.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

From what I've seen in the standard vacuum diagrams, you have everything connected fine. ( #1 brake booster and #2 vacuum modulator)
http://www.peachparts.com/shopforum/general-information/301855-mercedes-vacuum-diagrams.html

Yes, #3 is a vent to the interior. It what allows the vacuum to bleed off with increased fuel pedal.

What kind of problem are you having?

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Sam

84 300SD 350K+ miles ( Blue Belle )

Thanks guys.

SD Blue: Previous owner said the only way he could get it to shift was to let off the pedal, then repress. I found that I could get it to shift manually; by starting in Slope, then shifting to drive once I reached about 40MPH, it shifted late, with an occasional flair but she shifted. BUT...., I had/have to shift into neutral whenever I stop, and sometimes when I drop below 10MPH, or the trans refuses to engage. I adjusted the modulator a little, and got the timing better, but I still had to shift to neutral at stops.

Last night, I came to a stop at a light, she refused to move. I started pushing her (in gear) and the trans started to grab. I made it home without stopping, but I did have to shift back into third for a short time as 4th started slipping bad.

She really acts up when its cold.

I just want to make sure I have checked/ fixed everything else before I take her to the shop to get the trans worked on.

SD Blue, noticed that you reside west of Ft. Worth, I acquired my Blue Belle in Dallas.

Small world.

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MC Professor

1981 300D non-Turbo
with unknown 617. motor and 711.311 transmission

I don't think you have any kind of vacuum issue. From what I've read, vacuum is set up to soften the shift on these transmissions. Usually the symptom of a vacuum problem is harsh shifting.

Maybe a possible B2 piston problem? Although, this usually is a sudden catastrophic failure in forward gears, with reverse working normally.

The shop will probably need to put pressure gauges on the transmission to see if there is a pump issue.

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Sam

84 300SD 350K+ miles ( Blue Belle )

I was changing my P/S filter and accidentally pulled out the vacuum connection to the vacuum pump and didn't notice it. First time I drove it I definitely knew something was wrong cause it really bumped hard when shifting. After plugging it back in it was smooooth.

'92 300SD

Reverse works great, wish forward worked that well.

I just got went out and looked at the levers on the vacuum valves, they are worn. I think that might be the reason I get no vacuum to the VCV at an idle. On order now.

I noticed another curiosity. never paid much attention to it, but the vacuum line going to modulator is very thin, not what I would normally expect to see as a 'vacuum' line, windshield washer maybe. I'm thinking that at an idle (once vacuum is applied) there is about 18" Hg being applied, it may be enough to collapse the tube giving the modulator 0"Hg. I believe that would tell the modulator that the engine was at max RPM, and without sufficient rpm from the engine not enough hydraulic pressure to engage.

Does that sound right?

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MC Professor

1981 300D non-Turbo
with unknown 617. motor and 711.311 transmission

The tranny is designed to upshift with zero vacuum. And it will be smooth. As soon as you hit the accelerator, the vac is designed to go to zero so it can upshift.

It will also downshift OK with zero vacuum but it till clunk. Vac is necessary for a smooth downshift only.

Thanks. Guess I'll probably be going to the shop. I wouldn't know where to start with an automatic trans.

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MC Professor

1981 300D non-Turbo
with unknown 617. motor and 711.311 transmission