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Old 12-06-2013, 03:46 PM
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Join Date: Nov 2005
Location: taylorsville, nc
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Question vacuum control valve 90/91 350 SDL

Slightly confused here. Now I own two of these. The newer one had the vacuum amplifier pulled off it before I got it-don't ask me don't know. Anyway in fiddling around with these I can't figure out what the vacuum control valve on the side of the IP does. It has 15-20 psi going in and 3-5 psi coming out. Changing the throttle position does nothing to change the out reading. Is this normal? Seems like something should change with the throttle position.

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Old 12-06-2013, 03:59 PM
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smoke gets in your eyes
Join Date: Jun 2000
Location: SF Bay Area
Posts: 20,807
Even a new VCV isn't absolutely vacuum tight but in operation it should hold input vacuum, reducing to about zero with increasing throttle input. The purpose is to simulate gasser manifold vacuum. So engine with the engine idling, you should see as much vacuum through the green damper as entering the VCV. Hook up a vacuum pump or another car as a vacuum source to the VCV and a vacuum gauge downstream of the green damper. You should see full vacuum at idle tapering as you increase throttle. I'm guessing you don't want to stand on the throttle in your driveway.

87 300D
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Old 12-06-2013, 04:19 PM
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Join Date: Nov 2005
Location: taylorsville, nc
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must both be bad only 3-5 coming out at idle
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Old 12-06-2013, 06:12 PM
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Join Date: Jun 2008
Posts: 37
I have an 87 300SDL and an 87 300D. The SDL's VCV was shot when I bought the car, the 300D's has always worked like a dream. On the SDL, I replaced every vacuum line on the driver's side of the car and drilled out the rivets in the VCV and took it apart. There is a little black rubber disk on a plunger thingy that moves inside, according to the throttle position, at idle the disk is supposed to seal against a metal nipple that is attached to the vacuum source, then presumably bleed vacuum off as the throttle opens, to try to simulate what happens on a gas powered car. Over time the rubber sealing against the metal wears a hole in the rubber, I tried to replace my rubber disk by cutting one out of a piece of gasket material I had laying around.
I finally gave up on it, I took it apart multiple times, and fiddled with it, used screws when putting it back together, so I could take it back apart and mess with it some more. I tried to use gasket sealer to make it more vacuum tight etc. It just would not work right, had 15 going in virtually 0 coming out.
I ended up ordering the part through the dealer, they said it had to come straight over from Germany, cost me dang near $280 and took 6 weeks to show up. The new one works like a charm, you can hook up a vacuum gauge and loosen the bolts and adjust it so you get about 15-14 going in at idle. Tape the gauge to the windshield and go for a drive, as you step on the throttle the gauge will approach 0. A full throttle romp should have the needle pegged at 0. Around town you can watch the gauge move and feel the difference in the shifts according to how hard you get on it, adjust it so the shifts are firm but not harsh. Less vacuum applied to the transmission means harsher shifts, conversely more vacuum equals softer shifts.
I still haven't found a setting that I think is perfect, it either shifts too hard or to soft, and you don't have to adjust it very much. At least it doesn't snap your neck like it used to before, when the VCV wasn't working at all.

My local MB specialist that I used to go to (who has since disappeared and no one knows what happened to him, his shop has been abandoned for some time, but everything is still there) told me once he never saw a VCV that he thought worked totally correctly, and no 2 of them ever worked the same. I tend to agree, the 300D has 75k miles on it and seems to work better than the brand new one I got for the SDL.

Sorry for the long post and hope this helps.

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