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Old 01-30-2007, 12:02 AM
angst's Avatar
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Join Date: Feb 2002
Location: Chattanooga TN
Posts: 272
vacuum damper, diagnosing. also leaking vac control

Ive been going through my vacuum system trying to find what is going on with my intermittent not shutting off problem and funky shifting under full throttle.
I have mostly been using this mbz.org http://articles.mbz.org/transmission/adjust/ article. and also the flow chart one at petershmid. http://www.peterschmid.com/vac_p1.htm
Im getting closer to zeroing in on the problem. The mbz.org article says to check vacuum on the vacuum control valve and should hold vacuum for minutes with the car off. Conecting the MightyVac directly it would leak down but would not leak down when connected to the damper first.
I discovered the vacuum damper was clogged up. I blew compressed air through it but was hesitant to run a solvent through it. I can not find any solid intel on this damper from a thread search to tell me how to know if it is in spec or not and what to do or not do to clean and test it.
The car drove much better after blowing out the damper but still doesn’t shut off consistently.
Also a question regarding the vacuum control valve. Am I testing it correctly by connecting a Mightyvac to the top nipple and checking for leakdown on the gauge with the engine off? When I pump up the Mightyvac gauge it will not build vacuum at all when connected directly and when connected through the damper will leakdown pretty steady.
I have been messing with the hose connections and am using fresh hose but need to try some skinnier hose before I can be sure my connection is bomber.
If the vac control valve is leaking can it be taken apart and tweaked, or does it need to be replaced outright?

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84 300SD 274K
38K miles on flatplate heat exchanger and various diesel/veg blends. prior to that 4K miles on unheated veggie blends with kero and DinoD.
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Old 01-30-2007, 01:55 AM
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Join Date: Apr 2004
Location: Wisconsin
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Quote:
Originally Posted by angst View Post
The mbz.org article says to check vacuum on the vacuum control valve and should hold vacuum for minutes with the car off.
What the article means is that you should connect your vacuum tester to the black vacuum line that goes from the connector at the vacuum control valve back to the vacuum modulator on the transmission -- the idea being to test that part of the system for leaks. It shouldn't leak. Again, look for the black vacuum line.

The vacuum control valve, though, is all about leaking. It leaks off vacuum at a controlled rate to modulate shift damping based on throttle position.

Have you tried isolating the vacuum system managing the engine and transmission from the central locking (yellow lines) and climate control (blue line) systems? If not, that's the first thing I'd do. Unhook the accessory lines at the big connector at the firewall, plug the holes in the connector and see if the car shuts down properly.

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