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Old 09-07-1999, 01:58 PM
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I'm starting to worry a problem my car is having: with engine cold when braking and hitting the 30KPH (~20MPH) mark the transmission downshifts and a BIG "tunk" is heard from the back end, seemingly from the passenger side. This does not happen after the engine has warmed up and is much less noticeable if I am not braking. Upshifts are OK. Not extra smooth, but nothing to complain about.

The vacuum to the transmission is OK, there is but a small vacuum leak in the climate control (tracked it down to a solenoid activated valve in the dash, only leaks when powered, so I can take that out of the equation). Also, the transmission will not downshift under acceleration. I need to hit the switch at the end of the accelerator's travel for it to downshift.

Any help/experience/tip appreciated.


1982 300D -

Old 09-07-1999, 10:28 PM
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Join Date: Mar 1999
Location: Suwanee, GA, USA
Posts: 4,712
The most common thing is a vacuum leak or not enough vacuum when cold. The vacuum pump should be checked when the car is acting up.


Old 09-08-1999, 04:27 PM
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Thanks BenzMac. That was the problem.

I read your reply last night and, as luck would have it, I received one your competitor's (name withheld...) parts catalog yesterday. I'd ordered it just for the vacuum diagrams I'd been told it contained. I opened it up to the transmission vacuum diagram, compared it to my car's and voila! The setup was wrong!!!

I know it was still as it was when I bought the car. I'd replaced some parts and moved stuff around but I'd made a drawing of the setup and always put it back as it was. I guess the previous owner - a shop - wasn't as careful.

Thanks again.


1982 300D -

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