Automatic Tranmission Adjustment 85 300D - PeachParts Mercedes-Benz Forum

rmmagow · #1 10-31-2012, 08:47 AM

193,000 miles.
What can I adjust to help the transmission shift a little more gently when stuck in slow bumper-to-bumper traffic? Mine hits pretty hard 1 to 2, 2 to 3 when I'm going 5~20 MPH in crap traffic. In normal acceleration it shifts nice, firm, but no jolt. The bowdoin (sp) cable seems properly adjusted with a nice smooth downshift to third when traveling at say 45 MPH or so and pushing on the accelerator.
Overall trans is in good condition with zero flare and good smelling fluid at the right level.
Thanks for any and all advice.

vstech · #2 10-31-2012, 09:11 AM

sounds like a vacuum leak to me.
find the black vacuum tube that comes up from the transmission, and follow it through the devices and connections. odds are good you will find a damaged vacuum connection.

rmmagow · #3 10-31-2012, 09:39 AM

That the line that connects to the green (I think) modulator on the left side of the transmission?

Govert · #4 10-31-2012, 09:57 AM

Your 1985 300D has a complicated vacuum setup with a vacuum control valve and a vacuum transducer. You have to do some vacuum checks (with a guage) and inspect/clean/replace the vacuum parts.

vstech · #5 10-31-2012, 09:59 AM

yes. there should only be one vacuum line going down to the transmission. as noted, there are plenty of connections underhood for the leak to originate from.

rmmagow · #6 10-31-2012, 10:03 AM

That Vacuum control valve, blue thing on the driver's side fender under the hood? Has two lines going to it? Always wondered what that was as my 83's didn't have it.
Might there be a "cookbook" for diagnosing this system? I do have a vac gauge and a MityVac to help figure this out but having a "start here", then test this type approach would make life a lot easier.

Govert · #7 10-31-2012, 11:22 AM

The blue flying saucer is the vacuum transducer. It reduces vacuum to the transmission depending on turbo boost.

Do some searches here and on Benzworld about 1985 vacuum.

This thread is a good start:
1985 300D vacuum crazyness.

Junkman · #8 11-01-2012, 11:49 AM

There is lots of information available from Professor Google. Begin by putting the vacuum system in shape. You stop all of the leaks including dash climate pods, door vac, EGR leaks, connector leaks.

Next make sure that the VCV is working and adjusted. (I made a ~10MM spacer for the VCV by putting a 1/8" hole in a nickel.) Finally adjust the modulator if necessary.

rmmagow · #9 11-01-2012, 12:22 PM

Way too much variability of information out there for this system. I'm going to embark on a step-by-step and maybe see if I can get/borrow a smoke machine to help. Under the dash will be my biggest challenge I think. My A/C never worked but connection-wise and control-wise I think it could be made to work again. I need to pull a vacuum on it and put in some refrigerant to see if it tries to work. There a retro-fitted R-134 connector on the low side so I'll put in 134 for the test. I don't care too much about the A/C but if it helps to resolve the tranny shifting that would be great. My basic understanding is that hard upshifts are caused by low or lack of vacuum at the modulator. I am going to hook a long line to the modulator, see if I can pull and hold vacuum with the MityVac and drive the car to see if shifts go a bit softer. As a start anyway. I spent too much of my short summer fixing my Volvo.