Transmission Shifting Diagnosis - PeachParts Mercedes-Benz Forum

tobybul · #1 04-21-2007, 06:46 AM

84 300D. I'm trying to diagnose/tune the transmission shifting. I thought I found the problem when I replaced the modulator cap (cracked and leaking). Made some improvement but still too quick from 2-3 with some flaring and mild slam from 3-2.

I monitored the vacuum with remote gage whilst driving. Noticed holds vac (15") at idle but drops to zero whenever I gas-up. I really didn't know what I was looking for with the vac gage setup other than seeing the write up on using and connecting the tool. Would be nice to see a system description of how the vacuum system relates to the tranny shifting. I have the vac diagram.

Chris Bell · #2 04-21-2007, 06:53 AM

See this article...
http://www.peachparts.com/Wikka/TransVacTune

Ara T. · #3 04-21-2007, 07:23 AM

I've had mine for 4 years and it's never really shifted right. It's responsive and doesn't flare so I don't worry about it too much.

The shifts are inconsistent. I can go through days of hard shifting, then all of a sudden it will start shifting softer so I can just barely feel the shifts. When I give it minimal throttle and let it shift up at 1800 RPM or so it will shift hardest and sloppiest. So basically in parking lots it is most annoying.

I dunno what the deal is but i could probably at least start by replacing all plastic vacuum lines with new stuff, and the rubber connecting pieces. I've never even seen the vacuum modulator so I could check that too. That blue saucer is on my '85 and have been told that's rather pivotal as well.

tobybul · #4 04-21-2007, 08:14 AM

Quote:

Originally Posted by Ara T.

I've had mine for 4 years and it's never really shifted right. It's responsive and doesn't flare so I don't worry about it too much.

The shifts are inconsistent. I can go through days of hard shifting, then all of a sudden it will start shifting softer so I can just barely feel the shifts. When I give it minimal throttle and let it shift up at 1800 RPM or so it will shift hardest and sloppiest. So basically in parking lots it is most annoying.

I dunno what the deal is but i could probably at least start by replacing all plastic vacuum lines with new stuff, and the rubber connecting pieces. I've never even seen the vacuum modulator so I could check that too. That blue saucer is on my '85 and have been told that's rather pivotal as well.

Normally, the first thing to check with any vacuum related diagnosis (as with the tranny shifts) is that vacuum is holding well. A Mityvac is a good tool for this. I have confirmed that vacuum holds in both my Ds. The modulator rubber cap is always suspect with high mileage Ds. That was were the last leak was with my 84.

The blue saucer in the 85is a vac amplifier.

My up shifts are normally soft except mybe the 2-3 which is detectable but not harsh. The harsh one is 3-2 downshift (at least I think its the 3-2 because it happens as I slow really down but not when I abruptly slow down.

I have read Steve Brotherton's well written article. I just find it difficult to follow as he talks about various things and I don't deal with tranny shift problems everyday. **************.com is also another place for tech info but you gotta buy Kent Bergsma's manuals. I have a couple of them but the ones I have just touch up on this subject.

Brian Carlton · #5 04-21-2007, 10:47 AM

Quote:

Originally Posted by tobybul

The harsh one is 3-2 downshift (at least I think its the 3-2 because it happens as I slow really down but not when I abruptly slow down.

Find the Bowden cable under the hood. At the attachment point is the plastic adjustment screw. Turn the screw clockwise (inward) one turn and drive it again to see if the downshifts are improved.

They will clunk on downshifts if the downshift occurs at elevated rpm's........a characteristic of a Bowden cable that is a bit too tight.

The other characteristic of this issue would be slightly delayed upshifts..........but, you may not notice this........because the vehicle performs much better when adjusted to achieve this.

The SD has delayed upshifts with a tightened Bowden cable, however, it hasn't started banging it's downshifts as of yet.

tobybul · #6 04-21-2007, 05:38 PM

Thanks, All...

One thing I noticed with the vacuum is that it drops to zero immediately as I press the gas - not at full throttle as y'all say. Does that mean the Bowden is too tight?

Going out with the wife to a Fund Raiser right now. I'll be using the car. I'll back off the Bowden b4 I leave and see what happens. I'll report findings when I return.

Brian Carlton · #7 04-21-2007, 05:42 PM

Quote:

Originally Posted by tobybul

Thanks, All...

One thing I noticed with the vacuum is that it drops to zero immediately as I press the gas - not at full throttle as y'all say. Does that mean the Bowden is too tight?

That's unrelated to the Bowden cable. It sounds like the VCV is not functioning properly. But, I'm a bit surprised that you don't have rock hard upshifts with zero vacuum.........