87 300D transmission shifting late - PeachParts Mercedes-Benz Forum

mkrazy1 · #1 03-20-2012, 09:24 PM

On my car, it shifts okay from 1 to 2, and 2 to 3 will only shift once the rpm gets to about 4500, and it won't shift to 4 unless I move the shifter to neutral and then back into gear. I have gone through quite a few of the different postings for shifting issues and figure it has got to be either the modulator, or a vacuum issue.

I have link http://i323.photobucket.com/albums/nn447/Jeremy5848/1987%20Mercedes%20300D/Vacuum603.jpg for the vacuum lines.

I hooked up a mity vac to the port on the vacuum amplifier, "blue flying saucer", that goes to the transmission, and at idle it is at about 17". If I floor it, it'll drop to about 7". Is this correct? From reading, it sounds like it should drop to zero.

If I remove the vacuum line going to the modulator, it'll shift into all the gears, albeit real early and hard.

mkrazy1 · #2 03-21-2012, 09:01 AM

One other thing. This morning I drove it to work, 10 miles, and it shifted as it should. Yesterday morning it did also. (The problem started over the weekend). I didn't have the mity vac hooked up this morning, so I am not sure what the reading is when it's operating normally. So, it could be related to heat also?

scottmcphee · #3 03-21-2012, 02:34 PM

Vacuum controls mostly shift hardness, and your numbers sound OK enough to at least see all gears.

I suspect it's the Bowden cable, as this controls the shift point relative to "throttle" position (so roughly "RPM"). So for early / late shifting (or real late shifting as in never for 4th) check if the Bowden has slipped out of position. And it's adjustable by finger tightening at the engine side near the cruise actuator, no need to go under the car. Get the slack out of it. Then adjust 1/2" either way and go for a test drive, repeat the other way.... adjust to your likeness.

See if that helps.

mkrazy1 · #4 03-22-2012, 09:30 AM

Left work yesterday and headed home - by the time I got home it was just starting to act up. (10 miles)

I moved the bowden cable by hand to see if it was catching or stuck, and it moved fine and in the idle postiion, there is no slack. By looking at the fitting, it hasn't moved.

I was home for a bit and hooked up the mity vac to a tee and ran a line to the cabin, then left for a meeting which was about 25 miles away.

It seems like the vacuum is higher than it should be. It is really hard to tell what is going on though! I was going up a gradual hill on the interstate, doing 70 and it kicked down to 3rd. Obviously the rpms went up to about 4500. I only had it maybe 1/3 throttle and wasn't accelerating. Boost pressure was 10 and it looked like vacuum was about 10" or a bit higher. If I released vacuum with the mity vac, it shifted back into 4th.

This morning on the way to work, it shifted fine. There is even one fairly steep hill that I was doing 55 on and it stayed in 4th. It has been getting into the 80's here during the day (never had winter) so I am thinking it has to do with heat.

mkrazy1 · #5 03-23-2012, 10:25 AM

Definitely heat related. Does okay in the morning, but once it has been running for a while and the transmission is hot, it starts acting up. I got under it last night and pulled the cover of of the modulator to try adjusting it and kind of scratched my head and put the cover back on it. Is it adjustable? After pulling the cover, it had a metal pin going across the top. Since it is as such a hard angle to see, I couldn't tell if it had a screw under the pin or not, and I didn't want to pry the pin off of it and have it come apart.

So, do I just pry the cross pin off and will there be a screw below it, or is it not adjustable?

Is this whole issue maybe where the tranny is showing it is on it's last leg. If I remove the vacuum from the modulator, it does shift somewhat normal, just a bit early and hard.

Thanks for any/all help!

scottmcphee · #6 03-23-2012, 02:44 PM

The word "fluid" does not appear in this thread.

You've checked the ATF level, and it's clean, and reasonably new filter, right?

mkrazy1 · #7 03-23-2012, 03:55 PM

Thanks for helping... Yes, I had changed the fluid/filter a year or so ago, and did verify it is full at the onset of this issue.

I did find this link that helps explain how things should be adjusted, but it was doing fine up to a few days ago.

Another 603 vacuum adjustment/shifting thread... help Brian, Dave, etc.!

Also, I did find out that the "metal bar" I am seeing is actually the adjuster, so it sounds like I need to pry it up and then turn it. I saw another posting though that makes me wonder how it adjusts.

mkrazy1 · #8 04-04-2012, 12:00 PM

Okay - I turned the adjuster 1/2 turn ccw on the modulator valve. It did get back to the low 80's yesterday and it shifted okay. Time will tell when the temperature gets warmer as we approach summer.

I did get to thinking about this more though: Since I got the car about 3 years ago, it didn't have the acceleration it should and shifted hard. Last fall, I finally figured out the overboost protection valve was not working, so the pressure from the turbo was not activating the alda. I just bypassed the overboost protection valve and performance was markedly better. At that point I started getting 10" pressure to the alda. Well, this would mean the Vacuum Amplifier was now getting a different value. I am guessing the previous owner had adjustments made and the real issue was never diagnosed (bad Overboost Protection Valve).

So that brings on an additional question... What shoud the max boost be? The most I get is 10 to 11, so I haven't been too worried (haven't replaced it yet) about bypassing the OBPV.

Thanks for all your input!!

sleepstar · #9 04-04-2012, 01:33 PM

might it be vacuum problem going to the transmission itself? maybe when your engine heats up it's causing intermittant vacuum issues? have you checked to see how much vacuum is goign to the transmission when the car is warm? also in your first post you are correct. i think at idle it should be ~10" and at WOT it should be 0"

davidlee · #10 04-04-2012, 03:40 PM

I just went through the same thing on my 87 300D about 6 months ago , It wouldn't go into 4th gear unless i put it in Neutral then back into Drive. I took it to the trans shop and had the filter changed, when I got it back, everything seemed to work fine. I drove it for about 2-3 months then one day I was driving down the road and It felt like someone had engaged the parking brake while I was driving, so I pulled over and right before I came to a stop, What ever it was freed up and it felt normal again, so I pulled back onto the road, then at the next stop I came to, I got about 5 feet past the crosswalk and All my forward gears let go, I only had reverse. So I bought a used trans. had It installed and The mechanic is doing me the favor of pulling the old one apart to find out what let go, because all the gears shifted properly and strong before this happened, but it all started with It not wanting to go into 4th gear.

mkrazy1 · #11 04-04-2012, 05:20 PM

Davidlee - Don't scare me!

Sleepstar - I have a vacuum gauge hooked up and monitoring it. I also have a gauge hooked up at the alda to see the pressure from the turbo. I had made mention in a previous post. It is hard to really tell if the vacuum changes much depending on the temp. It may change a 1" or 2" but no more. I think it has more to do with the viscosity of the trans fluid, when its hotter, its not as heavy.

davidlee · #12 04-04-2012, 05:38 PM

You say you changed the filter a while ago, do you know how long before that it had been changed. It is possible if you changed it for the first time in a long while that you could have done more harm than good. the clutches over time of not keeping regular service become accustom to the old fluid and once that is finally changed out, The new fluid carries new friction modifiers in it and they can start eating away at your clutchs and re clogg the filter.
Im not trying to scare you, My apologies,
I'm just trying to help you figure out your problem thru my personal experiences with my 87 300D. My car has 320K on it. I suppose my trans. was allowed to go out after that many miles without getting too upset

mkrazy1 · #13 04-05-2012, 10:08 AM

Not sure if it had ever been done prior to me getting it. No records with the car. I did get it with only 90K miles on it though. I changed the fluid and filter at about 100k.

scottmcphee · #14 04-05-2012, 10:22 AM

Normal boost, you asked, is about 11 psi. So your turbo is good.

vstech · #15 04-05-2012, 11:06 AM

when you changed the filter, if it's been 100K, you may need to change the filter again. all the passages may be cleaned now, and the new filter is clogged up.
for giggles, I'd dump and change again... perhaps fill with redline?

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