transmission shifts and "clunk" - PeachParts Mercedes-Benz Forum

rg2098 · #1 10-30-2004, 09:22 PM

Heres the story, I replaced my transmission vacuum modulator last week because of inconsistent shift pressures. It still shifts at all the correct points, but the 1-2 shift is still extremely harsh all the time even though all the others are smooth as silk. What do I do now?

Also now I have a "clunk" now that I notice mostly when it shifts from 2-1 while braking coming up to a stop, could be with upshifts as well but I can't hear it over the engine revs. I unbolted the transmission mount to drop it a few inches, would that possibly cause my clunk if the mount is not torqued correctly or not alligned, or did I damage something in the drivetrain?

rg2098 · #2 10-30-2004, 09:22 PM

its a 83 300D, thats probably a good thing to add

fz500sel · #3 10-30-2004, 09:48 PM

but my modulator was good. I ended up having a plugged vacuum line to the modulator, but it was NOT at the modulator end of the vacuum line.

As for the "harsh 1-2 shift".....I was told that this is normal with the 300D's. It shouldn't be a head jerking kinda' shift though. Just be happy you have a nice smooth shift thru the rest of the gears.

rg2098 · #4 10-30-2004, 10:00 PM

unfortionatly it is a "head jerking" shift. I know that the 1-2 harsh is normal but this is way to hard.

Old Charro · #5 10-30-2004, 10:17 PM

Adam, I also have a 83 300D, and changed the modualtor a couple of weeks ago. Have the same symptoms as you. Hard 1>2 shift, and hard clunk when coming to stop 2<1 downshift.

I dont think you did any damage, just a matter of getting right pressure, and shift points adjusted just right.

What speeds is your tranny shifting at right now?

Andy

rg2098 · #6 10-30-2004, 10:43 PM

Under full "pedal to the metal" acceleration it shifts at around 4300. Tommorow i'll take her out and look a bit closer at the RPMS granted my tach decides to work (its intermittent) and play with the modulator a bit more.

rg2098 · #7 10-31-2004, 01:32 PM

GRR i'm getting fustrated, sometimes it will shift smoothly through all the gears, most of the time it won't. I've touched every adjustment and vacuum line in this system and it responds to nothing. I'm almost at the point thinking about just taking it to the dealer and tellin them to make the da** thing shift smooth. SOMEONE HELP!!!!!!!

psfred · #8 10-31-2004, 01:35 PM

Check the condition of BOTH flex disks -- any cracks or other signs of deterioration mean you need to replace them. Thumps on downshifts at trailing throttle are almost always bad flex disks.

The hard 1-2 shift has several causes, but a bad flex disk or center mount will cause the same sensation (bangs the floor pan).

Bad tranny mount (gone soft, usually) will also bang.

Peter

Eric Eliel · #9 10-31-2004, 01:38 PM

Sounds like your modulator needs further adjustment, its currently shifting too harshly and needs to shift a bit smoother.

rg2098 · #10 10-31-2004, 01:39 PM

Flex disks appear to be new, transmission mount is somewhat acceptable, I'll replace it anyway, they're not the much if I recall.

BoostnBenz · #11 10-31-2004, 01:49 PM

I think ya'l are barking up the wrong tree. Have you checked to see what vacuum your IP bleeder valve is putting out at idle? Anything below 13" will cause a clunk on the 2-1, get down near 10" and you won't just hear it.... Ideal is 15"hg or greater vacuum. As the valves get old they loose their wide range they used to have, and the part new is quite expensive ($250+) for something that costs probably less than $5 to make.

rg2098 · #12 10-31-2004, 03:47 PM

Everything holds vacuum fine, at the modulator I get about 12" of vac, and about 2" at full throttle.

BoostnBenz · #13 10-31-2004, 04:09 PM

Well there is your problem then. Now the question is why. Things I'd check;
-Does the EGR control box on top of the camshaft cover leak vacuum?
-Is the little dampening orifice (green thing above bleeder valve) clean? How about the orifice at the main vacuum line (be careful the old ones are brittle and will snap clean off)?
-Disconnect the vacuum source from the valve and pump it up by hand, how much vacuum will it hold? It should be 15-17". How much does it let through at idle? The same 12"?
....-Would you rather, live with the problem, adjust the vacuum up at the valve with the risk of causing a 2-3 flare or soft high rpm shifting, or go find a better bleeder valve?

Edit- I presume you've already verified the black line to the tranny doesn't leak. If not take the black line off and pump it to 15-17" and wait. Does it hold the vacuum? For how long? If over 3 mins I'd call it good.

rg2098 · #14 10-31-2004, 04:53 PM

The green orifice is clean. EGR is disconnected. The main orifice allows a steady 17-20" through. No vac leaks. The valve will hold 11" after shutdown. Is this a valve adjustment problem?

rg2098 · #15 10-31-2004, 07:44 PM

Ok, The valve hold vac fine, I just played with the linkage and have 15" at idle and 5" at full throttle. Without the linkage i can get a range from 1"-16" but the linkage only allows 10 (2"-12", 4"-14" etc.) I don't think the valve range is adjustable