yet another trans question 84SD - PeachParts Mercedes-Benz Forum

dynamitejacket · #1 06-06-2007, 02:28 PM

Thanks to the forum, i brought my SD back from the grave...but theres one piece missing:

The SD runs great, but the trans goes nuts when it gets warmed up. heres its current state-

Bowden adjusted.
Vacuum 10in at idle, VCV is working properly, no vac leaks anywhere.
Fluid level is spot on, no leaks.

The car shifts nicely when cold: first and and second are firm, the third change is like butter, but as the trans warms up, fourth gear slowly disappears, then as the trans gets hot third gear gets sideways... i havent pushed it any further than that.

I plan another fluid and filter this weekend to see what gunk has broken loose since the first fluid change. I will also be looking at the trans cooler (maybe it is plugged?)

Anyone have a suggestion on what else i might look for, or does this sound like the dying gasp of an abused transmission?

jack avant · #2 06-06-2007, 03:35 PM

Have you tried unplugging the modulator? I have an 84 300sd and I am still in the adjustment stage. I unplugged the modulator and the transmission worked better.

dynamitejacket · #3 06-06-2007, 07:44 PM

thanks for the suggestion, i dont think that will help, though since the change to fourth gets softer as the trans warms up, and the first two changes are pretty firm...

one think to add, when the tranny starts acting goofy, it veritably slams into third from second-- im talking tire chirping, like a shift kit in a 3peed powerglide!

SD Blue · #4 06-06-2007, 07:58 PM

Actually that vacuum reading sounds high it should be more like 5.5-6.5 in./Hg

Here is the procedure I used to get mine shifting like it should: http://articles.mbz.org/transmission/adjust/

Brian Carlton · #5 06-06-2007, 08:17 PM

Quote:

Originally Posted by SD Blue

Actually that vacuum reading sounds high it should be more like 5.5-6.5 in./Hg

Here is the procedure I used to get mine shifting like it should: http://articles.mbz.org/transmission/adjust/

10" is typical. But, the issue isn't the vacuum to the modulator. Losing fourth gear is a more serious issue that vacuum adjustments are not going to fix.

The only hope is that the filter is partially clogged resulting in low internal clutch pressures.

dynamitejacket · #6 06-06-2007, 09:31 PM

thats sort of the direction iv'e been thinking Brian.
on a short drive today, after 4th gear disappeared, the 1st and second shifts were also getting slowly smoother. third gear never disappeared, but it seems to have some relation to the temp of the fluid. How does hot fluid make a shift get smoother? the trans is not slipping at any point- ??

Brian Carlton · #7 06-06-2007, 10:17 PM

It's quite common for cold fluid to result in harsher shifts. In fact, M/B installed a "shift softening" circuit in the later vehicles ('85 and up) to compensate for this fact. Must be related to the higher fluid viscosity and higher resulting clutch pressures.

dynamitejacket · #8 06-06-2007, 10:56 PM

do you think then, that the trans is only shifting nicely when cold because the clutches are shot, and when the fluid gets up to temp they cant deal with the lower pressure?

or could the disappearing gears and softening shift be a symptom of too hot fluid alone? (plugged cooler or failed pump)

the dex in it is brand new, but already smells a little toasty...

SD Blue · #9 06-06-2007, 11:56 PM

Here are some things I noticed with mine a few years ago. However, they were not temp related so I don't know if it will pertain to your problem.

Erratic shifting - sometimes smooth, sometimes rough - with no pattern.
4th gear seemed to disappear

I disconnected the vacuum from the modulator and shifting was still erratic. When I replaced the modulator, the plastic pin behind it was broken and I had to replace it as well. While testing the modulator, after install, I noticed that if the trip pressure was adjusted too low (using the small key that comes with it) 4th gear would disappear. Once the pressure was adjusted correctly, along with checking the vacuum system, everything has worked fine.

I don't know if any of what I ran across will help you in your diagnosis, but just to let you know some of the symptoms and what I found.

Brian Carlton · #10 06-07-2007, 12:34 AM

Quote:

Originally Posted by dynamitejacket

do you think then, that the trans is only shifting nicely when cold because the clutches are shot, and when the fluid gets up to temp they cant deal with the lower pressure?

or could the disappearing gears and softening shift be a symptom of too hot fluid alone? (plugged cooler or failed pump)

the dex in it is brand new, but already smells a little toasty...

If the fluid was too hot, you'd get a burnt smell out of it and it would lose the very light red color.

If it already smells burnt, the temperature of the fluid might be a serious issue.

dynamitejacket · #11 06-07-2007, 11:44 PM

i think ill drop the pan again this weekend and see what the filter looks like... maybe all the old junk is in it now--

part of me is hoping the trans in done for, i get all dreamy-eyed reading those manual conversion threads...

635CSi · #12 06-08-2007, 12:00 AM

That sounds very similar to what happened in my SDL, it turned out to be a solenoid in the back of the transmission. If I remember right it was about $90CND for the solenoid.

chetwesley · #13 06-08-2007, 01:59 AM

Sorry to hijack the thread with the stupid question, but... What does burnt fluid smell like? I am having some goofy temperature related transmission issues too (though the opposite -- shifts wacky when cold, perfect and stays that way once warmed up).

I have smelled my fluid. It smells like something, but I don't know what it is supposed to smell like, so I don't know if it smells burnt. Would it be obvious if it was burnt (like burnt plastic smell, for example?), or is it a smell you have to know?

Visually, it is a light red color when I wipe it on a cloth.

dynamitejacket · #14 06-08-2007, 10:05 AM

burned fluid smells, well, burned. I mean, when its normal, it smells kinda sweet, and looks like grenadine in color-- when burned it gets dark and smells awful, not at all like buttered toast!

youll know if its burned, theres just something not right about it.

635CSi, can you elaborate a little on the solenoid?

dynamitejacket · #15 06-10-2007, 12:56 PM

Drained the TC and pan today... the fluid was a nice red, and didnt smell bad... but i did notice that after a 15 minute drive, the fluid in the pad was merely warm, while what came out of the TC was freaking hot!

I know that fluid in the TC is bound to be hot, but shouldnt the pad fluid be pretty hot too after a 15 minute drive?

i have not yet dropped the pan, as i am waiting on a filter kit, so i dont know what sort of junk washed loose since the initial fluid change. I havent yet given up hope that this trans is servicable, it operates so nicely for that first 15 minutes, seems like if it had bad clutches or bands it wouldnt operate properly ever...