300cd trans problem - PeachParts Mercedes-Benz Forum

BenzTurbo · #1 02-16-2013, 10:29 PM

the trans in my 1983 300cd has been acting up for a while. when the car is cold it "flares" in the 2-3 and 3-4 shift. 2-3 always seems worse and the 2-1 downshift is hard. once everything is up to temp the flaring goes away unless you're very easy on the throttle. when you accelerate hard, there is no flaring. My vacuum system does not have any egr crap and i have tried both disconnecting the vacuum to my trans (hoping for all shifts to be very firm) and swapping my vcv calve for a known- working one and no luck... with no vac, 2-3 and 3-4 still flare but once the gear "grabs", it is harsh. I have turned up the adjustment key on my trans modulator and it seemed to help a bit. when i really get on it, i get second gear rubber lol

. is my modulator bad? do i need those spring kits i have heard about? i don't think my trans is bad because once it is in gear it does not slip at all. thank you!

Stretch · #2 02-17-2013, 03:07 AM

Ideally you should measure the vacuum pressure and the hydraulic pressures to make sure the transmission is adjusted correctly. Do you need some links?

BenzTurbo · #3 02-17-2013, 01:36 PM

I have checked the vacuum with a mighty vac and all the readings are spot on. i have not checked the hydraulic pressures though... i guess i need to get that kit from mercedes source or make my own. if those pressures are not correct, what would that mean?

Stretch · #4 02-17-2013, 02:33 PM

Well if the modulator was adjusted with the aid of a vacuum gauge it doesn't sound like the modulator isn't working - you should have noticed a problem there.

If you are really considering buying some hydraulic gauges shop around for a good deal.

When did you last change the transmission fluid / change the filter / check the level?

unkl300d · #5 02-17-2013, 02:53 PM

Did you try replacing the filter and trans fluid? Maybe its just debri in the filter.
Or try some trans 'cleaner' stuff to clean up the valves before you replace the fluid/filter.

valves could be sticking.

BenzTurbo · #6 02-17-2013, 08:06 PM

the modulator is getting proper vacuum but i know is is out of adjustment. i did not adjust the vcv (the vacuum bleeder on the IP) because when i had the mighty vac all the readings were fine. What are the symptoms of a bad modulator? Both the fluid and filter were changed about 6 months ago but i have not run any trans cleaner. has anyone had success with that?

jimandsuzy · #7 02-17-2013, 10:26 PM

read about the "B2" valve. That may be informative.

BenzTurbo · #8 02-17-2013, 11:07 PM

from what i have read, with a bad B2 valve you won't have any forward- gears. My car drives and shifts but it shifts too hard in some gears and too soft in others depending on what kind of mood its in.

sleepstar · #9 02-18-2013, 12:28 AM

did you apply vacuum directly to the modulator to see if it held vacuum?

BenzTurbo · #10 02-18-2013, 12:35 AM

no i haven't. i will check that tomorrow. good question!

cho · #11 02-18-2013, 03:44 AM

Quote:

Originally Posted by BenzTurbo

My car drives and shifts but it shifts too hard in some gears and too soft in others depending on what kind of mood its in.

this is strange,
when I disconnect vacuum to the trans it bangs
in all gears.

.

jimandsuzy · #12 02-18-2013, 01:59 PM

Quote:

Originally Posted by BenzTurbo

from what i have read, with a bad B2 valve you won't have any forward- gears. My car drives and shifts but it shifts too hard in some gears and too soft in others depending on what kind of mood its in.

OK, maybe I meant K1 and K2. I think they have to do with soft shifts.

Brian Carlton · #13 02-18-2013, 02:30 PM

Quote:

Originally Posted by BenzTurbo

i have tried both disconnecting the vacuum to my trans (hoping for all shifts to be very firm) .............and no luck... with no vac, 2-3 and 3-4 still flare............. I have turned up the adjustment key on my trans modulator and it seemed to help a bit.

You have done everything that can be done with the vacuum system. Without vacuum, the problem remains, which typically means the transmission is at the end of its capability.

The only possible option is to further increase the modulator pressure so that it helps more than "a bit".

You must get to a condition where it does not flare with no vacuum applied. There is no assurance that increasing the modulator pressure will accomplish this with a severely worn unit.

BenzTurbo · #14 02-18-2013, 03:41 PM

im sure it's not the vacuum system at this point. can the bowden cable effect shift hardness? i adjusted all my linkage so the "arm" on my IP hits the stop when my foot is to the floor and i noticed that at WOT it would only shift at 3000 rpm, so i backed the plastic bolt out a bit until it shift around 4200 rpm WOT. when your easy on it, it will shift at 1400 rpm.

Brian Carlton · #15 02-18-2013, 04:22 PM

Quote:

Originally Posted by BenzTurbo

can the bowden cable effect shift hardness?

It has no direct affect on the firmness of the shifts. However, if the cable is tightened and the shifts are delayed, you might get some benefit from the higher rpm at the point of the shift. It "might" cause a reduction in the 2-3 and 3-4 flares, but not a certainty.