86' SDL, Gettin Angry at the Tranny! - PeachParts Mercedes-Benz Forum

Putty · #1 07-29-2008, 08:18 AM

I love this car and my girlfriend was slightly shocked to hear me say I was thinking of selling the car.

The cars stupid shifting problem is driving me CRAZY!!!

When cold, a good overnight cold...the tranny shifts smoothly...when up at operating temps, she starts shifting rougher and more erratic.

The shifts between 2 and 3 slip and sometimes and it wont downshift for anything sometimes.

Im almost positive that its a vacuum problem but I dont know where to start.

Ive been told that this particular tranny is referred to as the "neck snapper" because of the nature of the 1 to 2 shift...dont know if thats true or not.

Anyway, some tips would really help me keep this car from the auto lot...I would really appreciate it.

bbboomer · #2 07-29-2008, 08:32 AM

Do a search on vacuum testing. There is an excellent article on testing vacuum. You may find all you need there. If not do another search for trans shifting problems you will likely find some help.

mbdoc · #3 07-29-2008, 08:39 AM

That car has a somewhat complex way of controlling vacuum to the modulator.

I always start by connecting a vacuum Gage to the line that goes to the modulator & driving the car while watching the vacuum readings.

The controls TRY to mimic what happens on a conventional gas engine. The vacuum should be HIGH at idle & on decel. Low at full throttle & in-between at partial throttle.

herring · #4 07-29-2008, 08:40 AM

I've been having loads o' fun with my trans. First thing you need to do is get a mityvac if you think it's a vacuum problem. They sell them for $30 or so at Harbor Freight.
I'm surprised to hear the trans handles differently depending on whether it's warm or cold (and worse warm than cold) but I'm a newbie. The thread starts with "it's critical", I just posted in it yesterday.

Sterling30 · #5 07-29-2008, 11:34 AM

I've had the best luck with Trans-Medic.., might be worth a try.

herring · #6 07-30-2008, 01:53 PM

Quote:

Originally Posted by Sterling30

I've had the best luck with Trans-Medic.., might be worth a try.

Are you talking about this stuff in a bottle by GUNK:
http://automotive.hardwarestore.com/89-548-transmission-fluid/trans-medic--666379.aspx

TX76513 · #7 07-30-2008, 01:57 PM

Quote:

Originally Posted by Sterling30

I've had the best luck with Trans-Medic.., might be worth a try.

It's VACUUM related don't put any crap in your tranny

herring · #8 07-30-2008, 02:04 PM

Quote:

Originally Posted by TX76513

It's VACUUM related don't put any crap in your tranny

It sounds odd to me that you'd have a vacuum-related problem that only occurs once the tranny warms up.

Brian Carlton · #9 07-30-2008, 02:08 PM

Quote:

Originally Posted by Putty

...when up at operating temps, she starts shifting rougher and more erratic.

The shifts between 2 and 3 slip and sometimes

You'll need to define the problem with a more exacting description. "Rougher and more erratic" could mean just about anything.

The vacuum system regulates the speed of the shift. A slower shift, created by more vacuum, results in a "smooth" shift.........when carried to the extreme, it's almost imperceptible. However, the presence of high vacuum during high power situations will result in a "flare".........whereby the engine has a false neutral between shifts and it runs away briefly. It's mandatory to keep the vacuum level low when the engine power is high.

The opposite scenario is also true. During low power situations, a low vacuum condition will cause the transmission to shift very quickly. The symptom to the driver is a "bang" or a "clunk"..........a very hard shift. The vacuum level must be increased during low power operation to avoid this situation.

The most important data necessary to determine your issue is the vacuum to the transmission during various operating modes. Nobody can assist you without this information...........unless you want to take the advice of dumping a useless additive into it..........

TX76513 · #10 07-30-2008, 02:37 PM

Quote:

Originally Posted by herring

It sounds odd to me that you'd have a vacuum-related problem that only occurs once the tranny warms up.

Very common - heat up things expand (and leak). Old EGR Valves are vacuum time bombs.

vstech · #11 07-30-2008, 03:00 PM

oh, you need to get rid of that car... bring it down here to me. I will get it off your hands no problem...

pawoSD · #12 07-30-2008, 03:12 PM

Quote:

Originally Posted by vstech

oh, you need to get rid of that car... bring it down here to me. I will get it off your hands no problem...

Yeah same here.

When was the last time it had its fluid and filter replaced too? No one has mentioned that but I've noticed that shifting in general is much improved with good fluid and a new filter.....

I do agree its probably a vac issue in this case though.

TX76513 · #13 07-30-2008, 03:14 PM

Quote:

Originally Posted by vstech

oh, you need to get rid of that car... bring it down here to me. I will get it off your hands no problem...

TN cars have to go to either MS or AL. NC gets VA and DE cars

pawoSD · #14 07-30-2008, 03:18 PM

Quote:

Originally Posted by TX76513

TN cars have to go to either MS or AL. NC gets VA and DE cars

And michigan gets PA cars.

daw_two · #15 07-30-2008, 05:59 PM

Quote:

Originally Posted by pawoSD

Yeah same here.

When was the last time it had its fluid and filter replaced too? No one has mentioned that but I've noticed that shifting in general is much improved with good fluid and a new filter.....

I do agree its probably a vac issue in this case though.

I already sent him a PM --- I could ship him "baby polar bear" and then he could drive a 300SDL that does not have shifting issues.