Transmission has caught something deadly... - PeachParts Mercedes-Benz Forum

Mersadie · #1 08-27-2015, 02:41 PM

Alright, I really need some help.

I have been driving me 1983 300D Automatic for probably 5,000 miles now since I completed a full engine rebuild. The car now shows high 28X,XXX miles, although I am sure ir is over 300,000 as I just fixed the odo recently.

At the time of the engine rebuild, I replaced fluid and filter in the transmission and made sure the modulator held vacuum. (I have not rigged a pressure gauge to set the modulating pressure yet). The car has driven great since the rebuild, transmission didn't shift flawlessly by any means, but was pretty dang good since i had chased down proper vacuum, adjusted vcv, adjusted bowden cable, etc. All was well.

About a month ago I started noticing on the highway, especially when the bike rack was on, a hesitation at cruising. As if the driveline wasn't completely locked up. I couldn't quite put a finger on it, but it just seemed in efficient, and at some times i would not be able to accelerate much at all.

Well! Last night I had been on the road for an hour, coming home from school, stopped at autozone, and then I continued on my way and it started… Keep in mind the shifting never suffered all of this was in gear. While i was cruising in 4th gear the engine seemed to be spinning a little bit higher than normal, not much, and I couldn’t really go faster than 45mph. If I pressed my foot down, a little more noise was about the only effect. Then when I got to my neighborhood there is a big hill, and it rapidly lost speed in 1st gear, ad when I downshifted into 3rd, and even 2nd, it wouldn't go faster than 20 mph but the engine was spinning out to like 3 grand. Bottom Line there was some SERIOUS slipping happening.

When I got home I checked the fluid and it was fine, maybe a tiny bit high. Later today I plan on investigating the vacuum situation and maybe taking a little fluid out. But from the smallish amount of knowledge I have about these transmissions, I believe im correct in assuming that even if all vacuum signal was lost it wouldn't slip while in gear, would just have serious issues shifting, if at all.

Sorry for writing a book, but I seriously need some help here, and I figured as much background as possible helps.
Do you guys think it sounds like torque converter failure or just overall transmission failure? Im not sure how much of a roll this could play, about 2 months ago one of the transmission cooler hardlines sprung a leak while I was driving and I suspect that I drove with very low fluid for about a mile..

I really love this car.. Its incredibly clean, so charming, and on top of all that I just completely went through the AC and IT WORKS GREAT. Thats a big thing in ATL around August.

My biggest issue here is that if the Trans is fried, I cant fathom going through the effort of swapping in another Slushbox. Seems like a perfect opportunity for a manual swap in my mind. But the combination of being a full time college student, and there not being a good donor car around right now, seems like time would be my biggest enemy.

Any thoughts from you guys would help tremendously and mean a lot.
-Matthew

Heres some eye candy(sorry for crappy cell phone pics)http://i.imgur.com/ST5XkRf.jpg
http://i.imgur.com/S1lHKeL.jpg
http://i.imgur.com/ST5XkRf.jpg

leathermang · #2 08-27-2015, 02:50 PM

Use the search for ' trans x' ....
if the cause is built up varnish in the valve body that can help...
cheap to try with no downsides if it does not work... and you have eliminated that as a potential cause...

mespe · #3 08-27-2015, 04:12 PM

If the trans fluid is a little bit high, when the trans heats up, the trans fluid starts to bubble because there's too much trans fluid in there.

This is always the hope this is the cause of the problem as it costs nothing to lower the level a little bit.

mannys9130 · #4 08-27-2015, 04:46 PM

Plan for the worst, which is swapping in a tranny. Adjust the fluid level first, then go from there. No sense troubleshooting anything until the fluid level is in spec.

300dmichael · #5 08-27-2015, 06:37 PM

When you are loosing every gear and you feel it is slipping in every gear, the only thing that every gear has in common is oil pressure..

It takes pressure to push the 2 pistons to tighten the bands..It takes pressure to engage the 2 clutch packs

Try taking down the pan and changing out the filter..If it is totally plugged, that would keep the main pump from developing the pressure needed to create the shifting..because it can't get the oil it needs

I can't believe that the pump would wear out..It is constantly in oil..It is usually destroyed by a metal chunk when the tranny is destroying itself elsewhere..

That would be my best guess..Drain the torque converter while you are under there too

SD Blue · #6 08-27-2015, 07:03 PM

From your symptoms, and what I've read here that is similar, it may be a B2 piston failure. If it is, there is a lot of information, here on the forum, about what it takes to replace it. Every write-up I've seen states that removal of the transmission isn't necessary.

A new B2 piston, and seal, is ~$150.

Here are some links that may help you sort things out:
b2piston
http://www.peachparts.com/shopforum/diy-links-parts-category/142395-transmission.html

300dmichael · #7 08-27-2015, 07:20 PM

The B 2 piston and the band that goes with it is used in 2-3 gear..The OP is saying that it is slipping in all gears..

A lack of pressure would cause a slip in all gears..And several things could cause a lack of pressure..

Try disconnecting the vacuum line that goes down to the modulator..Just give it a try and only a try..it is not a fix. With no vacuum, that is like you are telling the transmission that you are flooring the pedal and it will cause the modulator to increase the line pressure

Mxfrank · #8 08-27-2015, 07:56 PM

That only increases modulator pressure, which will firm up the shifts. But it sound like he's losing working pressure, which is a different kettle of fish. It could be a governor problem, or the seals on the clutch packs could be hemmohaging. I'd bring it to a shop and ask them to read the pressures.

SD Blue · #9 08-27-2015, 07:57 PM

Hmm....
"Catastrophic failure of transmission in forward gears."
"Partial engagement of forward gears, with slipping progressively increased as transmission is shifting from 1-2, 2-3, and 3-4"
"Reverse gear engagement normal."

I'm just providing him research material from folks with a whole lot more experience and expertise. Now if he just wants to "shotgun", that's up to him.

300dmichael · #10 08-27-2015, 08:37 PM

Take a look at this page out of the ATSG manual..
On the left of the page...

Complaint:
Transmission slips in all selector lever positions..

It could be either the mod pressure or working pressure

Mersadie · #11 08-28-2015, 12:19 AM

First off, thank you everyone so much for your quick and helpful responses.
Tonight I sucked out quite abit of fluid... like Probably 600mL. That put me a little bit below the minimum line. I also adjusted the VCV linkage so that I have 15in of vacuum at idle and 0 at full throttle.

It is cured! I took it on multiple drives and couldn't replicate the issue. So I think that eliminates the possibility of any catastrophically failed parts. and honestly, I feel like it worked better than it ever has.

Thanks again guys, now I can focus on the vibration and suspension...

uberwasser · #12 08-28-2015, 12:56 PM

At what transmission temp did you check fill level this time around? You say you're a bit below the minimum line. If that is with the transmission cold, you might still be a bit overfilled and so in the long run you'll want to further adjust the fluid level.

You'll want to check the FSM to be sure, but my memory is that pretty much all the W123 transmissions are to be filled to 30mm BELOW the minimum line when the transmission is cold.

Then I'd suggest driving it on the freeway for, oh, 30 minutes - and ideally checking trans temp with an infrared thermometer aimed at the pan with a reading close to 80*C - to make sure that at operating temp it is within the fill lines.

Then you'll know you're in good shape!

Orv · #13 08-30-2015, 04:28 PM

I've found the transmission dipstick reading is also very sensitive to slopes, on these cars, so try to have the car level when you read it. Even a typical driveway slope to make water run away from the house makes a noticeable difference.

I usually make it a road job -- stop at a gas station or whatever, leave the engine running and check the fluid level before shutting it down to fill up the tank. It's the level with the car running at temp and the torque converter filled that you really want. When you shut it down, the oil in the torque converter will start to leak back into the pan, changing the level. How much of it leaks down depends on how old the seals are, which is why there's no "official" cold level.

The annoying part is if you have to add fluid, you can't check the level afterwards because the new fluid coating the sides of the dipstick tube will smear all over the stick, obscuring the reading.

uberwasser · #14 08-30-2015, 04:45 PM

Quote:

Originally Posted by Orv

there's no "official" cold level.

There actually is. I mentioned it in the post above yours. Note the excerpt from the W123 service manual below, noting that at ambient temp (between 68*F and 86*F) there are ways to measure the fluid level (either at min mark, or 30mm below min mark, depending on trans equipped).

Orv · #15 08-30-2015, 04:51 PM

My mistake! I assumed that was a rule of thumb. I hadn't seen it documented before.