300td transmission failure - PeachParts Mercedes-Benz Forum

westadamsvets · #1 03-28-2011, 12:04 PM

Hi,

The transmission in my 79 300td just failed suddenly. The forward gears are not there, only reverse. I was surprised by this since the transmission always shifted perfectly. The fluid level is normal.

The question is the following, do I need an expensive rebuild from specialist, or is there chance that a "normal" mechanic can fix it at lower cost? Is there even a small chance that this could be something fixed relatively easily? Or, if the transmission is even touched, I may as well go all the way and get it rebuilt?

The engine is strong and the rest of car ok, so I don't mind putting some money into it but I don't want to go all the way with the rebuild if it's not really necessary...

04 Diesel · #2 03-28-2011, 03:02 PM

Do a search on B2, I think that is your problem.

vstech · #3 03-28-2011, 03:29 PM

call ROY and ask what the kit for the 79 transmission entails.
B2 may only be for the turbo trannys.

westadamsvets · #4 03-28-2011, 06:36 PM

I couldn't search for "B2" because it's too short of string for search engine. It's a non-turbo car.
I'm driving out tomorrow (in my other 79 300td wagon, lol) to chat with these guys:
http://www.mercedesdismantlers.com/mercedes,wholeyard.html
to see what they can tell me...
thanks for any tips.

04 Diesel · #5 03-28-2011, 07:32 PM

Quote:

Originally Posted by vstech

call ROY and ask what the kit for the 79 transmission entails.
B2 may only be for the turbo trannys.

Sorry, just trying to help. What does the turbo have to do with the B2?

Palangi · #6 03-28-2011, 09:12 PM

I believe the 79 has the old style transmission. There is a bolt with a locking nut for adjusting the reverse band. If the nut comes loose, the bolt backs out and, presto, no reverse.

There are some old threads on here describing re-tightening the reverse band adjustment.

Start by inspecting the outside of the tranny for a bolt that is working loose.

westadamsvets · #7 03-28-2011, 10:17 PM

Quote:

Originally Posted by Palangi

I believe the 79 has the old style transmission. There is a bolt with a locking nut for adjusting the reverse band. If the nut comes loose, the bolt backs out and, presto, no reverse.

Start by inspecting the outside of the tranny for a bolt that is working loose.

I lost the forward gears, the reverse is ok, - does the above still somehow apply? The loss of the forward gear was very sudden, no griding or any other unpleasantness.
That's why I suspect that the transmission may be 99 percent ok with the exception of some relatively minor part.

Skippy · #8 03-28-2011, 10:20 PM

If I were the OP, I'd take this as an excuse to do a manual conversion.

vstech · #9 03-28-2011, 10:20 PM

that is classic symptom of the B2 failure, but I'm not sure if the 79 trans has it.
that's why I recommend asking ROY (whunter, click buy parts above, and email/call him)

Palangi · #10 03-28-2011, 10:21 PM

Quote:

Originally Posted by westadamsvets

I lost the forward gears, the reverse is ok, - does the above still somehow apply? The loss of the forward gear was very sudden, no griding or any other unpleasantness.
That's why I suspect that the transmission may be 99 percent ok with the exception of some relatively minor part.

Oops, I read it too fast. The reverse band adjustment only affects reverse. Not the same as your symptoms. Sorry for he confusion.

westadamsvets · #11 03-28-2011, 11:35 PM

ok cool - i just read up on the b2 piston... will check it out!

rs899 · #12 03-29-2011, 06:24 AM

The classic B2 piston failure is not seen in the earlier, 722.1xx transmission. However my dusty copy of the ATSG manual states:

Fault- Engine races in 1st, 2nd, 3rd ( and presumably 4th) gear
Diagnosis: B2 band has failed:

1. Mechanical destruction of brake band lining
2.Brake band does not grip since fluid pressure on lift side is not released
a) Seating does not close
b) Slot ring in sleeve shift box leaks
c) Lifting sleeve is jammed

So, if the ATSG is to be believed, it seems to be a B2 band issue, for whatever reason. You may be able to check the B2 piston, and it that's not it, pull the valve body and see what's causing the issue. You won't be able to fix it without pulling it apart, though

westadamsvets · #13 03-29-2011, 03:58 PM

thanks for all the tips, dropped it off to be rebuilt...

ditzola · #14 04-06-2011, 09:23 AM

Oh, I'm definitely not expecting to buy a perfectly functional used transmissions for sale by any means. My main fear is buying a car with some crippling, very expensive impending doom that is obvious to someone with experience with these cars.