Slipping Transmission/Vacuum related? - PeachParts Mercedes-Benz Forum

84greaser · #1 12-29-2005, 09:42 PM

I have a 1984 300SD. It has shifted hard since I have owned it.

About a week ago first gear started slipping when I started out, and it sometimes shifts into second a little funny (flare?) but once in second it goes fine from then on.

The tranny was rebuilt under the previous owner about 50k mi ago. I changed the fluid (including draining the torque converter) and filter about 20K mi ago. I dropped the pan yesterday and found nothing but healthy red Dextron III. I used a magnet and sloshed it around in the drained fluid and didn't find any fragments or shavings.

I suspected it could possibly be vacuum related. I checked the vacuum going to the transmission, and there is none while it is idling in P or D. Is there supposed to be? A friend who admits he knows nothing about Mercedes suggested there should be a blast of vacuum when I shift from P to D, but I don't get any. My vacuum pump provides good vac up to the booster. I haven't checked further on the transmission control lines.

I found a thread with a similar tranny problem on a gas car, and one reply suggested the problem could be vacuum related, or that the Bowden cable may need an adjustment. Is there a Bowden cable on a diesel?

Lots of other threads suggest a possible cause of the B2 piston. Do I need to drop the tranny to get at it? Is it difficult to replace for an average shade-tree mechanic that has never attempted to work on an auto trans?

Any help would be greatly appreciated!

Thanks!

Diesel Giant · #2 12-29-2005, 09:47 PM

You probably have a vacuum problem. Check to be sure you are getting vacuum at the tranny modulator, then check to make sure the tranny modulator is able to hold a vacuum, then check to make sure your vacuum levers are not worn, then check the tranny vacuum switch over on top of the injector pump.

Look at the vacuum pictorial to see how to check vacuum.

http://dieselgiant.com/mercedesvacuumtroubleshooting.htm

84greaser · #3 12-29-2005, 09:52 PM

I checked the vacuum at the modulator while in park and in drive (idling w/brake applied), and I don't get any. When should I get vacuum there? All the time, or just when it is supposed to shift?

Thanks for replying!

Diesel Giant · #4 12-29-2005, 10:00 PM

You should have vacuum there all the time, however the vacuum switchover valve "meters" the vacuum in an effort to mimick a gasoline vacuum curve. It is like a vacuum rehostat. You need to hook a vacuum gauge in line after the vacuum switchover valve and see if it has a smoothe vacuum curve. If no vacuum then the switchover valve may be bad.

sailor15015 · #5 12-29-2005, 10:02 PM

You probably did this but its not mentioned in your post. Are you sure you have the correct fluid level, checked as the manual said to? Mine started slipping one day and I found out I had a very tiny leak around the gasket. I added half a quart and I was good to go.

84greaser · #6 12-29-2005, 10:07 PM

Sorry, I forgot to add that to my first post. YES, I did check the fluid level, and it was good. It was actually a little on the high side (about a 1/4 inch above the normal range)--woops. I don't know if that had contributed to my troubles.

84greaser · #7 12-29-2005, 10:12 PM

Thanks Diesel Giant!

I checked out your site. Very helpful!

I'll check out the parts in the the advice you posted. But it already sounds like you hit the nail on the head. Like I said, I didn't have any vacuum at the tranny at all.

A million thanks

RAYMOND485 · #8 12-29-2005, 10:58 PM

1984 300d Turbo
Check Vacuum With A Hand Vacuum Pump At The Line On Top Of The
Injection Pump Pull The Green Dash Pot Off And Check If Line At Trans Modulator Is Pull Off May Need New 2 In Line At Modulator.
If Engine On Need 15-19 In Vacuum At Dash Pot Green You Will Find A
Leak.

JimmyL · #9 12-29-2005, 11:13 PM

Quote:

Originally Posted by 84greaser

Thanks Diesel Giant!

I checked out your site. Very helpful!

I'll check out the parts in the the advice you posted. But it already sounds like you hit the nail on the head. Like I said, I didn't have any vacuum at the tranny at all.

A million thanks

It sounds like yall are on the something, but, doesn't the vacuum help the transmission shift, and no vacuum would make the shifting hard, which is what he used to have?

84greaser · #10 01-07-2006, 03:44 PM

I have learned a lot about vacuum and control over the past two weeks, but I still haven't fixed the problem of slipping while starting out in first gear. There is sometimes some flaring going from 1st to 2nd.

Here is what I have determined since the last post:

1) I have checked the vacuum and found that I had no vacuum after the switch-over valve.

2) I checked the transmission modulator and found that it at least holds vacuum. (The transmission was rebuilt under the previous owner about 55K miles ago) I can not guarantee that it is adjusted properly or that it works, but I don't think it is contributing to my current problem of slipping in first gear or shifting out of first gear.

3) Using my MightyVac hand vacuum pump and a vacuum gage I found that both the switch-over valve and the 3/2 transmission control valves were bad. I replaced the switch-over valve with a used one from the local junk yard. I also replaced the 3/2 transmission control valves, but they too were bad. There is a fix to replace the levers for these valves as DieselGiant pointed out, but I don't think it would help my valves because they don't hold vacuum anyway. I temporarily bypassed these valves by unplugging the lines that go to these valves and plugging the T's to which they were connected with golf tees. I NOW HAVE VACUUM TO THE TRANSMISSION MODULATOR!!.

4) My understanding is that the modulator and the vacuum that goes to the modulator control how hard the transmission shifts, not when it shifts (that is the bowden cable's job)

5) I adjusted the bowden cable (it was a little loose), but I still have the same trouble. In fact, now I noticed some minor flaring from 2nd to third.

6) I dropped the transmission pan and found no sign of metal shavings or broken pieces. The fluid, which I had changed within the last 18 months still looked red, but was slightly dirty. I replaced the filter and gasket, but still have the same trouble with slipping.

7) I checked the fuse that controls the transmission (and many other things) but it is okay.

I have NOT tried TransX, and I have looked into the possibility of it being the B2 piston. I found a thread from February '04 that gives me some clues that mine is bad. It also talks about how to fix the problem without dropping the transmission, but the links that are supposed to show pictures no longer work.

Can anyone help me figure out if it is the B2 Piston or something else that I might be missing?

What does a B2 piston look like? I know it is on the passenger side of the transmission, but there are, I think, 3 round gadgets there.

The new version of the B2 piston seems to be bigger that the old one because the thread I read talked about having to cut into the sound insulation for clearance. Although my transmission was rebuilt, I don't know if the B2 piston was replaced or not.

Thanks in advance for any help!

84greaser · #11 03-26-2007, 10:58 PM

Just thought I would do a quick update. It WAS the B2 piston (Brake Band). Fixed that and the car shifts liked it used to. YEAH!

cjl1973 · #12 03-26-2007, 11:11 PM

I'm pretty sure that I need to replace my B2 piston as well. The other day I walked out to go to work, and noticed a large streak of transmission fluid, running away from the car. I crawled as far as I could under to see where it sis coming from and it looks like it is coming from somewhere around the very back edge of the transmission pan. I know that the B2 piston is above the pan, but I really can't see past the exhaust pipe. Does this sound about right?