'84 300SD Transmission Issue (1=>2) - PeachParts Mercedes-Benz Forum

dafuzball · #1 11-16-2012, 06:38 PM

Long time lurker, first time poster!

I know there are TONS of posts on this and other forums on transmission topics, and honestly, I feel like I've read through all of them, but to no avail.

I've got a 1984 300SD (722.303 transmission) with ~170k miles. I just bought this car (in near perfect body shape), but it came with one issue, the shift from 1=>2. If I give it any more than, say, 1/4 throttle, the transmission will get past its shift point and then flare (I believe I'm using this terminology correctly) into high RPMs (basically as high as I will let it go). As soon as I back off the throttle some, it will shift into 2nd; and the rest of the shifts are just beautiful. I have been conscious of this, and by keeping it at about 1/4 throttle until I get into 2nd, I have no problem (except that the shift is still very stiff); and I can then give it through full throttle.

Since I wasn't sure of the maintenance records, I have been going through and doing most everything that 'may' need done. I changed A/T fluid and filter (fluid did not have any noticeable wear material, emptied both trans and T/C), engine oil/filter, 2 oil filters and under tank screen, ran a diesel purge, replaced the wastegate hose (the old one was barely hanging on), and cleaned out the banjo bolt from the switchover valve (both to the intake and to the ALDA). I also went ahead and replaced the modulator, as it was original and even if it had not failed, I read that it was a good PM item. After I replaced that, I adjusted it to the recommended level with no vacuum (I don't have it in front of me, but I think it was 48PSI), and it maintained that amount while giving it throttle in park. Of note is that there is a small amount of fluid leaking from under the T/C, but I'm assuming a seal leak shouldn't be a problem if the trans is adjusted to the right pressure?

Here are the transmission specific checks I've tried:
-After replacing the modulator and adjusting pressure, I drove the car with the vacuum unplugged to rule out vacuum problems. Same issue.
-I reviewed several pictures of the adjustment on top of the valve cover for the bowden cable, and mine looks to be correct with a slight amount of slack and a very tiny space between it and the throttle linkage. I adjusted it a small amount both directions to see if that made a difference, and it didn't seem to.
-I took some pictures (couldn't see otherwise) of the bushings connecting to the selector rod, and those appear to be in tact.
-I unhooked the kickdown switch from under the pedal, and that did not do anything (however, I have not verified that there is movement in the solenoid when it IS plugged in)
-I took a spin around the block with the intake banjo hooked directly to the ALDA (bypassing the switchover valve) to assure that my switchover valve wasn't preventing pressure from getting to the ALDA. No improvement.

Here are my next steps:
-Adjust valves
-Check turbo pressure on road test

And if neither of these are the problem, my best theory thus far is the B2 piston is sticking / has some wear and needs to be replaced.

I know there is a lot here, but hopefully one of you experts out there has read through this and has some insight for me, things to try, encouragement/disagreement with my theory. Thanks in advance for the help.

kerry · #2 11-16-2012, 07:04 PM

I believe there are shift kits to help alleviate this problem. Search on Superior shift kits. (I think that's their name).

dafuzball · #3 11-16-2012, 07:07 PM

Kerry,

Thanks for the quick reply! So you believe that the springs in the valve body have relaxed over time, and thats what may be causing this? BTW, I love that you've got a motorhome with an old 3208; thats my employer

dafuzball · #4 11-16-2012, 07:51 PM

lets be clear here.. its not just a HARD shift that I'm trying to address. it won't go into 2nd unless its 1/4 throttle or less. I was under the understanding that the shift kits / spring kits were mostly used to soften shifts?

kerry · #5 11-17-2012, 12:34 AM

I'm pretty sure the kits do the reverse--stop flaring.

Stretch · #6 11-17-2012, 04:33 AM

So long as the first gear is engaging and you don't have any trouble with 3rd gear or going from 3rd to 4th then I see little reason to suspect the B2 piston.

It should "stay on" until 4th =>

Now you might suspect the B1 piston / system.

The superior shift kit might help but I would first check out the Bowden cable - exactly as described in the FSM - and check the modulating pressure at the prescribed road speeds first.

If you need some pictures showing the positions of the springs and valves in the 722.303 valve body then I've posted some here =>

http://www.peachparts.com/shopforum/diesel-discussion/323259-722-303-bits-photo-shoot.html

dafuzball · #7 11-17-2012, 10:10 AM

Thanks Army! I actually have your breakdown bookmarked already, it was going to be my reference for any internal work I have to do if it comes down to it. Thanks for the advice and for such a detailed write up!

BillGrissom · #8 11-17-2012, 10:45 AM

I have yet to fool with my 300D transmissions, but have rebuilt 2 Chrysler trannys. A flare between shifts, usually seen from 1 to 2, is a classic sign of worn clutch plates. I had that in my Chrysler A-413 and indeed some clutch plates were worn to bare steel. The inner teeth were almost worn away too.

Shift kits I have installed (Mopars) give faster, firmer shifts. That is desireable to avoid wear and give better performance. Unfortunately, consumers think a soft, smooth shift means a good tranny, so manufacturers design them slow and slippy. In most shift kits, you take out accumulator springs, some valve body balls, and even drill out the wall in places. Don't know about Mercedes, but I expect the same idea.

The main complaint with 300D transmissions is a hard kick from 1 to 2. I understand that is usually a problem in the vacuum controls around the top of the engine bay. My 1985 CA vacuum stuff is such a complicated rat's nest that I haven't dug much into it. I just let up the throttle as the 2nd shift approaches. Actually, I haven't driven it for 3 years since the engine failed at 330K, but have another engine in now and should be back on the road soon.

BillGrissom · #9 11-24-2012, 02:01 AM

I happened to run across exactly what sounds like your problem. Read:

The (infamous) B2 Piston

and

B2 Piston Repair

It sounds like the B2 piston often breaks around 100,000+ miles, which causes flares from 1st to 2nd. You can fix it with the transmission in the car, but need to lower it a bit in a W123. I will be on the lookout for this in my cars.

dafuzball · #10 04-16-2013, 02:35 PM

just wanted to close the loop here. i adjusted the valves and road tested the turbo pressure. neither made any difference in the slippage during 1=>2 shift. i will note that although i hadnt noticed it until it was fixed, it WAS taking a while for it to shift from R to 1 as well. i left the trans in place, dropped the pan, and pulled out the B2 piston. sure enough, it came out as several pieces. for good measure, i also replaced the K1 spring while i was in there, refilled all, and the trans now shifts like new. always great to have $150 in parts fix what would be >$1000 component swap!

Stretch · #11 04-16-2013, 02:59 PM

Great result - thanks for letting us know (I've just had a learning moment - so thanks for that too!)