W126 300SDL Transmission Sudden Failure
 

Hello all-

Seeking some troubleshooting input for a 1987 300SDL that experienced sudden transmission failure.

I had been driving it a lot putting 1100 miles on it in a few weeks after a period of having only a handful of miles put on in several years. The transmission had been acting fine, had the typical jerkiness with low speed shifts, would shift smoothly under load. Never felt any slipping, no other issues.

Took it for an afternoon drive, experienced no problems. Put it in park at a restaurant on a slight incline, decided to reposition and put it in reverse, got nothing, no forward gears worked. No strange noises accompanied this. Had it towed home and began troubleshooting.

Checked the fluid, was a little bit low, not burnt smelling, bright red color, dipstick had small amount of black residue on it. Added fluid, no change.

Suspected linkage failure so lifted it up, checked linkage on outside of transmission, it moved smoothly with shifter movement.

No engine rpm change in any gear, no engagement when revved. In park vehicle is held secure (cogs are catching, flex discs intact, differential working), in any other shifter position vehicle rolls freely, in reverse the reverse lights go on. So the internal linkage seems to be working. Car starts in park or neutral, does not start in other positions, so NSS is working.

Loosened cooling line near radiator, had a dribble with vehicle off, good pressure when engine running out of both lines. Main pump would appear to be working, no blockage through the cooling lines anyway.

Dropped the drain pan, fluid was red and clear. Filter bottom has black film on it. One small metallic looking flake. No sludge to speak of, no other particles. No stray parts. Internal linkage that is visible moves cleanly with shifter movement.

I am running out of additional ideas to test short of dropping the transmission for an inspection and rebuild. The symptoms (sudden failure with no issues prior, not moving) has me thinking it is not a catastrophic failure, but I’m not sure what else it could be. So far the only thing I can find wrong is the park positions lockout is broken, you can move the shifter without applying the brakes. Where is this device located? If the linkage is moving to the right position, could this cause has of clutch plate engagement?

Your B2 piston may be hung up? Fixable with trany in vechile.

Thanks for the reply!

With B2 piston failure the reverse gear normally works right? That is definitely a likely candidate and the W126 is the one good chassis to work on this problem.

I attached a photo looking up towards the B2 piston and believe I can see the end of it next to the brown piece. Does not necessarily mean that it is functioning, just is in place.

Update:

Tried a few last things on the transmission before scheduling it for a rebuild.

Refilled fluid to proper level, lifted all four wheels up and checked for engagement with no resistance. All gear positions resulted in forward movement of driveshaft/wheels. In park the pawl stops it from spinning but you can still see pressure on it as you rev it up. In reverse/neutral/drive/low, the wheels are stopped at idle, when you rev it all those positions move wheels slowly forward.

One shop suspects something broke inside during the attempted shift to reverse, now it is bound up in the forward position and not actually shifting. Another rebuild shop thinks it may be a pump failure; I know there is pressure in the cooling lines, don't have a way to check operating pressure, he said pump might not be getting to high enough pressure.

My problem seems similar to an old thread by Rollguy, not sure what the resolution was there.
http://www.peachparts.com/shopforum/diesel-discussion/301324-something-broken-no-power-engine-wheels-2.html

Someone will chime in with a rebuilder. I think it is Sun Valley and am remembering they have been helpful and have an outstanding reputation. Of course, action plan depends on how long you want to keep the car. $1500 is what I remember for 722 transmissions with complete rebuild and warranty by people who know.