Originally Posted by goldstone

BACKGROUND:
Last month, I swapped a new German Star rebuilt tranny into my '89 420SEL. Too big a job for me to handle on my own, so I turned it over to a HIGHLY experienced Benz indy (highest references.) As long as he was going to be in the neighborhood, I provided him with new flex disks and a replacement tranny mount to install as well.

New transmission seems okay (great actually), BUT here's the problem I'm now having:

PROBLEM:
Only when the engine is fully warmed-up, and only upon take-off (in 2nd gear), there's a VERY obvious thump-thump-thump that you can both hear and feel coming from the floor above the driveshaft in the vicinity of the firewall. If I back off on the accelerator, the problem stops at once.

That's it... No other symptoms: Only when engine is completely warm; only when starting from a dead stop. Otherwise, car rides like new--no vibrations or other telltale signs of faulty driveshaft balance--whether driving in town or on the highway.

My guess is its the replaced front flex disk, and I intend to get under there this week to check it out, but wanted to ask all of you for any insights you can offer...

Thanks for any guidance here!