So... I've had this '83 300d turbo w123 for about a year and have put about 50k miles on it commuting. (90 miles one way, over a 4000 foot high mountain.) I drive it fairly hard and do some towing on that route. I have a truck for the heavy stuff but really appreciate the mileage of the Mercedes. (2-3 mpg drop for a 1000-1500 pound trailer.) I recently tried to tow closer to 2500 pounds and ended up letting a little smoke out of the transmission. I removed the load before it got very bad but I did smoke it enough to smell/see a bit of smoke (not out of the tailpipe)

My first question: How do these transmissions fail? Is it likely to be sudden or can I run it for a while as symptoms increase? (Drove to work ok today) basically I want to know if it's likely to leave me stranded or if I'll be able to limp it when it finally goes. I'm not worried about running the trans into the ground, I just don't want to have to pay for a tow truck or rent a car to go get my truck and trailer.

Secondly: I am planning to try to put a M5OD-R2 light truck transmission (available in junkyards) in it with an adapter kit from Mercedes Diesel 4x4.com (no affiliation). Has anyone on this forum ever tried to use one of their kits in a Mercedes chasis? The vendor told me that someone had once asked about it but they have no info on such a swap. (They put om617 engines in jeeps mostly) I've browsed this and other forums, but never seen anything posted about this type of swap.

Any info would be helpful.

Thanks