Dead Tranny? What to Check? (1981 300D)

I’m wondering if my automatic transmission in my ’81 300D W123 is completely dead or just needs some good diagnostics. I’m asking for help on what to check to conclude what needs to be replaced.
When accelerating from a complete stop I heard a loud bang coming from under me near the transmission. I coasted to the side of the road to find that every gear acted like it was in neutral, except for park which acted like it should. There were no grinding sounds, no slippage or flaring, or any other symptoms leading up to the incident (other than the typical hard shifting when engine was cold), just a load bang and then no torque getting to the drive shaft. I pulled the pan off the tranny and the fluid color looked good, no burnt smell, and only marginal metal flakes in the bottom of the pan, no more than I would have expected for the age of the transmission. There was plenty of fluid and it was changed according to recommendations.

Is the tranny dead or might it hopefully be something less severe? What steps should I take to narrow down the possibilities?

Forums

Transmission / Suspension Specific Aid

Chassis Model Number

W123

Not likely a simple fix

If there is not rotational vibration inside the transmission it might mean the pump shaft sheared, or the cogs that connect to the torque converter broke off. If it were a broken flex plate at the flywheel there would be rattling and clanging. If there is no movement at the out put shaft to the drive line then you may have to pull the transmission for further diagnosis. I have seen this happen before and it is usually fatal. Sorry....

Used tranny question

I may have found a used tranny. Two questions:
1) How do I match it up so I know it will fit. I have a non-turbo 1981 300D. The match was pulled from a 1981 300D. Need I worry about any oddities?

2) Any way to bench test a used tranny?

Thanks

1980 or 1981 non-turbo 300D only

As long as you are sure it is a non-turbo you will be ok. There is no way to bench test. If fluid smells burnt or you find metal particles in the pan don't use it. Be sure and refer to my Transmission Tuning Manual for proper adjustment procedures when you install it.

'78 300D into an '81??

Would a '78 300D non-turbo work in an '81 300D?

Do I need a kickdown?

I acquired a 722.118 automatic transmission from a '78 300D non-turbo to replace my 722.118 in my '81 300D. Alas, the '78 tranny has a manual kickdown whereas the '81 uses an electronic kickdown switch. Two questions:
1) Is it possible to use the '78 tranny and just not have a kickdown, instead just manually shifting if needed? Or maybe I can convert eh manual kickdown to an electronic kickdown?
2) If so, then can I install the vacuum control valve that controls shift points from my '81 into the '78 that has the port for it but is plugged with a bolt?

Thanks for any help.
VM

1980 or 1981 non-turbo 300D only

I think you are beginning to understand why I said in the above post to "find a 1980 or 1981 only." Unless you have a very thorough understanding of how each transmission works and what linkages will need to be changed I do not recommend this swap. It could be very difficult to get it to shift right.