Is a W123 300D automatic directly compatible with a W126 300SD?
 

I'm thinking that the answer is no (electric speedometer in the W126 vs. the mechanical one in the W123), but could I swap over the speedometer parts from the 300SD transmission into the 300D transmission and use it in the 300SD?

Oh, and what about the cruise control? Could I just leave that disconnected for a while?

Is a W123 300D automatic directly compatible with a W126 300SD?
 

Swap tail shafts

Does that deal with the cruise control issue as well?

There is enough difference that I don't find myself using any parts from a W123 except for OM617 related parts on my 300SD. The cruise control seems to be the same. The computer is different, though. Same actuator and same switch. The wiring seems to be the same, too. Perhaps there is a sensor on the W123 as well as the mechanical speedo. If the differential ratio is different, the computer will be different. I believe the computers are calibrated for a certain gear ratio and sensor input. I have yet to meet an actuator unit that did not work with a W126 as well as a W123.

My 300d has a sd tran in it and could see a way to swap the reader

I think I'm correct in saying that the "swap the tail ends" myth has been busted by the likes of mach4, rollguy and deliveryvalve.

Have a look for mach4's diesel SL (107) conversion thread - I think there's evidence somewhere in that mega thread.

x2.

I was about to get a trans from another member to go 126 -> 123 and decided to run it by my local indy Mercedes shop first.

The owner showed me the exploded parts diagrams for both. No way in hades I can see it working, from difference in the shape/footprint of the tail shaft housings alone. I was looking at the swap for a 1980 300td.

nope, not TOTALLY busted... a 123turbo auto trans will work in a 126 if the tail shaft and sender ring is swapped. however, it will NOT work the other way around.

this is assuming both trans are the 722.3xx the 2xx and 4xx will not fit parts to the 3xx or each other.

the 123 mechanical sender fits on the 126 output shaft, but there is no provisions for oiling the parts... so only the electronic sender can be swapped...



your 1980 722.2xx trans is totally different from the .3xx in the 126's...

So in the context of this thread you are saying yes then?

You could use a 722.315 - remove the tail cone and the mechanical speedometer part and put the tail cone part from the 722.303 in its place.

That's exactly what I've done on my 617->107 swap, although I used the tail cone from the 722.3 out of the 380SL. I also swapped the yoke in order to keep the SL's larger flex disks and driveshaft. It works perfectly.

Well it looks like I'll have to read through that thread again!

Confused - that's me...

I went back and checked the various threads and i don't think I ever detailed that part. When I replaced the 722.4 with a 722.3 I took the tail cone from the SL's 722.3 and swapped it to the 722.3 of the 300D. This is what gave me the electric speedo and the option to keep the larger yoke.

I considered this fixing a problem not part of the swap. Guess I should have documented it in the thread.

Do you mean the Bowden setup or something else? I was doing a complete linkage swap to use the Bowden setup on the earlier car.

Actually my '80 has a vacuum-control-only trans -- .117>? if I recall correctly.

When I saw the parts diagrams it was apparent how completely different the shape of the housing is in the tailshaft area. I could see a difference by looking at the pic of the trans I was supposed to get, but for some reason I still thought the other tailshaft housing would bolt up....

Hey that's fine - at least I'm not loosing my marbles faster than I thought I was!