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Ian White 09-29-2009 04:55 PM

w123 transmission in a w126?
I want to use my w123 parts car transmission in my w126 300sd, bu my sd has and electronic speedo, whereas the 300d has a mechanical. Is there any way to do this while still having a speedometer? THANKS

bsmuwk 09-29-2009 05:57 PM

Hopefully Paul will chime in here with what he did.

I sold him a good W123 transmission out of an 85 300D. He put it into his 300SD by swapping a few things here and there. Apparently, it's wonderful. :vbac47679

Graplr 09-29-2009 06:07 PM

You will need to swap the tails. Apparently it is a special Mercedes tool. Some on the forum have it and will loan it to you. Someone offered it to me when I was looking to put a 116 tranny into my 126 but ended up just getting a 126 tranny.

Ian White 09-29-2009 09:56 PM

Well, where do you get this special tool?

pawoSD 09-30-2009 12:02 AM

I did this project successfully as was mentioned above. :)

The "special tool" is only needed for the older (82 and earlier) transmissions as they have a 4-tab locknut on the output flange, and the newer transmissions use a 12 point 30mm socket, easier/cheaper to obtain. Since your SD is an '81 it will need the 4-tab style socket to remove the old nut. The replacement nut will be a 30mm 12point. Also needed will be a rear housing gasket, and a razor blade to get all remnants of the old gasket off the surfaces. The "special tool" can be borrowed possibly, or you can make getting metal pipe in the correct size of the nut, then cutting out the sections with an angle grinder to obtain an end with the 4 tabs that fit in the slots...then a pipe wrench can be used on the pipe to loosen the nut. I found it easier to find/borrow a proper socket....

After the rear output flanges on both units have been removed, the cover is unbolted and removed, then the internal parking prawl/speedo transmitter piece is swapped, with care to keep the original spacers intact and in the right places. The actual swap is pretty easy. Then just set the new gasket in place on the tabs, and tighten all the bolts in an alternating fashion to about 15-20nm.

So far I've driven a little over 2,200 miles with my converted 300D trans and it works great!

A search of the forum will reveal more in depth threads and explanations of the whole process.

DeliveryValve 09-30-2009 01:03 AM

Yes do a search.

This is what your swapping out.

turbobenz 09-30-2009 08:02 AM

The old nut is not hard to remove. Usually a chisel and a hammer on the edges can do it. The 12 point nut is horrible though. I tried looking everywhere for a an ultra deep 27mm star socket but eventually welded my own together. The dealer sells them though but their super expensive. Replace the rear seal while your there too and put a thin coat of RTV grey on both sides of your new gasket (and youll have to figure out which gasket to use, idk if you can use the 300d one or the sd one or if you can use either one. Just look at the edges and passages.

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