Tailshaft (Transmission) Swap - PeachParts Mercedes-Benz Forum

Graplr · #1 09-05-2008, 12:07 PM

I am putting a 722.120 transmission (from a 1980 300SD W116) into my 1984 300SD (W126) that originally had a 722.303 transmission.

I know I can it to bolt in (as it is already bolted to the engine that is going in).

My biggest concern right now is getting the speedometer to work. Since the 120 is mechanical and the 303 is electrical I have two options.
1-Change the speedometer out
2-Change the tailshaft out (as suggested to me by vstech in another thread)
3-Purchase an add on unit that converts the mechanical signal to an electrical one.

I did some searching on here and found a couple posts about people stating that they swapped the tailshafts of an electrical and mechanical tranny with sucess but no real discussions on the ins and outs of it.

I already have the 120 out and in my garage and am hoping to get the 303 out and in my garage today. So I should get started on the swap tonight.

Anyone have any insight/pointers? Do I need any gaskets or such?
Thanks!
-Ryan

DeliveryValve · #2 09-05-2008, 01:38 PM

Yeah I like to find out too. The closest I've seen is from DieselGiant's website. But it was just swapping a damage part with a good one. So no real conversion.

Here are pictures from his site:

When you do it, you should make a writeup and pictorial.

Graplr · #3 09-06-2008, 02:21 AM

Well I got my engine and transmission pulled out of the 84 tonight. I set the two engines and trannys next to each other. I really don't see how I would get a tailshaft swap to work.

If you look at the pics, from front to back both transmissions are the same length. The problem lies in that the tailshaft on the mechanical one is about 6-8 inches and the electrical one is only about 2 inches. Even if I could get them to bolt up on the tailshaft swap I would have to modify the drive shaft.

I don't think it will work...what do you guys think?

The first two pics are the electrical one (303) the 3rd and 4th are the mechanical one (120).

DeliveryValve · #4 09-06-2008, 02:33 AM

I guess you need a transmission around the same year to make it work. The 120 is clearly a different housing. If you had a tranny from a turbo w123 1981-1984 then the swap should work.

Graplr · #5 09-06-2008, 03:10 AM

Quote:

Originally Posted by DeliveryValve

I guess you need a transmission around the same year to make it work. The 120 is clearly a different housing. If you had a tranny from a turbo w123 1981-1984 then the swap should work.

Yep, guess I gotta decide whether I want to transplant the mechanical speedometer into my cluster. The only problem was that the mechanical speedometer was off when I drove it in the 116. I don't really know why because that was the original car it was in but it had a 952 engine. I didn't think that would matter.

turbobenz · #6 09-06-2008, 03:14 AM

ive done the swap from a w123 tail to a w126. Your out of luck with you trans.

turbobenz · #7 09-06-2008, 03:15 AM

And I really doubt it will bolt up to the car

Graplr · #8 09-06-2008, 10:06 AM

What won't bolt up? The transmission (120) was bolted on to the back of the 952 engine in the 116. So all the tranny has to bolt to is the drive shaft. The rest is just support.

Johnhef · #9 09-06-2008, 10:20 AM

My 83 300CD had a 116 617 engine bolted to the original trans- the bellhousing bolt holes didnt all line up. That however, didnt stop whoever did the swap because as I found out when I was removing it, the thing nearly fell on me after removing only 4 bolts

You should be able to swap the back plate from the old engine to the new one and have it work with the 126's trans.

Graplr · #10 09-06-2008, 04:05 PM

Quote:

Originally Posted by Johnhef

My 83 300CD had a 116 617 engine bolted to the original trans- the bellhousing bolt holes didnt all line up. That however, didnt stop whoever did the swap because as I found out when I was removing it, the thing nearly fell on me after removing only 4 bolts

You should be able to swap the back plate from the old engine to the new one and have it work with the 126's trans.

The 126 motor and trans are shot. I bought a 116 with original tranny and a 952 engine in it and bolted together. It appears nicely done as I have had it out for a week and a half working on it.

So the 952 and 120 are going into the W126. I don't see any reason why it shouldn't drive. The only problem I see is getting the speedometer to work.

turbobenz · #11 09-07-2008, 12:41 AM

the problem I foresee is the tailhousing mount location lining up with your support plate. Unless this same tranny was also used in w123's, I doubt it will bolt up to the bottom of the car. Not to mention your drive shaft will probably be to short.

cth350 · #12 09-07-2008, 08:55 AM

I just went through the EPC last night looking up this info for somebody else yesterday.

The W116, W123 and W126 engine backplates are interchangable up to the end of the 1983 model year. There are two part numbers, an A115 and A615 part to 1983 and an A617 part number for the 1984 model year.

The two transmissions you have will swap around easily. If you were to unbolt the transmissions, you would see that your newer transmission has two locating pins. They match holes for bolts in the other bell housing. Also, the old transmission has a metal lip on the bellhousing missing from the new one. But the A615 backplate has the hole necessary for it.

In the DIY section of this site there is a page with transmisison info. One of the pdf's listed is the transmission service manual for the G class cars, and it shows the step by steps for the 722.3 transmission tailshaft removal.

Somebody somplace should have the identical manual for your older trans.

I know a fellow who has done it before. I will reach out to him and get some advice.

-CTH

Graplr · #13 09-07-2008, 01:41 PM

Quote:

Originally Posted by turbobenz

the problem I foresee is the tailhousing mount location lining up with your support plate. Unless this same tranny was also used in w123's, I doubt it will bolt up to the bottom of the car. Not to mention your drive shaft will probably be to short.

If anything the driveshaft will be too long. If that is the case I'll simply swap over the 116 drvie shaft but I measured the front to back of the tranny's on both and they measured the same.

I think the support looked to be at the same location as well. I'll have to check.

Graplr · #14 09-07-2008, 01:42 PM

Quote:

Originally Posted by cth350

I just went through the EPC last night looking up this info for somebody else yesterday.

The W116, W123 and W126 engine backplates are interchangable up to the end of the 1983 model year. There are two part numbers, an A115 and A615 part to 1983 and an A617 part number for the 1984 model year.

The two transmissions you have will swap around easily. If you were to unbolt the transmissions, you would see that your newer transmission has two locating pins. They match holes for bolts in the other bell housing. Also, the old transmission has a metal lip on the bellhousing missing from the new one. But the A615 backplate has the hole necessary for it.

In the DIY section of this site there is a page with transmisison info. One of the pdf's listed is the transmission service manual for the G class cars, and it shows the step by steps for the 722.3 transmission tailshaft removal.

Somebody somplace should have the identical manual for your older trans.

I know a fellow who has done it before. I will reach out to him and get some advice.

-CTH

Thanks for all the info.

Johnhef · #15 09-07-2008, 04:33 PM

Quote:

Originally Posted by turbobenz

the problem I foresee is the tailhousing mount location lining up with your support plate. Unless this same tranny was also used in w123's, I doubt it will bolt up to the bottom of the car. Not to mention your drive shaft will probably be to short.

It was- the 240D was the last to use it in the early 80's