This may have been covered before, but I couldn't find it. So forgive me for asking what should be common knowledge.

Can the 3spd AT be swapped with a 4 spd AT on the '79 through '82 US cars? is it a fairly straight forward swap?

Thanks in advance

 

'87+ 4sp has larger torque converter and those require mod to body possibly including changes inside at rear seat area.

Earlier 4sp will only need TT and few other parts. Possibly rear subframe too, not sure about this.

 

Different bowden cable routing and fitting. Later version of torque tube. Don't use later then 86 and make it easier on yourself.

Different flx plate at the back I think too. I did this and was very happy with the car afterward.

 

No Rear subframe change in my job.

 

The early 4 speed also starts out in first gear which is a plus when you switch from a 3 spd to a 4 speed it is nice to actually have all 4 gears all the time. As mentioned torque tube , linkage and transaxle ...will be different

 

I think m21sniper has a 4-speed auto in his early car, an '82 IIRC. I haven't seen him around in a few days, but maybe you could PM him and talk him into crawling underneath to get the # off the tranny.

 

Thanks everyone for your responses. I have parts from an '84 and '86 donors, and it sounds like it would be a good thing to pursue when the time comes. And yessss...1st gear start is one of the objectives.

But just to be clear on some terms. The subframe you are referring to is also known as the cross member. Correct?

Thanks

 

The early 4 speed also starts out in first gear which is a plus when you switch from a 3 spd to a 4 speed it is nice to actually have all 4 gears all the time. As mentioned torque tube , linkage and transaxle ...will be different

I have the 4sp in my 84S and I am pretty sure it starts in second. Is there an adjustment I need to make to get it to start in first?

 

Yes, large aluminium casting under gearbox where rear suspension arms mount to. Seems 3sp and 4sp automatics use same piece according to PET. There are few different part numbers but they are interchangeable. Early manual gearboxes (G28.03) also use same part.

 

Start in 3rd or 2nd and it will start in 1st. In 2nd will stay in 1st till the 5K+rpm if not it needs an ajustment for sure.....

 

In driving our Mercedes, with the four speed, I like my 3-speed more and more. The four speed is always trying to find a gear, if driven hard (granted the 3.0L I6 Merc doesn't have the same torque as my V8). First is way too low, too (wagons have a 3.27:1 gear ratio).

With the three speed, there's no hesitation, first goes to 60, second to 90, and third to 160 (dyno)!

 

To continue with Sterling's Comments, there are actually NO markings on the dash in t he 3spd cars, so therefore if you wish to KISS, you can just ignore it. Now, if you WANTED to make sure you had the proper notice, you could have something to accept the detent info.

 

I actually like the three speed in my 81 better than the 4 speed in the 84. I was considering switching them out before I sell the 81. Am I crazy?

 

4 speed autos wake up a stock 3spd car more then you guys can really imagine. The first two ratios are infact way shorter or at 2nd just even with the 3spd 1st.

 

Thanks everyone for the info. I'm not concerned with gear selection because as Brendan mentioned, there are none in the dash of the '79

There is a strong possibility I will tackle the job this weekend