Ok, there are some noticeable differences, electronically speaking. I need some help understanding if blinding off the sensors on the newer ‘88 trans is ok, while installing it in place of the original ‘86.5.
there are two photos below showing two discrete sensors on the ‘88 (first 2 pics) that are not on the older ‘86.5 which is being replaced (last pic).
installing my ‘86.5 trans Cooler line will eliminate one sensor. The second lower sensor will be eliminated by the blind plug from my ‘86.5. Are there problems here? Beyond not getting improvements that the sensors may have provided on ‘88 cars this trans was actually installed on?
and to be clear… the car drives otherwise great. But it’s a driver. Much more than a good “original” ‘86.5 trans would be required to get it into purist category.

 

The lower is a pressure sensor for shift detection which is used by the S4-up ignition computer to retard briefly during shifts to protect the transmission.

The upper sensor is a temp switch for the S4-up fans. This may fit in the plugged port on the S3 cooler.


The fan switch on the S3 cooler was only fitted as part of the trailer package. It connected to the empty plug near the jump post.

 

Thanks Ken!

 

mj1pate, I am going to assume you have not physically installed the replacement 88 transmission yet, correct?

Get a good measurement of the torque converter housing. It is my understanding that the A28.12 automatic for 88 had a larger torque converter and corresponding larger torque converter housing than the A28.07 for 1986(.5).
Meaning I am not sure if it will fit under your floor pan. It might with hammer massaging...

Good Luck!

 

hmmmm… so question is: was the underside of the 928 changed between 86.5 and ‘88 to accommodate the auto trans with larger torque converter..anyone?
…and this thread doesn’t give a clear answer. Does anyone authoritatively know if my recently acquired ‘88 auto trans will fit in my 86.5?
https://rennlist.com/forums/928-forum/363962-transmission-replacement-advice-needed-2.html

 

So many changes were made in the 1986.5 928's, in anticipation of the 1987 model, that chassis details can be difficult to confirm.

However, the early 4 speed chassis had a more confined area for the smaller bell housing for the '83-'86 transmissions. The larger bell housing transmissions ('87-'95) require some "chassis alterations" in order to fit.

You are also going to trade a transmission which starts in 1st gear for a transmission which starts in 2nd gear, resulting in a certain loss in performance.

The '88 transmission additionally has a lower stall converter, meaning converter lock-up will occur at a lower rpm. This will also result in a loss of performance, as the lower torque '86.5 engine will be forced to operate at an rpm below where it is happy.

 

I'm confused by this.

I put an 89 in my 86.5 and all I had to do was retrofit the late model connector at the transmission for the kickdown cable.

That was like 20 years ago. I'm pretty sure that was the sum total of what I changed.

Kevin

 

Wow… what type of operations may be involved in “chassis alterations”? Do I need to be a metal worker?

 

S3 have a pretty high torque peak and at a few hundred lower rpm than S4. 2700, IIRC.

At some point the 86.5 rear hatch has indents for the S4 wing. I could imagine the body stampings cross over at some point, too.

If not, I seem to recall reading that for earlier cars (pre-86) that a little massaging with a ball-peen hammer was all that was required.

 

sounds reasonably like I’ll be able to get it done. I’ll post with pics what I needed to do to make it happen.
are there simple mods that can make the ‘88 shift more like the ‘86.5? Are the vacuum modulators interchangeable? If they are, is it wise to use the vacuum modulator from the ‘86.5 if it was slipping?

 

Would strongly suggest to have your 86.5 tranny rebuilt and reinstall it. This would be a positive for you and for any other person in the future if you plan to sell it.

During the rebuild you can discuss some upgrades to enhance its performance too.

 

I agree that it would be best to keep all 86.5 vehicles as "pure" as possible, not to mention the decreased performance of the '88 transmission in this application.

However, I also understand the desire to find an economical solution, which a good used transmission might provide. (The trick is the "good" part. "Good" 35 year old transmissions are pretty rare.)

Rebuilding any of these "early" 4 speeds properly, with Mercedes parts and performing all the updates, has gotten to be frighteningly expensive, as the Mercdes parts prices have skyrocketed.

Yes, there are "offshore" parts, and terribly shoddy rebuilding practices, which greatly decrease the parts required, but these pieces have extremely limited life expectancy and make the "second" rebuild incredibly expensive.

That entire "Pay me now or pay me more, later" thing is incredibly appropriate for automatic transmissions.

 

I see… so is it possible (while transmission is down, TC bearings being replaced), to install a more performance optimal torque converter, like from below?
http://www.hughesperformance.com/tc-stall/
Bear in mind, I’m not advocating this. I wouldn’t know where to start. From those who might know where to start, is such a replacement even realistic?

 

forget it. I now see you’ve all been down this path before:
https://rennlist.com/forums/928-foru...l-speed-3.html
I’ll go with my S3 on ‘88 MY transmission Thorazine, so it’ll still be at least street-able. I need to get it back on the road as the little garage has much higher priority projects waiting, soon to be followed by down-sizing

 

It will be tight but should fit. You will need to use S4 bowden cable at trans, not sure if it is long enough to reach front 86.5 front throttle connection. You might need to improvise.

For gear selection, use S4 shift cable if possible. S3 cable is on verge of being too short to clear the 1 inch larger diameter TC. Must use S4 TC and rear housing.

S4 body cavity has more clearance than mid year cars. if your trans mounts are new, might ride too high. Might need to bash a bit with sledge. I had to do very little of that, handful of hits near tc cover. Very tough strong area, you wont move it much.

I installed 89 S4 trans and TC as a replacement for the mid year 4 speed. My swap is in 84 car. 83,4,5 6 and 6.5 are same 4 speed. Same body cavity. Beginning 87 the box changed as did cavity apparently. Fantastic swap for me. The lower gearing of 89 is perfect match for the low hp 16V car.

iirc you can use either electrical connector, old or new. Plug and play. Either torque tube also. Can leave S4 trans sensors in place.

Again length of Bowden is the only issue I anticipate. On 84 the bowden at throttle is located at throttle body. Had to cut some armor off 89 cable to shorten. You know where it connects on S3, near hood latch, I think that's a lot farther than where S4 cable hooks up. Theres probably a simple workaround though, some sort of linkage you could use to extend cable.