evozine

I have a 84 928s (US spec) automatic transmission with an inoperable 4th gear. I purchased a used 84 (euro spec) automatic transmission, which is sitting in my garage staring at me every day, asking to be installed in the shark. SO here's my question:

Has anyone ever tried to bench test a 84 automatic transmission: meaning, attempt to move through the gears with an electric motor connected while manually changing gears? I hate to spend two weekends to install a transmission that is not fully operable, as I already have one of those in my car now...

thanks in advance...,

IcemanG17

there might be a local-ish transmission specialty shop that can do this? I would imagine its like a reverse engine dyno?

mpesik

That's a question I've never come across before on this forum. Good-one.

evozine

Ok, it's time for plan B:

-I spoke to two shops, they both said that the cost of removing, then completely rebuilding and resinstalling the transmission will set me back around 5,000.

So my plan is to spend the time in January swapping out the transmissions myself, with some qualified help, which will answer the question "is my used trans in working order". Total cost for me 100 dollars for fluid, filter and pan seal.

bummer

evozine

The guys at another porsche forum suggested I replace the A/T vacum regulator. Here the text from the add of the vendor who makes the part:

"This is the new factory genuine transmission vacuum modulator valve with adjustment key and cap. Failure of this part can lead to harsh or jerking shifts and in some cases prevent the transmission from shifting into 4th gear. There are other things that can cause problems with poor shifting, so there is no guarantee this new part will fix them. But due to the cost and ease of installation this is a good place to start..."

It looks like 928Int has the same part for half the price..., I have a spare transmission, so I will try swapping out that part first...

Has anyone had success with this quick fix:
https://mercedessource.com/node/7640
http://www.928gt.com/default5.htm

G Man

I had shifting issues with the trans in my 84'. It shifted into each gear but shifted way to early and the shifts were clunky rather than quick and firm. Replacing the modulator solved both issues for me. I had to fiddle with the adjustment a little to get it to shift to my liking. Have you tested the modulator to see if it will hold a vac? Mine did not prior to being replaced. If you test it be sure to have the rubber cap in place. FYI...that part shown in your pic is not what the modulator for our cars looks like. Ours is like the pic below. Also, you might want to check with resident trans expert Steve C, and get his take on your issue before you dive in. Good luck with the project.

borland

That won't work on your car. Since your up to swapping parts between transmissions, try swapping out the valve body. Not sure if the shift points will be correct, unless they both have the same gearing.

928mac

If you have no training on transmissions I would not play around.

There are to many things you can do wrong including removing and not seeing what will fall out.
When I rebuilt trans, I air tested each component as it went together.
A hair in the wrong spot can screw up a valve moving, they have to be cleaner then clean.

The only trans dyno that I have used was in collage now called LeTourneau University in Longview Texas.
Two parallel pipes with sliding mounts for a engine a trans and the dyno on the end.
You could connect any engine and or trans combo to the dyno.
he he we used the instructors alki fueled sprint car engine he he

Landseer

Have you gone through the A28 troubleshooting information?

There are ports on the side of the transmission for hookup of hoses/gauges so you can measure operating pressure and diagnose certain issues.

Might be worth some study? I don't see a 3-4 shift complaint / resolution in a skim through it, but it might be repairable without removal.

Besides filter change, you can change the modulator valve, B2 piston/sleeve and governor without removing transmission. Modulator valve should be changed anyway if you haven't done it. Have you tried increasing the pressure by turning the key that is under the rubber cap? The cap sometimes cracks and gives a vac leak.

These transmissions are in the heavy mercedes sedans (different case, same innards). There are a bunch of them out there. Key is finding a shop willing to diagnose it and share knowledge without charging you a fortune.

Withholding diagnosis and knowledge is what I'm finding out here when dealing with shops, especially transmission guys. But I think there are also VHS tapes available showing the step by step rebuild. Might even be youtube videos?

One other beautiful thing: you have a spare. You could swap governers, valve bodies, all kinds of parts before you drop the trans out of the car. Don't have much to loose, do you?

blown 87

Amen

danglerb

Can't believe you haven't installed that transmission yet. I know it basically works, unfortunately the only driving I could do was inside my condo complex, so I don't know if it ever shifted into top gear, but it would kick down and go back up to something, and nothing felt odd about the shifting.

84 Euro will drive differently, higher stall speed and lower final drive ratio should make it quicker.

IcemanG17

That sounds high to me.....

I have a deal for you.....I just removed a 1984 4 speed auto with open diff from my Lemons racer..... it has a few leaks but otherwise works fine.....it needs a main pump seal and a filter change..... I will sell it silly cheap.....

The shop that rebuilt the 1988 4 speed auto with LSD I installed charged me $2400 and it was nearly a total rebuild..... I'm sure they could rebuild my 84 4 speed for that much or less

shoot me an email icemang17@aol.com and we'll discuss

evozine

thanks, but I already have an extra 4 speed trans. from a 84 euro spec car, which apparently ran fine. Too bad you are in NorCal, I could use your advice on how the to remove this beast of a trans..

Then again, how silly are we talking about here...I may need your trans if mine extra one is bad...

Landseer

Not brain surgery.
Plenty of documentation here.

evozine

How did the 928 perform in the lemons race...?