Maybe missed an earlier post but where are you located?
Vacuum yes, but the cables are also important to check and have to be adjusted together. I fought this problem for many months to get it okay.

 

Calgary, I am in The Netherlands (Europe).

I checked the vacuum and also eliminated slack on the throttle cable going to the transmission.

The gear selector cable seems fine, there is a noticable difference when driving in D, 3 or 2. Dont know if there is further adjustment required


Any thoughts are welcome

 

In conjunction,take a look at the throttle cable. Its the one closest to the radiator on the cable mount plate. Slack can be removed at the screw-type connection on the firewall. I think what he is saying is that there is some bit of interactivity in setting the cables just right, and setting the throttle rod length slightly longer or shorter (also at the mount plate). The other interactivity is with the idle switch. When that hard throttle rod just starts to be pulled forward, the throttle body "TPS" switch should be heard. A faint click. Read more about the dual function of that switch as you continue to sort-out your ride. The switch needs to just click with slight movement, plus, return-click when released (when turning the mount plate by hand, for instance)

This is interesting "tuning" and when its finally right the automatic car is very, very responsive.

You won't hear it, but at something like 75% throttle motion the WOT part of the TPS switch makes contact. Separate switch within the TPS, looks like "points" on an old car. When the contacts close under heavy load, the brain makes a large timing shift electronically, adding to you acceleration pleasure. Again, read whats written about it.

I did intake work to replace rubber, but neglected the TPS, so yesterday I spent all day adjusting and tweaking the above stuff. The 85 now runs more better! Still hot, but more better.

I used the Porken blink'r tool to sense the opening of the TPS. Genius.

 

Well my hands were itching today to take a look at the Porsche.

So I took it for a drive, jacked it up, removed the transmission oil reservoir metal thingy and took a look at the oil level.

There was barely any oil in it with a running engine, so I filled it up with some fresh atf dexron IId oil. I was able to fill approx. 900ml (!)

Then I took it for a drive again and it was significantly quiter, the whining sound is completely gone, and the downshifts seem faster.

However it still upshifts quite harsh, with a real 'clunk' throug the drivetrain. Not really as a Porsche GT should be. Especially 1-2 is harsh.

Next thursday I can use the lift in a workshop nearby and plan to change transmission oil and filters.
In case this still does not improve the harsh shifting, I plan to adjust the modulator valve. Can anyone give me a quick instruction how these adjustments are done?

In case this also fails, is it possible to replace the modulator valve with the transmission in the car?

Thanks again
cheers

 

Have searched the forum a bit more and found many postings regarding the modulator valve, but no posting with very clear instructions how to set is.

I ordered a workshop manual also, so I guess it will contain some info on this topic as well.

As far as I understand the valve can be set by removing the cap and adjusting a nut?

Is this correct, and what direction should I move it (eg is left for softer shifts / harder shifts??)

Thanks again for all your input

 

The cap is rubber. It pops off. Beneath is a little metal T-handle that flips out somehow. It is turned clockwise or counterclockwise. Doesn't take much to change the feel of the shifting. I can't remember, but I think clockwise is for more abruptness. Somebody will chime in I hope. Proper way to set is with gauges. Mercedes shop might be able to do this for you if you need to go beyond tweaking it slightly.

I can take a picture of one later today if you want.

The WSM set from Rog100 is what you really want, it has lots of tech publications with it that aren't in other CD sets usually.

 

Last evening I decided to take the 928 for a spin and miraculously it upshifted wonderfully through all the gears!

Apparently the oil-fill up did make a substantial improvement. As I drove some km's the oil got warmer and the upshifts became somewhat harsher, but never as harsh it was before.

So I am now really looking forward to getting the car up the lift tomorrow and changing the gearbox oil and filter. I am really wondering how bad/deteriorated the the fluid looks.

Feel confident the oil change will solve the problem completely.

928 felt great during the drive yesterday, put a big smile on my face :-)

 

Great! Way to dive-in. Rewards accumulating already!


These gearboxes seem very well built. Be sure to get a CD set from Rog100. It has the workshop manuals, but also a couple of extremely useful transmission documents. The WSM's are thorough, but technical and disjointed due to various updates made as cars evolved. The separate tech documentation helps me pull it all together, whether its transmission, engine, or something else. That's the genius of this compilation. (Its also called the Morehouse CD.)

Its a bit much to copy here, and its not mine to copy, but it has several pages of practical road speed / throttle position tests to determine shiftpoints and set cables, for instance.

 

Counter Clockwise for softer shifts, and clockwise for harder. If it's close, I'd start with maybe one complete turn. There's also a very detailed and professional way to set it perfectly to spec, that requires a method of testing fluid pressure, etc, and I'd be willing to bet that spec settings aren't necessarily ideal for a 20ish year old car anymore, so IMO adjust it to the feel you like, but only after you've eliminated all the other possible issues first, otherwise you're just masking the symptom.

Basically, if you're under the left hand side of the car (driver's side for US models), then look up at the side of the transmission, you should see a plastic cab about the size of a soda bottle cap. You'll probably need a flash light, because it's up toward the top a ways, and it's difficult to see in that little pocket. Under it is that little metal t-bar, and you'll be able to see that it fits into little notches cut around the piece the cap covers. Pull it out just enough to clear the notches and turn it. Like I said, if you're close, start with one full turn. Made all the difference in the world to mine!

 

Under the little black cap. This modulator valve is green, but yours is probably red plastic.

 

Thanks! I already found the modulator, mine is indeed red plastic.
Will take the car for a drive tomorrow after replacing oil+filter and adjust if neccessary.

Will off course keep you posted!

 

When you get the Tech CDs from Roger, it will give you the modulating pressure that should be set for your MY car. I made and adaptor and fitted it with a pressure gauge and screwed it into the modulating pressure connection on the gear box and I adjusted the pressure to this value at idle with car on jack stands in drive with vacuum line disconnected. I acutually adjusted the modulating valve many turns to get the pressure correct and this removed the harsh shifting.

Tails 1990 928S4 Auto

 

Well today I spent a couple of hours under the car changing the transmission oil + filter and also releasing the tension on the flexplate to prevent the dreaded TBF.

The oil in the transmission looked like new, so did the filter. No surprise the gearbox did not feel any different with the new oil and filter. But fortunately I am now sure it is not the oil :-)

Also turned the modulator valve counter clockwise 1,5 turns. This made the gearshifts somewhat softer, but still not as soft as I would like them to be.

Perhaps I will expiriment with it some more. How many turns could I make counterclockwise without damaging anything? Thanks
Michel

 

You can keep adjusting the modulating valve by this method until you get smooth shifts without a harsh uptake, however, if you adust it too much you start getting flared shifts, that is slipping shifts. IIRC this can cause clutch wear.

If you wish to adjust the modulating pressure correctly, get the pressure reading for your MY car from the WSMs and take it to a Mecedes Benz auto workshop and have them adjust it for you to the correct pressure.

Tails 1990 928S4 Auto

 

Today I doublechecked the vacuum and found no problems, so I decided to turn the vacuum modulator a few turns counterclockwise.

After approx 3-4 turns the car is running GREAT.
Perfect upshifts, no flaring, just BRILLIANT!!!

I am one happy 928 owner right now!

Thank you guys for all the input!

I am just wondering how it is possible the modulator got so much off-spec? Is this an age related problem?

I will now proceed with other things to make it perfect, such as timing belt and water pump, electric windows, passenger side electric seat, and some other minor stuff.

I cannot express how thankfull I am for all the support and helping me fix this problem so quickly!

Cheers
Michel