Auto trans modulator questions

[marlinspike]

I noticed when looking at the Porsche 928 auto trans vacuum modulator it looks different than the MB part. Anybody know if it is functionally any different?

Also, what would be the symptom of that o-ring on the vac modulator if it wasn't making a perfect seal?

I ask because I have a souped up MB (also uses that same 722.3 trans, just not as a transaxle) and I recently had to replace the trans modulator (went to adjust it to make it slightly firmer, but the rubber cap and the pin were gone). MB has subbed out to a new part (I don't just mean the new sub they did several years ago, but now an even newer sub). With this new one in I can make my shifts firm how I like them, but they're late on full throttle (and no, it's not a bowden cable adjustment issue), late enough that on the 2-3 it will hit the fuel pump cut off before ****ting. MB has had some issues lately subbing in new part numbers with the new parts working, but not quite (or in the case of some hardware not working at all), so I got to looking up the 928 part.

[Opelotus]

Not too familiar with the automatics, but if the o-ring is leaking, it may not be getting enough vacuum to do it's job properly. Not sure if a vacuum leak would cause a later or earlier shift though. Also, by "souped up" do you mean that it has a shift kit? If so, I believe that changes the shift points, so that may be a contributing factor to your problem.

[marlinspike]

Close. It does have a screw-like thing in the center that is used to adjust it, but timing of the shift is done by the bowden cable, not by the modulator. The modulator controls firmness (though I'm told that there may be some inter-relatedness)

This is the MB part Pelican Parts - Product Information: 126-270-90-79-M22

And the porsche part
1983-1991, 1993-1995 Porsche 928 Automatic Transmission Modulator Valve - Transmission - OEQ 83-91, 93-95 928 Automatic Transmission Modulator Valve - 104-01034675 - *********

[marlinspike]

Originally Posted by Opelotus

Also, by "souped up" do you mean that it has a shift kit? If so, I believe that changes the shift points, so that may be a contributing factor to your problem.

Shift kit?
By souped up I mean the car now has far more power than it left the factory with, but your talking about a shift kit has me intrigued.

[Opelotus]

I believe I read somewhere about there being a shift kit for the 928 auto trans. This is basically just a new set of valves/different strength springs, etc. which replace the originals in the trans valve body. By changing these components, the shift points and firmness can be tweaked. Again, I don't know much about the MB auto trans, so I could be wrong, but I believe that is how it works in most other transmissions.

[marlinspike]

You know what...I was playing with this the other day, thought I updated this thread but I guess not. I think it's not running into the fuel pump shut off, but rather the car is bucking on the 2-3 shift. However, if I dial back the modulator it will flare...then you said shift kit, so I started googling.

Then I saw there is a K1 spring upgrade for slip/flare on the 2-3 upshift...dang it, I just drained all the fluid a few months back to do the b2 piston, now I need to waste all that fluid all over again for the spring kit.

[Opelotus]

If a replacement spring solves the problem, it will be worth it. You shouldn't lose too much fluid when removing the pan; alot should remain in the torque converter. Is the pan easily removed without removing the transaxle?

[marlinspike]

Last time I did it I think the pan was about 60% of the capacity. In there right now is Mobil 1 ATF. Mobil 1 is now a new formula. Granted it is still good for Dexron III (but is not good for Dexron IV). This trans calls for Dexron II (and Dexron III is backwards compatible) - can I mix the two Mobil 1 ATFs even though they're different formulas if they're both Dexron III compatible?

MB put the same trans in their cars, but didn't set it up as a transaxle, so it's not particularly difficult to get to, just as much a pain as any trans job is, plus I'm busy studying for the bar exam.

Should I change the filter, even though it was changed with the fluid just a few months ago? As someone who just finished grad school and has not yet started working, money is...well, it is what it is.

[Opelotus]

Not sure about mixing the two ATFs, you would need to ask Mobil about that or someone who has more experience with that sort of thing. I would say leave the current filter in place, if it's only a few months old, I don't see a need to replace it unless it is cheap and easy to obtain. Just make sure you keep any parts you remove from the transaxle clean for reinstallation.

[marlinspike]

I talked to a tech btw, and in case anybody in the future comes across this thread, if that o-ring were leaking the symptom would be a slow leak of trans fluid.

Also, to be clear, my car is an MB, so I don't have to deal with a transaxle (MB put the same trans in a traditional place in their own cars that Porsche was putting in the axle) I just come to the 928 forum because they shared parts and because the 928 guys tend to be a bit more tinkering-friendly.

[Opelotus]

Haha ok, thanks for clearing that up, I thought you were just referring to the fact that the your 928 has a MB transaxle, but now I realize you don't have a 928, so it all makes sense. You should have no problem removing the pan then, since your MB doesn't have a huge crossmember under it. Good luck with the work!

[marlinspike]

I asked ExxonMobile about the compatability of the new ATF. I expected the typical over-involved answer you get from large corporations. Nope, just "Yes, it is backwards compatible." Not even a signature line lol.

[Opelotus]

cool, well at least you now know for sure it's ok to mix!

[marlinspike]

How's this for a headscratcher. Changed out the K1 piston/spring assembly, and now my 2-3 shift is good, but the 1-2 shift, which was amazing before, has slip now.

[marlinspike]

Just test drove it again, without making any changes, and it works fine...I'm so confused.