ihoe

I have an automatic gearbox in my 86.5. I think it works well in the 928, but I have one complaint, I think the upshifts are a bit too harsh, especially at cruise(low throttle). Downshifts are generally ok.

To make sure everything is ok with the gearbox I have done the following:
- new ATF and correct level
- new filter
- replaced the modulation valve
- adjusted the modulation pressure according to WSM (vacuum disconnected at 50 km/h)
- checked that I have 0.5 bar vacuum at the modulation valve at idle
- idle speed 680 rpm

In addition to the harsh upshifts the box generates a jolt when the gear lever is moved to R or D. This only happens when the car is warm, cold it is much better.

I have seen other people also have some complaints about the upshift quality of the 928 automatics. Is it to some extent inherent to this gearbox?
What is other peoples experience? Any suggestions on further adjustments to improve the upshifts?

Speedtoys

My 87 was super smooth for a week after I bought it.

Then between parked Tuesday night, and Wednesday morning, started doing the same dump-truck style upshifts..overnight.

Modulator valve is one of the few things not on the R&R list for the car since 2001 in the receipt stack, so Im changing it tomorrow.

SMTCapeCod

Harsh shifts often come about when a vacuum leak creeps in to the mix.

Sometimes that leak is the modulator itself, or the hose to it.
There are a number of modulators for the different trans versions- be sure to source from an expert.
Also, in my experience at least, seems like few places are equipped to verify/calibrate the operating pressure post-install.

MainePorsche

Attached are some docs/links which may be helpful.

You say you have 0.5 bar vacuum at the modulator. You need to be sure you have adequate vacuum of the line that feeds the modulator. This line is off the manifold and is not a single continuous line to the modulator valve. Also when testing the modulator, it is important that the modulator holds vacuum otherwise it will lose its function - easy to do with a Mityvac. After you know you have good vacuum to a functioning modulator, you can do a final adjustment of the modulator through the 'T' key that is under the modulators rubber cap - see the attachments for the how to. You also must be sure the modulator has a good rubber cap for the seal otherwise you will lose vacuum and proper function.

CapeCod is right, there are different modulators (different colors) so you need to be careful when sourcing these. Also the operating (working) pressure is easy to measure with the right equipment, but it is not an adjustable pressure.



http://jenniskens.livedsl.nl/Technic...x%20akp722.pdf

http://www.w124performance.com/docs/...722_repair.pdf

ihoe

It is the line that feeds the modulator that I have checked, 0.5 bar vacuum. I measure the same on the vacuum line attached to one of the fuel dampers. So I have vacuum. The modulator holds vacuum as well, checked with a Mityvac equivalent tool. When I attach the vacuum hose to the modulator with the engine running I can measure a sudden drop in the modulation pressure. I think this is how it should work. When it comes to the versions of the modulator I am a bit uncertain. Currently I have a brand new green modulator installed, part no. 126 270 92 79. However, the part I removed was brown and had part.no 126 270 27 79. Can I use the green one? I have looked in PET and find no reference to the part nr for the brown one. I have A28.07 but this is also not mentioned in PET. By the way, the car shifts exactly the same way with both the old brown and the new green one. Maybe I am to critical and that it shifts the way it shall?
I will try to lower the modulation pressure below the specified value and see what happens.

MFranke

How many miles on it? I have similar thing. Smooth when cold but harsh at low throttle when warm.

ihoe

69000 miles Mine is shifting the same hot or cold. However, it makes a jolt when I shift in to D or R only when hot. There is no significant delay when I put it in D or R.

Mrmerlin

NOTE this info applies to 1985 , 86 , 86.5 32V cars.

My suggestion would be to see where you have your vacuum source coming from,
if you have them mixed up the engine will run OK but the shifting will suffer.

NOTE there are two ports on the upper rear side of the throttle body.

NOTE the port that feeds the dampers and the trans is on the driver side of the car,

NOTE ref WSM for a proper diagram, its easy to mix these up

ihoe

I have used this schematic for the vacuum lines. Should be ok, but will check.

Speedtoys

Swapping my leaky..as I found, modulator did nothing...but I did find something Im waiting to hear back from Greg about..will update.

Speedtoys

What I found when I removed my modulator to replace it, is that the pin under it (1262779375) had it's snout snapped of.

Greg says many are broken, and its not a huge deal..it all still "works" because the parts are captive, etc.

Fixed my harsh shifts today, vac line was good one day, bad the next. I ran a separate single piece line..

MainePorsche

Yep, as I mentioned before the vacuum line is not a single continuous line as it goes from manifold attachment to the modulator. It goes from rubber line, to fixed metal line by the torque tube, then back to rubber as it goes to the modulator. Multiple sites for vacuum loss. I ran this line as a single unit. Size 3 is the one. https://www.pegasusautoracing.com/pr...p?Product=3260

OTR18WHEELER

+1, when I did my IR, I replaced the line from the 7 way splitter to the hard line. I also replaced the section from the hard line to the modulator.
The original line had chaffing , due to fail.

MFranke

What length did you use to run a single separate line?

Speedtoys

..or 7/32 hose from generic parts store.