On my 92’ when the auto trans shifts from 2nd to 3rd, under light load, the revs of the engine increase slightly, like its to slow in engaging 3rd.

This happens only from 2nd to 3rd, and at all temperatures, but perhaps more so when cold.

ATF oil and filter is like 1000km old.

Any pointers..?

 

Shift flare. First , check the fluid level. If the fluid level is correct, replacing the K1 valve can alleviate the symptoms.
https://rennlist.com/forums/928-foru...rt-number.html
I'm not sure if I consider this a permanent solution, but it has helped many times I've replaced it.

 

Would also suggest test / set the modulator base pressure - as it relatively easy to do, the process is detail on here, If that was low you would get flare as well although that fact you only have in one scenario might point to what soontobered84 has suggested

 

I've had this issue before and fixed it by adjusting the vacuum modulator on the transmission. You can adjust it to make the shifts harder or softer and mine was set to be too soft, hence the flares. Adjusting it to be a little bit harder made my shift flaring disappear. If that's what your problem is it's a simple fix.

 

There's a repair kit from Mercedes that addresses the "flare problem"....except your '92 already has the kit installed, from the factory. The spring sometimes breaks and I've seen a fair amount of damaged pistons for the 2nd-3rd shift....that's worth checking.
You can try increasing the pressure in the modulator and see what happens. Give the adjuster one full turn and see if that helps.
There is a "shift" kit made by Superior which has stiffer springs to make the transmissions shift firmer, but I shy away from this approach, as it is a "patch" for the actual problem.
Generally, the "actual problem" is that the teflon seals inside the valve body have worn down to the point where a higher percentage of the fluid passes around these piston rings, slowing down the shift signal. (You can buy a kit of these seals from Mercedes, but not from Porsche.)
(I always replace the teflon seals, in the valve body, when I rebuild a transmission....some people "insist" I also install a "shift kit". The combination of the two makes the transmissions shift so hard, it ruins the car, in my opinion.)

 

Any write-ups/ tips on the adjustment of the vacuum modulator?

 

Lots of good tips. Thanks.

I will try to give the adjuster one full turn. Clockwise or anti-clockwise?

 

Clockwise.

 

Hello Greg, slight tangent (apologies to the OP). These teflon seals in the valve body, are they for the later A28.** transmissions only? I don't recall seeing any teflon seals in the valve body on my 1986.5 A28.07 the two times I have had it apart. Or, more likely, I don't know what I am looking for.
Can you give any enlightenement? Are they listed in PET?

Thanks!

OP, good luck with your flare problem. I am curious as to what fixes it.

 

Yes, you have them.
On the pistons inside the valve body....behind the side plates. Not listed in PET. Only available from Mercedes.

 

Did a turn Clock-wise on the vacuum modulator. That seems to have cured the issue. No flaring. And, it seems to shift much better overall.

On a sidenote I would like to mention that there was no cap, or T-handle on the modulator. I took this off a spare box I have on the shelf.

Also, atf level is spot-on.

 

Glad to hear! It's always best when the fix is easy and you don't have to open up the transmission.

 

Yeah. Cheap fix. Hope it lasts.

 

Good to hear of such an easy fix... alway Greg’s invaluable input.

Get the cover for the modulator or you might have a slight vacuum leak there.

 

I suspect your improvement was as much from getting a cap on it as it was from the adjustment. The cap is required for it to hold vacuum and work correctly. The caps don't hold up spectacularly well over time.