3sp AT vs idle
 

Hello,
I have a question regarding my 3speed Automatic of my 928S of '80 :

When AT selector is in R,N, D, 2 or 1 and slightly revving, it will drop rpm under 500, barely keep engine running but then catches up again till 800rpm.
It does not do this in P.
Has it something to do with my idle ?

I noticed that the ATF level was a little bit low, so I filled it with about 0,5L. But this did not provide any improvement.
I checked stall speed of my AT , it was about 2000rpm. I handles quite well, shifts up and down without hesitation.
I only have a clunk when speed goes under 35mph and the gear in D automatically goes from 3 to 2 (via brakeband). So I suppose tension brakeband is not 100%, causing slip.
Remark : when starting engine cold, my RPM barely goes to 1000rpm...shouldn't it go straightly to 1200rpm ?

Gerrit

Cold idle speed is controlled by the AAV, and these can become unreliable. Mine did - car stalling when cold. I opened it up, drilled out rivets, bent spring post until it closed hot but opened more cold. Bolted up with M6 bolts. Dont know why some gears would affect idle, but suggest you check the vacuum line to the box - it shouldnt clunk when it changes down as you slow down.
jp 83 Euro S AT 57k

As for the AAV, when checking it when cold, it is almost closed except for the square hole :
https://cimg3.ibsrv.net/gimg/rennlis...bba16dc32.jpeg

This should be correct, not?
Or is the square hole too small?

As for my AT, when checking the WSM and troubleshooting, the clunk is related to the brakeband....I do not know how to solve this

Here's how much mine is open at Thai ambient temp around 34°C.
if I remember right it was about half open when I put it in freezer.
Not sure if this is correct.

https://cimg6.ibsrv.net/gimg/rennlis...b5cff80530.png

AAV should be 90% open when cold - fresh out of the freezer cold, not Thai ambient. I get idle ~ 1100rpm when very cold, and drops quite quickly after moving off.
The reverse band can be adjusted externally, but I dont think others.
jp 83 Euro S AT 57k

If you get a rough or early shift then you could have a vacuum leak,
so you would be wise to replace the short flex lines at the hard line on the TT

then mine is not sufficiently open when cold.
And should it be fully closed when warm? or may the square opening in the disc be visible?

Shift is rather smooth and also good shifting (rather a little bit late than early, also kickdown is late)
But when turning the idle screw CCW , it will not higher my idle significantly (max 1000 rpm)...

Mine is fully closed warm

https://cimg1.ibsrv.net/gimg/rennlis...ec7b3a1b17.png

I checked my auxiliary air valve and seem to be working fine : opened further when I have put it in the freezer , here is a pic :
https://cimg9.ibsrv.net/gimg/rennlis...2d590e603.jpeg

when the engine was warmed up after about 5 to 10 minutes, the AAV was completely closed.
However, rpm with cold engine start will not rise significantly, not even when turning the idle screw CCW.
Max rpm at cold engine ( environment temp was 23 degree Celsius ) was 1000 rpm.

However, I noticed that rpm would go up when detaching rubber hose from AAV to inlet ( suction of vacuum was quit strong)
Vacuum measured at the WarmUp regulator is -0,4 bar or 400 mmHG.
When detaching the rubber hose from AAV to inlet, vacuum raised till -0,6 bar.

Another device that controls the rpm and vacuum is the vacuum limiter at the ignition collector

What should I check for my low idle issue? Is it a vacuum issue, can vacuum be too low? I cannot see any direct vacuum loss
How should I test and check my vacuum properly?

From your pic AAV needs to be open a lot more when just out of freezer. Also 23C is not COLD - I would not expect idle to be over 1000 at 23C. Highest idle I get in winter at start up is ~1100rpm, and it drops within minutes as the block warms and AAV also warms.
AAV needs to be opened and mechanically adjusted IMHO.
jp 83 Euro S AT 57k

What should be the opening at 23°C then ?
I am asking this because if I am adjusting the opening (about 90% open) when AAV comes out of freezer, will it still close with warm engine ?

If it doesnt fully close when hot, just close down the idle screw in front of the spider body. There will always be some air going through the AAV because of the shape of the hole.
jp 83 Euro S AT 57k