Hi,
I've just purchased a 1985 Porsche 928 fitted with the same 4 speed slush box that is fitted to many an MB.
I havent had the car high enough in the air yet, but it really feels like the vacuum pipe to the modulator has been pulled off! The shifts at anything below 2/3 throttle are super hard.
So either the pipe is off.... easy fix, Or the modulator needs tweaking.

I've read on here that it shouldn't just be a hit and miss adjustment, and that it should be to spec. I've looked through the 126 service manual and can't find a thing on adjusting the transmission!
Can anybody give me a procedure on doing this?

Regards,
Ben

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Ben

'85 500 SEL
'85 928S2
'88 560 SEC
'90 750i

I wouldn't worry too much about it until you know how the vacuum routing is on it, if you connect it chances are you'll be fine. Porsche manuals will probably give you the spec you need, but really htis rarely needs adjustment, and you really can do it by feel if it does need adjusting. Make sure you don't open the adjustment up too much or the shift will take too long and you wear out the friction discs faster than normal, you want it to feel firm but not harsh. CCW= softer, CW=firmer, but again don't mess with the adjustment until you know you have vacuum to the modulator.
Gilly

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Thanks Gilly,
I gave it a try today. First of all, on the 928, there is no dipstick as such to check the level, there is a tank like a large brake fluid reservoir screwed to the side of the transmission. When cold the level was perfect, I took it for a good drive. Had to add just over 2 quarts to get it right!!
I adjusted the modulator by one full turn CCW. This gave me shifts slightly firmer than my W126, but still no where near as hard as it was! It would almost kick you in the back before.
Next task was the bowden cable. I was getting shifts at around 4k rpm under really light acceleration before (way too high, even for a sports car). Adjusted it longer, and got it spot on the first time. Shifts at just under 2k now. I'm glad it was right the first time, the adjustment is right in the valley of the engine under the intake plenum.

Regards,
Ben

__________________
Ben

'85 500 SEL
'85 928S2
'88 560 SEC
'90 750i

The modulator pressure for that car is 4bar/ 58lbs at 45mph, with the modulator hose disconnected and plugged.

The feed line for the modulator is located on top of the intake manifold, ¼’’ steel line with a banjo fitting. Normal manifold vacuum at idle for an engine in good condition is about 18 to 22 in.-Hg.
Vacuum at the modulator should read 18 to 22 Hg.

At normal operating temperature the fluid level should be between the MIN and MAX .


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