Danny B

My 84 US has been shifting harsh (all gears ) since I got it so I went ahead and changed the small 3.5 mm vacuum hoses at the cat and the one that connects to the vacuum modulator, thinking I had a vacuum leak.

I still have harse shifting. It also shifts quickly (it Always did )..
I'm in 4th by time I hit 40mph.

I was assuming I had a vacuum leak and was hoping these line changes would

What should I be testing now? The vacuum modulator?
Are the any other vacuum lines I should be testing?
Suggestions?

Thanks
-Dan

Avar928

Check the bowden cable, which I believe should be taut. If there's slack, you'll shift early. Tighter = shift later.

Tails

If you have a WSM or the Technical CD, there is a procedure to set up your cable to the auto box, however, harsh or flared shifts usually mean the modulating valve needs adjusting. The pressure to adjust to are also included in the WSM. If you are not a competent DIYer take the car to a Mercedes Benz auto transmission specialist and he can adjust your modulating valve to the correct pressure a the auto box is 722.3 series MB transmission (IIRC).
Tails 1990 928S4 Auto

WallyP

Did you check to make sure that you actually have vacuum at the modulator? Not too unusual to have problems with the vacuum line.

Danny B

I'm obviously a rookie but..

I connected my vacuum gauge to the modulator end of the hose and started the car. It held pretty steady at 18 on the gauge.

dcrasta

Check your modulator for leaks. It should hold 18-21hg vaccum easily. .

Bowden cable must be adjusted correctly.
Vacuum must be on the modulator
Modulator must be adjusted correctly.
Trans fluid levels must be correct.

Danny B

I tightened up the Bowden cable to be taut. I took it for a spin right after with minimal difference.
I just checked the modulator and it appears to hold 21hg well. (checked when not running ).
18hg vaccuum coming down the pipe ( TT ) steadily.
Fluid level is accurate.
Was there anything left I missed?

Thanks for the tips btw..
-Dan

Barry Chan

Mine would be at 4th at 30 mph if left on D to safe gas that's why if i want performance from the tranny,
try shifting it, start at 2 (first gear) then shift to 3 then 2 (second gear) from there you go from 2 to 3, then 3 to D, if shift at red line each time, you will run out of road or go too fast by the third. I f you want to have some fun driving our auto, you can up shift and down shift.

dcrasta

@Barry > A Bowden cable that is too loose may end up 'short-shifting' the transmission. If you tighten it up 1 or 2 turns you may get the 2.5k shift point (I prefer between 2.5k and 3k for my 'normal' shiftpoints.)

WallyP

Somewhat simplified normal operation:
- Speed at which shifts occur is largely controlled by Bowden cable adjustment.
- Speed at which shifts occur is also seriously affected by the kick-down solenoid. When the solenoid is activated, the transmission is more reluctant to upshift, and more eager to downshift.
- Firmness of shifts is largely controlled by vacuum modulator.

Non-normal operation:
If there are problems with the internal governor, shifts may be delayed, prevented, or occur early.
If fluid levels are low, shifts will be affected.
Aged, hardened O-rings and seals will cause late shifts, flare during shifting, and slippage.

thezonster

When I changed my modulator, the harsh shifts went away.

Bertrand Daoust

If the modulator doesn't hold vacuum, it might be only the rubber cap. Not the modulator itself.

https://rennlist.com/forums/928-foru...ing-fixed.html

91 S4
Pressure control valve (modulator) part#: 126.270.927.9
Cover (rubber cap) part#: 126.277.008.1

Tails

From all of the above and your answers I would suggest that you check the pressure of the modulating valve and have it adjusted to specification relating to your vehicle.

This adjust should be checked whilst driving the vehicle as described in the WSM, however, it can be adjusted with the vehicle stationary on jack stands by installation of a pressure gauge and nipple into the vacuum modulating test pressure port.

My car had very hash shifts and it was due to excessive pressure at modulating valve. I reduced the pressure to specifications via the "T" adjustment located under the rubber cap on the modulating valve and my shifts are now extremely smooth and have been for years now. To low a pressure will cause "flared" shifts.

Tails 1990 928S4 Auto