adjusting transmission vacuum modulator
 

Hold the reprimands! I've looked and looked in the archives and cannot locate the answer to my question. I want to soften the shifts on my 300D. First replaced a leaky vacuum modulator. This was an improvement but she still shifted a bit hard. Next I fiddled with the bowden cable. Got it to shift softer but at the expense of shifting too soon. I had read that the bowden only affected timing, not softness but it surely seemed to influence both. My question at this point is ------ when adjusting the vacuum modulator counterclockwise for softer shifts, are there just three positions (in the slots) or can I turn it several revolutions (essentially 6 or more slots)? I've read and tried to understand some of the excellent advice about orifice restrictions in parallel vacuum circuits and am coming closer to understanding --- but I could surely use some dumbed down advice. Again, thanks in advance for the help. This forum is great!

You can turn it many revoultions.

Oversimplification of the day: Before tweaking the modulator make sure the rest of the vacuum system is sending it a proper vacuum signal. 15 mmHG or so at idle, diminishing to 0 at full throttle.

Make sure you have your bowden cable and your throttle linkage adjusted correctly. There is an article on this in the FSM.

I've tinkered with the bowden cable. Checked the vacuum and it's about 13 at idle and diminished to 0 with some revs. Will beef up on linkage adjustement and maybe give the modulator a few ccw spins and see what happens. Thanks. I'm really enjoying learning how this thing works.

You should not be indiscriminately playing with the modulator pressure. Consider it a last resort if you cannot get the vacuum system to provide sufficient vaccum to the modulator.

The adjustment of the VCV should be your first mode of attack. Adjust the VCV for more vaccum to the modulator and the shifts will soften. 13" at idle does sound reasonbly close to the spec.

You can test that valve with the engine shutdown if you've got a Mityvac. You can also adjust the valve for more vacuum.

If the shifts occur earlier, they occur at a higher vacuum level. So adjusting the Bowden cable can indirectly affect shift quality.

Is the VCV the green thing on the IP? How do you adjust it?

I've found mine to shift best at around 9" at idle...

Different vehicles react differently. The SD is currently at 8" and I really prefer it there. But, if a vehicle is causing hard shifts at 10" or 11".........it's time to see if the vacuum can be increased to compensate for the hard shift.

I have a suspicion that something is amiss with his readings. 13" should not generate hard shifts.

hydraulic pressure
 

new modulators are not adjusted.
The book says (for 722.303) 2,9 bar at the port near the modulator.
I took a holow screw M8x1 and had >6 bar pressure when I started.
Several turns of the T-pin and set to 2,9 it shifts like a dream.
AND now I know that the tranny is still in good condition (with correct vac coming from IP).

Tom

OK, here's what I've learned since yesterday. Vacuum coming from the valve on IP was 13" at idle. When I revved it up quickly it dropped to zero. If I steadily increase the throttle it was 7" at 4,000 rpm. I didn't have the heart to rev it any higher. I backed off the modulator (sorry Brian, this was befroe I read your post) and I think there's some improvement, but it's still pretty stiff. I had read that it might be desirable to change the orifice to soften this shift. I'm not even totally certain which component the orifice is --- sorry, this is all new turf for me. Is it the green cannister looking this coming out of the IP valve?

It's difficult without a Mityvac, but, it does appear that the VCV is working correctly.

You can adjust the VCV if you get a Mityvac.

However, your results via adjustment of the modulator seem to indicate some success as well.

The orifice is a very small plastic nipple that is easy to miss. Look for a round piece of plastic........just about the same diameter as the vacuum hose.........in the vacuum hose line from the main vacuum to the VCV. If the orifice was slightly larger, the rate of vacuum falloff with increasing throttle position would be lessened.

Thanks, I think we're getting there. So --- if I want to soften the shift, then I would want to restrict the orifice a bit more. Correct, or backwards?

You'll need more vacuum to soften the shifts. So, the orifice would need to be slightly larger and the VCV won't be able to dump vacuum as fast.

That was my second guess. Thanks. I'm sort of beginning to understand this. Once I get it working smoothly I think I'll comprehend what I'm seeing. Thanks.

Where can I get an M8x1 screw? I have been meaning to do this forever...

Is there a spot somwhere I can find one on a benz in a junkyard?