Rough Transmission Shifting - PeachParts Mercedes-Benz Forum

Sdsbenjamin · #1 01-30-2017, 09:26 AM

Replaced glow plugs on my 1995 E300 Diesel and the plastic vacuum lines connected to the intake manifold connections are brittle and got broken. After that my transmission began to shift with jolts. Is this because of the broken vacuum lines and would replacing the line remedy this problem?
Are these plastic lines readily available?

Diseasel300 · #2 01-30-2017, 10:35 AM

Yes to both questions. The transmission has a vacuum-based modulator to control the firmness of the shift quality. With low vacuum (or no vacuum) to the modulator, the shifts will be VERY firm. You may have to order the plastic vacuum line, our forum sponsor should have it on their website.

Sdsbenjamin · #3 02-01-2017, 08:09 AM

Thanks for the response and explanation. Was able to find and order the plastic line from Mercedes Benz parts.

tyl604 · #4 02-01-2017, 08:55 AM

The up shifts should be fine with no vac. It is designed to bleed off vac when you hit the throttle and shift smoothly. Only needs vac for downshift.

Diseasel300 · #5 02-01-2017, 02:07 PM

Quote:

Originally Posted by tyl604

The up shifts should be fine with no vac. It is designed to bleed off vac when you hit the throttle and shift smoothly. Only needs vac for downshift.

Vac is necessary for coasting downshifts and upshifts at low RPM/load. Lack of vacuum results in a firm/abrupt shift. The same applies for a gas or diesel transmission in the 722 family.

tyl604 · #6 02-01-2017, 02:29 PM

That may be true generally but it is not true for my 300SD. I have personally tested it when I replaced the tranny. It goes to zero vac as soon as you hit the throttle and gives very smooth upshifts. I doubt if you could drive as fast at 10MPH and still have any vac distributed to the trans modulator on mine.

It clunked badly however on downshifts. When I pumped up the Mityvac, the downshifts were perfect. If there is some difference with very show coasting downshifts, I did not notice it at all.

Diseasel300 · #7 02-01-2017, 02:42 PM

There are 2 types of downshift. One is as you reduce speed, the transmission shifts down. The other is under WOT or acceleration to allow a faster rate of speed increase. There are very different operating conditions for both.

On the transmission shifting down through the range as you decrease speed, usually the throttle is mostly closed and you will have "high" vacuum to smooth out the shift. Under a WOT or acceleration downshift, there will be little to no vacuum to firm up the shift.

If no vacuum is present for speed reduction downshifts you will get clunks and jerks as the transmission shifts down (very abrupt shifts). If too much vacuum is present on a WOT or acceleration downshift, you will get a flared shift or a slipping transmission.

The vacuum situation is complicated in these cars. On a gas engine the vacuum is a direct proportion of load and throttle position. The diesels have to "fake it".