'95 E300D 722.4 trans question on hard shift - PeachParts Mercedes-Benz Forum

dtf · #1 11-01-2017, 04:18 PM

The 4 speed transmission on my beloved '95 E300 is shifting hard when going slow from 1-2 and then from 2-3. However when I goose it, it shifts fine. I 've done the searches here and read the Bentley E Class bible about this transmission too.

The question is I don't know if it is a vacuum problem or a modulator problem. I don't have the set up to check the modulator or check the vacuum. I read that one needs to check the vacuum where it connects to the transmission? I'd like some prior information before brining it to my car guy in town.

Oh - the thing shifts great going from drive into reverse too - nice and clean, fast.

thanks!

Diseasel300 · #2 11-01-2017, 04:46 PM

Likely a vacuum issues. There is a vacuum modulator on the side of the transmission, the line going to it can have a crack or split, or the rubber boots on the ends of the line may be leaking or failed altogether. A lack of vacuum will cause harsh/abrupt shifts at low speed. You should also check your vacuum lines to/from the VCV on the IP and check your blue UFO (vacuum amplifier) for the same things.

Given this is a diesel, you're a lot better off posting in the Diesel Discussion forum, you'll get a lot more views.

Zulfiqar · #3 11-01-2017, 04:48 PM

there is no ufo on the 95 E300

the black line goes to the little unit on the injection pump that varies vacuum. Hard shifts are probably due to the points mentioned. i.e. perished rubber pieces on the vacuum line.

It is also a very good idea to have proper fresh ATF in there, it also helps a lot in proper shifting at low rpm. This transmission was tuned to take Dexron III or Dexron III-H (this is why mercon V works a treat in these later hydraulic shift mercedes boxes)

carl177 · #4 11-01-2017, 08:53 PM

I had a slightly rough 1st to 2nd at a certain RPM on the 95 E300, I changed the transmission Bowden cable when The time came to service the transmission as these tend stretch over time (Recommended by a forum member here) and that went away. The cable is like $60 bucks from the dealer.

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dtf · #5 11-01-2017, 10:17 PM

Fluid change is a good idea and going to happen any way. I bought the car w 155,000 on it and immediately had that fluid changed. It's got 206,000 right now so it's due. I looked at it today and while it doesn't smell burnt - it could be a little more red

I'll have the vacuum lines and modulator checked this week.

thanks guys!

dtf · #6 11-01-2017, 10:23 PM

Are the shift points completely controlled by the engine vacuum the 4 speed in the car?

tyl604 · #7 11-01-2017, 10:59 PM

Loss of vac will not give you harsh upshifts. It is designed to bleed off vac immediately as you accelerate and give smooth upshifts with practically no vac. You will start off with about 15 IHG of vac and as soon as you hit the pedal it goes pretty quickly to zero. Technically there may be a little vac still there at upshifts but very little. On the flip side, it will downshift quite harshly with no vac.

I know from personal experience as I have hooked up the Mityvac between the VCV and the trans modulator on my '81 300SD and have tested it both with and without vac on upshifts and downshifts. My assumption of course if that your works the same way and I think it does.

Diseasel300 · #8 11-01-2017, 11:42 PM

Quote:

Originally Posted by tyl604

Loss of vac will not give you harsh upshifts. It is designed to bleed off vac immediately as you accelerate and give smooth upshifts with practically no vac. You will start off with about 15 IHG of vac and as soon as you hit the pedal it goes pretty quickly to zero. Technically there may be a little vac still there at upshifts but very little. On the flip side, it will downshift quite harshly with no vac.

I know from personal experience as I have hooked up the Mityvac between the VCV and the trans modulator on my '81 300SD and have tested it both with and without vac on upshifts and downshifts. My assumption of course if that your works the same way and I think it does.

Loss of vac WILL cause rough upshifts at part throttle. At low throttle input, the VCV will still be supplying vacuum to the modulator to soften the shifts. At half-throttle or more, the vacuum is bled off fully and the horsepower is sufficient to prevent a rough shift. The modulator is there for a reason, it is not just for smooth downshifting while coasting.