I have searched and followed many suggestions. I have a 1981 300SD.

Very harsh shift and down shift. I have adjusted the vacum modulator all the way in and all the way out to no avail, never once did the tranny flair. I have adjusted the transmission vacum at the IP pump to no avail. Plugged up the vac. line going into the car for ignition shut off, hvac system and doors. No difference. Plugged the line going to the valve cover vac lines to no avail.

I read that not enough vacum will cause hard down shifts. Is the vacum modulator on the transmission the culprit? Yes I changed the hard vacum line and the rubber hose to the control modulator to make sure I didnt have a leak.

I do not have a vac gauge. Sorry.

Thanks for your help.

I don't personally know this transmission but typically this can be caused by an over filled transmission, linkages out of adjustment, needing to be set to the length specified in the FSM....or a problem with the Bowden cable....

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No further diagnosis is possible without a vacuum gauge. A Mity Vac is the tool most people prefer. You need to T in the Mity Vac to the transmission modulator line, run the hose to the Mity Vac out the cowling and rest the Mity Vac on the windshield where the gauge is visible and drive around recording the vacuum levels as the car accelerates and shifts. Report back the results.

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1984 300SD 315k--daughter's car
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You do not need vacuum for upshifts as I remember, only for downshifts. Isn't that right Kerry?

Tee the Mityvac in like suggested and do the reads. Then you can just connect the Mityvac to the tranny by itself. As you are shifting up it should be smooth with no vacuum. When you get ready to come to a stop, pump up the Mityvac and it should downshift smoothly. It has been 2-3 years since I got mine adjusted (with Kerry's help) but that's what I remember.

In normal driving when you go WOT - wide open throttle - the vac is supposed to drop to zero quickly and then you get good upshifts. (Good thing to check when you tee in the Mityvac; just put it in neutral and give it some gas; vac should go from say 15 to zero immediately.) Vac comes back when you let off the throttle and come to a stop. So vac is needed only for downshifts. My recollection is that when I disconnected the vac, the downshifts at a stop sign were like a slide hammer, really jolting.

As I remember you only adjust the vac modulator a few clicks either way. They say the new ones which do not have a black cap are not supposed to have the cap removed and then put back on but I did mine several times; seems to work fine. The original ones with the black cap supposedly were the only ones with a cap that could be removed, adjusted, and then snapped back on. Opinions vary about that.

However, Kerry is the expert. Listen to him.

The vacuum controls the modulation pressure inside the transmission. That determines the shift quality (harsh or soft shifts). The modulation pressure needs to be high when the power output of the engine is high and low when the power output of the engine is low. MB uses vacuum to signal the power output to the transmission. It isn't just there for downshifts.

When you have shifting problems (slipping/harsh shifts) than you should start by measuring the vacuum as described above by kerry. You need a vacuum gauge for that.

The vacuum modulator is the last port of call. You should only adjust it with a pressure gauge attached to the transmission. The modulator for the 722.3 and 722.4 transmissions has a black rubber cap. That cap holds the vacuum, so if it is damaged or loose, replace it with a new one. It also covers the adjusting key. The vacuum modulator for the 722.1 transmission has a white cap and a lock ring. If you remove the cap, you can see a rubber O ring, that is there to hold the vacuum. Replace the ring if the cap is loose.

Most likely a vacuum leak somewhere. I had the same issue twice and fixing a leaking vacuum line fixed them both. Follow this link for a good how-to on troubleshooting and fixing a vacuum leak Mercedes Vacuum Troubleshooting

Glenn

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1983 Mercedes 300D 376K (diesel commuter)

Goverts post hit on a problem that I had not thought was a problem. The modulator cap is badly deteriorated!

Just ordered a new one. If no improvement I will get a vacum gauge as Kerry suggests and let you know what the gauge says.

Thanks for the help.

Scot

Well, the new modulator cap has been installed and has delivered my long sought after smooth shift!

All the reading I have done in the past I had never run into a comment about the modulator cap. Shifts like new now.

Thanks for the help and tips everyone.

Scot

When I experienced extremely Hash down shifts to the it was because I had accidently disconnected the Vacuum Line at the Transmission Modulator (722.315).

All of My forward shifts seemed only slightly different.

It could be your modulator is not working.
As said before a Hand Held Vacuum Tester a.k.a. Mighty Vac is a valuable Tool For testing Vacuum Leaks and testing Vacuum Components for leaks like the Trans Modulator or the Vacuum Shutoff Valve/Servo on the Fuel Injection pump.

If you are just checking to see if the Lines have Vacuum you can get by with one of those Fuel Pressure/Vacuum Gauge combinations.

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84 300D, 82 Volvo 244Gl Diesel

Mityvac's can be had from ebay for about $25 ... just be patient

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