300SD vacuum information

Vacuum to the transmission's modulator is controlled by a vacuum valve on the throttle linkage - it's on the injection pump. That vacuum valve is not repairable.

The principle is: high vacuum to the transmission on light throttle - smooth shifts - declining with throttle opening to no vacuum to the transmission at full throttle - firm shifts.

Suggest you check the physical condition of the vacuum line between the vacuum valve and the transmission's modulator - a leak there will make all shifts hard.

My 1980 M-B manual lists the following test.

At the modulator vacuum control valve (on the injection pump) is a "Y" fitting. Disconnect it and connect the vacuum gauge and source. Run the vacuum source until a .5 BAR reading is obtained. Then move the throttle lever slowly to the full load (open) position. The vacuum must drop continuously to zero at full throttle.

Hope this is the answer you seek.

It does! I did check the vacuum line to the modulator and it held 20" of Hg. So I think it is OK. Your numbers are a little higher then I was getting ie.) 11 inches or 375 milibar at the "Y" for the transmission vacuum take off hose. The actual pump has a white plastic fitting that goes to a single line then has green barrel filter like looking device in line before they divide into the black "Y's". I checked the distal side of the green device and got the same reading. Should I go all the way down the while plastic on the pump? does that mean I need to rebuild the vacuum pump? wolf1

Well I did not get to 500milibar got my usual 375 reading ( My gauge may be off, I will have my garage check this too) when I accelerated as you said it did drop to zero as I got into high RPM's. I really do not want to tackle the pump itself. Spare me Oh Lord! wolf1

I'm not exactly following what's been tested. The idea is to connect the vacuum source and gauge to the (white plastic) vacuum modulator control valve and then see if the valve holds vacuum at closed throttle (engine idle) and then that the vacuum reading declines straight line as the throttle is opened (w/ engine OFF) to full throttle.

It was a good idea to test line to transmission with the vacuum gauge.

If the car's vacuum powered brakes work normally I'd leave the engine's vacuum pump alone.

If the vacuum control valve is OK I'd go on to the transmission modulator.

Incidently, for future reference, some of these engine vacuum pumps can be rebuilt with a kit. If you have any doubt about the engine's vacuum pump it's best to replace it, I've read of cases where the pump failed and its broken parts dropped into the timing chain and ruined the engine.

If you have oil in the vacuum lines replace or rebuilt it immediately. There are a couple of really bad things that can happen.

11" is bad. 23" is what a new pump makes.

I am not sure how to check the vacuum control valve. I think it is item #65 on the vacuum line layout. I checked the "Y" where the transmission vacuum line comes off both at idle and with full throttle and it went from 11" to 0 (as you predicted). I also went back closer to the vacuum control valve just ahead of a green item (identified as the vacuum damper on the diagram on my the car) to see if I had more or less pressure there. Same reading 11". I note that the pump is actually in front and the control system sits atop the injector. It may not be that hard to rebuild as I feared. Have any of you done this rebuild? I really need to pay someone to see if the values I am getting are accurate or not. This all started because I wanted my transmission to shift smoother between 1st and 2nd gear . WOW I really got off on another "project" all together. Thanks for the good suggestions. wolf1

STOP!

The system works, leave it alone. Accept the fact that some 1 to 2 shifts are hard, look for causes when ALL shifts are hard.

You seem to be looking for something to do.

You are right again! This is my project car and I enjoy figuring out various aspects of it's inner working. I think you are right again when you assert that the "system is working" ie.) vacuum. I did slow down since it made no sense to have it all working and my readings be low. The hard shifting is for real and as part of that research I read to "make sure the vacuum system is OK" since the modulator needs that to work properly. The throttle adjustment aspect I have yet to learn about. Again THANK YOU for "slowing" me down. wolf1

There's a little calibration device you get from MB that makes setting ghe gap in the VCV an easy job.

Here's what you need:

If you have any questions write to the author. His last name @mbz.org will work and he's very helpful. Probably too much so as I have his VCV adjustment thingy that I really need to return to him.

Excellent article for Wolf to follow.

I sent a reply yesterday but it did not appear in the list. Thank you for ALL the information. wolf1

OK we are back to basics here. I decided to adjust the valves (needed to be done 8 out of 10 out of adjustment) but in doing so I had to take apart to throttle linkage which includes the cable on the right hand side that goes to the transmission. It is literally "snaked" on itself in a sigmoid fashion. As I accelerated the car (after the valve adjustment) with the hood open I could see that as I revved up the engine there was no tension on that cable. How should that cable look at "rest" and as you start to get to say

1500 RPM. It seems you shift out of first at 1500 to 2000 rpm and I would think that cable should have some use during that phase of acceleration but mine has so much slack in it I cannot see how it is even involved. My guess is it should be and that may in fact be the main hard shift problem cause. wolf1

The tranmission modulator cable should be adjusted as follow...

First of all, the tranny cable should have a spring like tension on it... when you pull it, it should retract by itself too.

With engine off...On full throttle, the tranny cable should have no more extra slack when you pull the tranny cable out further.

It seems to me your shift point is too low... so what you have to do is adjust the cable so there is more tension (adjust the cable tighter).

Do a trial test... after every adjustment, do a normal driving... the shift point should rise as you do the adjustment. I am not sure on diesel... but I think the shift point should be around 2300RPM during normal acceleration.

As you adjust and gets to the point where you like it... do some spritited test drive... like from stop light... do a heavy acceleration to see if tranny shift right and aggressive.

While cruising... do a sudden acceleration... see if tranny downshift immediately and accelerate accordingly.

Thanks Tiger, I have read several archived posts on how to do this and they all indicate that there should be a fairly straight connection on this cable. Mine is as loose as a goose. I will work on this and report back wolf1

>

acceleration.

Boy this really helped a lot. I will now profess my ignorance..what does the throttle cable have to do with the transmission shifting smoother? I can see the logic of the vacuum modulator but what does this cable "hook" into the transmission that make the shift work better? wolf1

Hehehe... a bit tricky... okay... one step at a time.

Vacuum modulator helps tranny shift smoother at all RPM... depending on the available vacuum pressure, it determines how firm (low vacuum) or how soft (strong vacuum) to shift. The shifting firmness is necessary when accelerating or otherwise the friction plates inside the tranny will burn out... sort of like manual tranny...

The modulating tranny cable tells the tranny how hard you are accelerating so it adjust when the tranny should shift gear. When driving normally, tranny see light throttle and will proceed with quicker shifting... meaning you willl see 4th gear very soon. When you accelerate hard, it will tell tranny to hold each gear longer so you can accelerate faster.

So by adjusting the cable tighter, it kinda fools the tranny that you are accelerating harder than normal... and hold gear longer. Then again, the adjustment goes out of whack as the cable stretches due to age, so it need proper adjustment once in a while.

Now, as for heavy acceleration... tranny senses you stomped on your gas pedal... so it tells itself that it has to downshift so you can drive the bat our of hell... hehehe.

Lastly, when tranny age, the valve body will go bad due to leaking seals... and the tranny will shift very funky and fast that no matter how tight you adjusted the cable, it won't make a difference... then you have to decide whether to do a valve body service or new tranny.

Thanks Tiger. I think you know my level of car expertise. :-) wolf1