From hard shifts to flaring
 

I followed the instructions to remove the EGR vacuum lines from the top of the valve cover on my 1981 300 SD.

The hard shifts disappeared, but I now have SEVERE flaring. From what I have learned from Brian Carlton, this is caused by too much vacuum. This makes sense, as one of the pieces of tubing I removed exhibited a lot of little dry rot type cracks. Perhaps those cracks made it so it was shifting hard, but not flaring?

I am stuck at work but I have sent my wife to Sears to buy a Mighty Vac for me. As you can tell, I have no experience with one and have not had the need for one until now.

I have been searching the archives, but I’m running out of time, as my employer’s computer system only allows us 2 hours of internet time a day. I am looking to find exactly where the vac modulator is located and how I go about adjusting it.

Modulator is located on the driver side of the transmission. It's green and should have a black cap on it. You remove the black cap and their should be a little metal knob. I believe you turn it clockwise to firm up the shifts.

You're not interested in the vacuum modulator.

You're interested in the vacuum control valve. This shall henceforth be known as "VCV".

This valve is white in color and sits atop the IP. It has a vaccum line going into the top of the valve with a green damper inline. You'll see this green damper and will know that this is the valve.

The line with the green damper is connected to the main vacuum line with a T. One side of the T goes to the supply vaccum and the opposite side of the T goes to the transmission.

What you will do is to remove the supply vacuum line and connect up your Mityvac. Now, you've got a Mityvac on one side of the T and the transmission on the other side of the T and the VCV on the bull.

Pump up the Mityvac and see if it holds vacuum. I presume that it will. Get a reading and report back with the result.

Watch the Mityvac and slowly open the rack. The Mityvac gauge should fall gradually the further the rack is opened. When the rack is fully open, the reading should be close to zero.

All these tests are performed with the engine not running.

Report back with results.

Brian,

How do you "open the rack"? It that by moving the throttle linkage from idle position to WOT?

Oh, BTW, thanks again for your help. I ordered the orifice #63 from the diagram. It will be in this week. MB showed it to be 2mm inner diameter for the Federal emissions group. Also, MB does not list the part as an orifice any more. They now call it a nozzle. FYI, thier vacuum diagram was a bit different than the diagram posted here, and they had to find an older parts guy who knew about the vacuum systems on out older MB's

I will post a part number when I get it.

Brian, I'll be watching this thread as I need to do the same with mine.

Yep, that's how you do it.

Good job. It's quite difficult to find a parts man that knows what you're speaking of.

Please post that p/n. It's desperately needed by many members.

Reporting Back
 

Brian,
I believe I followed your instructions to the letter. I left the first hose from the "T" to the damper and on into the VCV in place. I left the second hose from the "T" to the transmission in place. I removed the third hose from the "T", the one coming from the vac. pump. Into this hole on the "T" I placed the Mity Vac.

When the Mity Vac handle is squeezed I read 11 on the guage, but it does not hold. I start losing vacuum immediately, down to 0 over the course of 23 seconds. When I open the rack, the guage falls to zero almost instantly.

I take it these results are a little different than you had expected?

For my drive home from work today I disconnected the vac line at the damper above the VCV. The flaring went away, but the hard shifts were back, as would be expected. Of course, I reconnected the vac line in order to turn the car off.

What do you recommend I do from here?

Yep, unfortunately, you've got two problems.

The first, but not all that severe, is a vacuum leak at the modulator on the side of the transmission. This is usually caused by leaks in the rubber cap. But, 23 seconds is not a catastrophic leak. I'd like you to fix it because it affects how we do the remaining adjustments.

But, in the short term, until we do get it fixed, we will change the procedure and use the engine vacuum as the source, because it's unlimited.

Disconnect the line to the transmission and connect the Mityvac to the output from the VCV.

Start the engine.

Get a reading on the Mityvac gauge (don't pump it......just read the gauge).

I expect that it will read 11" as you previously reported. We need to reduce this to about 8" or so.

Look on the side of the VCV for a domed cap. You may or may not have it. If so, then carefully pry the cap off the VCV. There is a locknut and a small screw beneath. Release the locknut and turn the screw clockwise by about 5 degrees (NOT MORE). Watch the Mityvac gauge. You're looking for 8". When you get it, tighten the locknut and snap the cap back on.

Reconnect the transmission line and take it for a drive. Report back.

I must be doing something wrong
 

On the three way connector I have the vac pump source line hooked in. I have the line to the VCV hooked in. The port for the trans. is empty. I disconnected the line coming from the VCV, the lower line coming off of the side. This is where I hooked the vac. guage. I started the car and nothing happened on the guage. It stayed at zero.

I tried different things such as occluding the trans port on the connector with my finger, hooking the vac guage up to the input of the VCV and so on. The guage stayed at zero. The only difference was that when I did reconnect the trans line, I could hear air moving somewhere around the VCV.

Now on top of all this I see that the "U" tube for my turbo has fallen off AGAIN.:mad: No matter how tight I get those clamps it pops off, but first things first.

The leak at the modulator, is this best fixed by replacing the rubber cap on the modulator? Is it a simple task?

The port to the trans must have the Mityvac hooked up to it.

The lower line coming off the side is not relevant here. Just leave it connected as usual.

I wish to see the vacuum to the trans at idle in the current configuration.

The information you requested
 

With everything hooked up with the exception of the vac. guage being in the transmission port of the 3 way connector, it reads 11, just as you predicted. Now I'll try to adjust it as you instructed.

OK, now see if you can find the small dome on the side of the VCV, facing you, just forward of the hose connections.

Follow the instructions above.

from bad to worse
 

I was able, with some difficulty, to pry off the plastic dome. I loosened the small nut and then started the car.

I don't know what I may have dislodged in doing the above, but the vac guage then read 15".

Behind the lock nut on the threaded post is another nut that is flat just on two sides. It seems to move rather freely, bouncing around as the car idles. I did move it a few degrees clockwise. Vac stayed at 15".

Thinking that I may once again misunderstand the directions, I tried to move the threaded post on which both of the nuts are on. I was unable to do so, as I could not get a purchase point of any type for leverage.

Should I now try to tighten the lock nut back down and see if that some how reduces the vacuum?

You're in the right place. But the description is not what I'm familiar with. The valve should have a locknut and a slotted screw. The locknut gets released and you turn the slotted screw slightly to adjust the vacuum.

If the screw is not reducing the vacuum then the valve is not functioning properly, or, the orifice is not in the line.

You must get this vacuum down to about 8" to correct the flaring.

Brian, thanks for the confirmation on moving the rack.

I'll get part number for all of them, including the CA emission cars. IIRC, when I looked at their monitor the Cali cars called for 1.1 mm diameter for #63. I do remember the part number started with 116. I found and copied the part numbers posted by Sam Ross, but they did not match, according to the parts guy, what is on the MB part file. I did bring a copy of the diagram to show them exactly what I was looking for. At first they told me it was just a connector. I told them that was incorrect, and after some back and forth, they called out the older guy. The younger ones apologized and said they just learned something new. The older guy also told me that part names in the FSM are sometimes different than what the parts dept has recorded and looks at.:eek: