Vacuum modulator installation problems.......'81 617 - PeachParts Mercedes-Benz Forum

i-osprey · #1 08-19-2007, 10:24 PM

I recently replaced a faulty transmission vacuum modulator on my '81 300D and immediately after I did so I noticed that it would shift WAY too early. I figured that the modulator needed to be adjusted so I tried that. No improvement.

Before I replaced the modulator it shifted very hard and very late with or without the vacuum line attached to it. That is one way I knew it was bad.

Now when I remove the vacuum line, it get the same early, low rpm shifts.

What gives?

Could the transmission have shifted when I removed the cross member that was supporting it? If so, would this cause this problem? I also can't get it to downshift, ever. I never had either of these problems before and they both started IMMEDIATELY after I replaced the vacuum modulator.

Any ideas?

LUVMBDiesels · #2 08-19-2007, 10:29 PM

The bowden cable determines when to shift based on the throttle position. You might have slipped it off when you were working on the transmission.

It runs from the top of the valve cover to the transmission. I know that on my car after I adjusted it the early shifting issue went away. Loosen it to make the trans shift later.

Brian Carlton · #3 08-19-2007, 10:52 PM

Quote:

Originally Posted by LUVMBDiesels

The bowden cable determines when to shift based on the throttle position. You might have slipped it off when you were working on the transmission.

It runs from the top of the valve cover to the transmission. I know that on my car after I adjusted it the early shifting issue went away. Loosen it to make the trans shift later.

Does the '81 W123 have a Bowden cable? I was hesitant in responding because I was unsure of the year of the changeover.

i-osprey · #4 08-19-2007, 11:52 PM

It doesn't have a Bowden cable.

Is there something else similar like a pressure control rod or something like that?

I can shift through all the gears but I have to do it manually.

LUVMBDiesels · #5 08-20-2007, 05:57 AM

My bad... I thought the '81 had a cable. It does on the W126.

i-osprey · #6 08-20-2007, 02:10 PM

I am completely befuddled by this.

What would cause this to happen?

Brian Carlton · #7 08-20-2007, 02:18 PM

Quote:

Originally Posted by i-osprey

I am completely befuddled by this.

What would cause this to happen?

On the earlier transmissions, the modulator performed the function of delaying the shifts under higher load. Presumably it does this task when it sees lower vacuum levels.

I believe you'll need a vacuum gauge for further diagnosis. If the transmission will immediately shift up without any delay, my guess is that it has full vacuum all the time and there is no modulation. But, removing the vacuum from the modulator should provide the exact opposite effect. The fact that it does not seems to indicate that the modulator is not functioning (improper installation).

However, I don't have any experience with those systems.

i-osprey · #8 08-20-2007, 06:00 PM

I do have a gauge and I have tested the modulator.

Going back to when I had the bad modulator on the tranny. It would shift very late and at very high rpm only. It did this whether the vacuum line was attached to the modulator or not.

Immediately after installing the modulator, it started shifting very early and at way too few rpms.

I agree that it seems that something in the installation went wrong but what? You just unscrew the old one and screw the new one in. The only thing I can think of is the spring and rod that goes between the modulator and whatever is inside the transmission. I remember removing the old one and the rod and spring fell to the ground. I then attached them to the new modulator and installed it.

I just don't know what I could have done or not done that would cause this problem.

i-osprey · #9 08-21-2007, 08:36 PM

Could I have tightened the modulator too much?

Would this change the internal tolerances too much?

tobybul · #10 08-21-2007, 11:47 PM

Quote:

Originally Posted by i-osprey

Could I have tightened the modulator too much?

Would this change the internal tolerances too much?

I didn't read you say that you turned the T-adjustment of the modulator. Try turning it CCW one notch at a time and see how it reacts. If the mod of the 81 is the same as an 80, thats what I did to mine and it stretched-out the shifting.

JimmyL · #11 08-22-2007, 12:04 AM

Quote:

Originally Posted by tobybul

I didn't read you say that you turned the T-adjustment of the modulator. Try turning it CCW one notch at a time and see how it reacts. If the mod of the 81 is the same as an 80, thats what I did to mine and it stretched-out the shifting.

I second this info. Pull up on some ramps, pop off the cover, pull out a little and rotate the adjustment tab and go for a drive.
Do you have the 3/2 valve on top of the valve cover? I had to change locations of the vac line to the tranny on my 3/2 valve.
You are shifting early because you have too much vacuum. Simple as that. "Why" is the hard part, but Tobybul has you in the right direction I believe....

i-osprey · #12 08-22-2007, 05:50 AM

There is NO VACUUM!!!!

When I remove the line I get the same early shifts. This happened after I removed the faulty modulator and installed the new one.

I repeat, I have removed the vacuum line and still get early shifts.

What the hell did I do to it?

dieseldan44 · #13 08-22-2007, 09:54 AM

i-osprey,

If you have installed a new modulator and you are having problems you need to get your T handle set to the correct position. I dont know if this is your entire problem, but this should be part of your process.

http://www.peachparts.com/Wikka/Trans722VacMod

Hope this helps,
dd

Brian Carlton · #14 08-22-2007, 10:24 AM

Quote:

Originally Posted by i-osprey

There is NO VACUUM!!!!

When I remove the line I get the same early shifts. This happened after I removed the faulty modulator and installed the new one.

I repeat, I have removed the vacuum line and still get early shifts.

What the hell did I do to it?

I've been thinking about your situation and have a theory:

The first modulator was simply stuck. It would not function whether supplied with vacuum or not.

The new modulator works fine, however, as you mentioned, you have no vacuum supplied to it. Therefore, you'll get early shifts all the time. Can you test this theory and confirm that the line to the transmission reads zero vacuum under all conditions? If you can confirm this, then you can forget about adjustment of the modulator and we need to find out the location of the vacuum leak.

It's entirely possible that you had two issues. One being a faulty modulator and the second being a massive vacuum leak.

i-osprey · #15 08-23-2007, 05:10 AM

Quote:

Originally Posted by dieseldan44

i-osprey,

If you have installed a new modulator and you are having problems you need to get your T handle set to the correct position. I dont know if this is your entire problem, but this should be part of your process.

http://www.peachparts.com/Wikka/Trans722VacMod

Hope this helps,
dd

As far as I know, if you don't have the vacuum line attached then adjusting the modulator will do nothing.

From what I understand hard and late shifting is exactly what is supposed to happen when there is no vacuum. I have no vacuum (because I detached the line) but I am getting very early shifts. I did adjust the t-handle first but it did nothing. I then decided to remove the vacuum line and nothing changed.

In a nutshell, I removed a faulty modulator and replaced it with a new one. Just doing this caused hard, late shifts to turn into way too early shifts. Again, as far as I know a hard late shift is exactly what should happen if there is NO vacuum as is the case with my car at the moment.

I have no vacuum going to the new modulator but I am still getting very early shifts.