Upgraded vacuum modulator - PeachParts Mercedes-Benz Forum

1983/300CD · #1 04-03-2018, 12:44 PM

I've been reading about fitment issues with the upgraded vacuum modulator for the 722 transmission. Apparently it leaks, o-ring doesn't seat, etc. Before I go through all the trouble of replacing this thing, does anyone have a success story with this part? Any tips or tricks? Or should I send this back and get an old style replacement? Any help would be appreciated.

tangofox007 · #2 04-03-2018, 01:10 PM

Quote:

Originally Posted by 1983/300CD

Before I go through all the trouble of replacing this thing, does anyone have a success story with this part?

Perhaps you could identify the exact part you are inquiring about. I am not sure that "the upgraded vacuum modulator" has universal meaning.

kuene · #3 04-03-2018, 01:14 PM

I had the original style green modulator. I tried to replace it with a new style and over the course of 3 days and probably 4 hours of grueling work on my back, I could not seat the replacement in the trans. It just wouldn’t go no matter what. In fact I destroyed one new style just trying to get it in. So, back went the old one with a new black rubber cap.

It was really frustrating.

Diseasel300 · #4 04-03-2018, 02:06 PM

Is your unit leaking or failing to hold vacuum? If not, let sleeping dogs lie.

SD Blue · #5 04-03-2018, 04:46 PM

What issue are you having to warrant replacement of the Vacuum Modulator?

Most vacuum leaks are due to the little black cap, which is easily replaceable, or the flex line attached to the valve. Vacuum leaks cause harsher upshifts and downshifts.

It is sort of a PIA to replace due to the adjustment procedure.

1983/300CD · #6 04-03-2018, 10:25 PM

ATF in the black vacuum line at the VCV. I need to replace it.
All I hear is the same story from everyone. I think I need to return this.
If the old style replacement lasts another 35 years, I think I'll be okay.

kuene · #7 04-04-2018, 01:23 PM

May I ask where you found the old-style green modulators? All I could find were the new ones. I even ordered on that looked like the old on in the pic - but when it arrived it was a new one.

1983/300CD · #8 04-04-2018, 03:05 PM

On Peach Parts.
I thought I had bought this new replacement from Peach, but after reading the box, I guess not. More fun than I wanted to have already. I can't trust somebody else to send me the original style, so I need to get a straight up refund, then order from Peach. Porsche also uses them, I believe.

Is the main issue the o-ring seat? Could one put some JB Weld around the housing, then use needle files to create a nice groove, simulating the conditions of the original? Or is there some other problem?

1983/300CD · #9 04-04-2018, 03:15 PM

To install this unit, you stick it in, then twist, correct?
Because there is a little wedge stamped onto the side of it that looks like it's telling me it needs to be turned.
Or does that just refer to the valve adjustment?
The unit's housing is asymmetrical, so maybe it does twist?
I might be spending $50 to find out...

1983/300CD · #10 05-02-2018, 09:53 PM

No, no twisting to install. The slope apparently just shows that turning the cap adjusts the pressure.

It took some pressing, but eventually it slid into the hole. I just tightened it up and crossed my fingers. I also replaced the flex discs while I was under there since they were 36 years old. Took my time, took all day. I got the front wheels up on ramps, and the rear wheels off the ground on jack stands. I unhooked the fan shroud, disconnected the linkage, lowered the transmission until I got reasonable access to the modulator, etc. and I remembered everything putting it back together. I then put the tester into the hole and tightened that up. Then I started it up and heard a squirting sound. I tried to shut it down, but the vacuum leak in the door decided to play its little game. By the time I pulled the hood and pushed the button, the transmission was squirting light pink airy fluid. It lost about 3/4 of a quart in 20 seconds. I guess I'm lowering the transmission again and seeing if I can get the modulator's o-ring to sit better.

kuene · #11 05-02-2018, 11:01 PM

This is what happened to me with new style green (minus the shut off problem). I tried a few time but the modulator seal would not take - leaked like nuts. I gave up and went back to old with new cap. But if yours is leaking fluid through line - not sure what.

Are you sure you have old style as the replacement. Old style has metal T pin that can only be rotated after taking off a black rubber cap. New style has a similar pin, but you can adjust a few degrees by turning the the white plastic cap - and not having to remove the cap for small adjustments within a range. That style modulator would not fit in my trans.

1983/300CD · #12 05-03-2018, 04:01 AM

I'm replacing with the "new style" modulator. I'll remove it again and see if I can file a little groove into the housing to hold the o-ring while it slides into the transmission hole. The original modulator has a deep groove to hold its o-ring in place. Maybe that's what this one needs. I can't ruin it.

MTUpower · #13 05-03-2018, 05:57 AM

pics!

1983/300CD · #14 05-03-2018, 12:28 PM

Well, it looks like I pulled a bonehead move. First of all, did I mention it's really cramped in there and hard to see anything? Yeah, well I put the screws back into empty holes last night, but one was just a threaded hole about a half inch away from the modulator bolt hole. I guess that one has a use on another version of the transmission, but on mine it's just a sucker hole. I might have been able to just move the screw, but I took it all apart again to check the o-ring. It seemed to be seated fine. It was a LOT easier with the flex disc removed. The thing didn't want to drop very far anymore, so I had to wedge a 2x2 in the driver's side.
It is hard to push the new modulator in, but I rocked and twisted it a little, and it seemed to go in. Starting it up again, no leak. Now to check the pressure...

1983/300CD · #15 05-03-2018, 08:54 PM

Some things I learned:
You don't want to put the cap on the modulator until you get a reading. I initially got a reading of 30 PSI, then with a slight touch, it shot to 70. I then had to fingertip-turn the T down about 10 turns to bring it down to 42.
Also, the banjo bolt needs to be facing forward toward the modulator, not backwards.
And you might want to keep a catch can under the transmission when testing.

Now I have a slight drip on both sides. The seal wasn't perfect before, but it looks to be leaking worse now. I'll try to tighten the bolts and see if that stops it. The leak on the other side is either the modulator or the test port. I hate to put too much torque on the bolts going into the transmission body. Should I use blue threadlocker/sealer on the test port to eliminate that? That 12MM bolt can always get cracked open again.