PeachParts Mercedes-Benz Forum - Is this the Transmission Vacuum modulator?

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- - Is this the Transmission Vacuum modulator? (http://www.peachparts.com/shopforum/diesel-discussion/306919-transmission-vacuum-modulator.html)

The vacuum release valve on the IP is to mimic the inlet manifold vacuum of a gas engine, as soon as your pedal goes to rest maximum vacuum is routed to the modulator. And vacuum varies with pedal position. Pedal position is inverse to vacuum - most pedal has least vacuum, least pedal has most vacuum with a variable in between.

I tested mine too when I had hard shifts - and the fault was a leaky vacuum line from the junction behind the brake booster. The transmission shifted hard literally shocking with no vacuum - some vacuum helped soften it, thats the only way the tranny glides into gears at low speeds and low pedal application.

Quote:

Originally Posted by tyl604 (Post 2819324)

Nope, you guys have it backwards. The vav is designed to dump the vac when you hit the throttle; the tranny upshifts quite smoothely with no vac (at least mine did - time after time). I know this because I drove mine around the neighborhood with the vac modulator connected to the mityvac. With zero vac my new tranny shifted fine. But the downshift was harsh. Drove it around some more and tried pumping it up with vac (after full upshift) before coming to a stop. It downshifted perfectly with vac from the mityvac as I came to a full stop. I did this many times and it worked the same each time.

Brian - my experience with my new tranny is that it did not upshift too fast without vac. In fact it shifted about right. My experience was driving around with the mityvac hooked to the tranny. The tranny was blocked off from the vav. I reread your explanation about how the vac slows the shift 0.75 sec and that makes sense. However, I did not get anything like the necksnapping shift you mentioned during my trials, not even once. And my shifts varied between WOT and more normal driving. Brian - also I question the WOT idea. When I tested my vac line from the vav to the vac modulator, it dumped the vac all the way down to zero almost instantly at just a touch of the accelerator pedal. (Actually the mityvac has a 5-zero range and it always ended up there - so I could not really tell if it was zero but it was 5 or below).

Unfortunately, the folks who rebuilt your transmission did a poor job. The modulator isn't performing properly and you don't get the very quick shifts that should be present without vacuum.

This is very undesirable under high power conditions where the design of the system requires a quick shift to protect the clutches. If you have soft shifts under all conditions, why bother with a modulator at all? It would simply be a wasted addition to the transmission.

No I believe you are wrong no vac hard shifts

Brian and Zulfiqar's explanations seem to be the most learned to me. I go with them. But for sure you do not need vac for an upshift. Maybe a bit of vac helps but that was not my experience. And it sounds just like the old time MB engineers to figure out a way to slow the shifting by 0.75 seconds - they just used to do value engineering like that. My gut feel is all that value engineering went out the door in about 1992 or so. Quite happy with my shifts now.

If you are getting hard/bad upshifts with no vac, someone should be able to figure out why. I am going with bad modulator so suspect you may be on the right track. After you get it all sorted out, why not try the trick driving around with the mityvac hooked up to the new modulator. Test the upshifts with no vac, then pump it up and come to a stop. Be interested to see if your does the same as mine did.

While getting my head around and fiddling with the tranny of more than one MB with this type of 4 spd setup, the shift shock is also affected by case pressure, meaning that if for some reason the VRV is goofy, causing erratic shifts the modulator T handle can be shifted to reduce shock too. Some mechanics have tuned gas engined cars that they shift soft at WOT redline (scary but they have done it as the customer asked).

Keeping that in mind, it is paramount that the transmission case pressure be set to factory recommended reading by tuning the modulator T handle with a pressure gauge on the trans, only then can the shift quality be tuned correctly on these transmissions.

Can someone tell me what size tool is required to remove the two hex bolts supposedly holding this modulator on. Its very tight and not sure if I need a special head that I dont have. Thanks

Off the top of my head I'm thinking 10mm. You should not need anything larger than a 1/4" drive socket/ratchet and maybe a short extension.

Can someone who has recently removed their transmission vacuum modulator give me their tip on how they removed the two bolts that holds it in? Its not a 10m but a hex head bolt. and my hex is too long to get a ratchet on it plus my neck and back are shot and cant stay in one spot for too long in removing this.

the space to work is extremely tight. Thanks

Quote:

Originally Posted by jmbeam (Post 2821396)

I feel your pain, I remember that job being a pain too. IIRC I used one of these:
Amazon.com: VIM Hand Tools SHM400 9 Pc. Metric Stubby Hex Socket Set: Home Improvement

Use a small flex-head 1/4" ratchet (I think mine came from Harbor Freight). I also used one of my favorite tricks for dealing with hard-to-see fasteners---use a strong magnet to hold a dental mirror to the underbody of the car in such a way that you can see what you're doing in the mirror.

Alex

Quote:

Originally Posted by Zulfiqar (Post 2819623)

This is very interesting - would be nice to see a DIY on this sometime.

For the record, when I lost vac to the modulator, it felt like there was an angry mule kicking in the trunk . . . . . on the upshifts.

Quote:

Originally Posted by tankowner (Post 2823408)

This is very interesting - would be nice to see a DIY on this sometime.

Not much to write up. Search for "transmission pressure tester" on eBay. There's only one place to plug it in. The pressure spec is not listed in the regular FSM but in the transmission service manual, which someone here should have a copy of. You just measure it at idle and turn the little tiny t-handle until the gauge reads correct. It's not likely to be off, though, in my experience. IMHO 50% of diesel Benz shifting issues can be traced back to vacuum leaks and 49% to improper adjustment of the VCV and/or accelerator/upshift linkages by incompetent mechanics. 1%, if that, are caused by actual internal transmission failure. If I had a nickel for every time someone on these forums wrote "My transmission wasn't shifting right so I spent $3,000 on a new one, and it still doesn't, what am I doing wrong?" I would be a rich man.