Vacuum Modulator Cap Question - PeachParts Mercedes-Benz Forum

Bengoshi2000 · #1 03-30-2019, 11:01 AM

Still chasing down vacuum leaks, but slowly getting better. I do have one question I can't seem to find the answer to... does the cap on the vac modulator hold any vacuum? Mine is dry and split.

If the cap needs to hold a vacuum then I've got to replace/repair it before doing much more.

BWhitmore · #2 03-30-2019, 11:23 AM

I believe the cap on the modulator is simply there to help keep dirt out of the modulator.

Bengoshi2000 · #3 03-31-2019, 07:15 AM

Here's the full issue I'm having: Hard shifts that seem to be related to the vacuum system (and the vacuum pods for the climate control).

What I've done:

Verified my vac pump is pulling 21+ hg.
Replaced 4 way connector and associated hoses (just off the vac pump)
Replaced the Y (above the master cylinder) that feeds the ccw vacuum manifold)
Replaced the Y and associated vac hose coming off of the VCV.
Replaced vac hoses on the top of the BFS.
Replaced the rubber hose that connects the hard line to the vac modulator at the tranny.
Returned the vac modulator adjustment to starting point (I had previously adjusted it 1.5 turns ccw).

The transmission was shifting very nicely... in fact, it's never shifted so well in the 18 months I've owned it. Throttle was MUCH lighter to the touch with better acceleration. CCW was working properly (as it never had before... specifically, I was getting heat at my feet). It was fine for about 5 days of daily driving. Then over the drive to work one day, shifts started getting hard, and the ccw stopped working correctly. Throttle response didn't change much (if any).

Obviously another leak has opened up somewhere. No visible splits or cracks anywhere (that's not necessarily conclusive, I know). The only thing I noticed was split was the cap on the modulator.

I'm loathe to wholesale replace all of the rubber (and maybe even the hard lines) under the hood, but I's the only way to be sure. I was just hoping for an easy/ier fix.

dieselbenz1 · #4 03-31-2019, 08:36 AM

So you had replaced the 4 way connector which supplies vacuum to egr system, climate control and transmission. Diagnostics maybe easier if you can live without climate control and unplug and seal that port. You will loose boost if you disconnect your egr system but maybe worth a try to determine where the origin of your problems come from.

How free is the slide on the bowden cable and throttle linkage, mine was fused when I had gotten mine but after some steel wool and silicone spray it shifted real smooth.

dieselbenz1 · #5 03-31-2019, 08:46 AM

Is your car fitted with the blue flying saucer?

Diseasel300 · #6 03-31-2019, 09:31 AM

Buy yourself a Mightyvac or a clone and go to town finding the leak. The VCV/Modulator connection is a controlled leak. Be looking somewhere else for your vacuum leak, probably the climate control having a torn pod, or something else in there leaking. If the modulator holds vacuum when you pump it down with the Mightyvac, it isn't the problem - move on. You'll go mad replacing parts and guessing until you shell out the $15-20 for a Mightyvac clone (or more for a real one) and start testing things so you know FOR SURE where the leaks are.

5cylinder · #7 03-31-2019, 11:18 AM

First and based upon your latest post, the vacuum modulator cap is the least of your worries.

Second and to re-state Diseasel300's post more succinctly:
Of all that you have posted, what I have not read is that you have methodically gone through the entire vacuum system isolating each small portion. Stated even more succinctly: You have failed to find the fault(s). The practical and sensible way to do this is the very same way that professional technicians do it: Use a Mityvac hand pump and reference a vacuum diagram from the applicable Factory Service Manual. And please, for your own sanity and pocketbook, do not be foolish and use a golf tee to plug the end of a vacuum line. This is classic DIYer stupidity. Leave golf tees for the driving range or course.

Bengoshi2000 · #8 04-08-2019, 06:49 AM

Follow up for posterity...

I've now gone through the system (at least under the hood) with my mityvac. I found the problem pretty quickly (though I took the time to check the whole system). I started by individually blocking off each branch of the 4 way (off the vac pump) and taking her for a drive. When I blocked off the "rat's nest" line, I was rewarded with a car that shifted smoothly and and showed a slightly improved throttle response. Bingo.

On inspection, I found a hardline had pulled out of one of the rubber Y's in the rat's next. The Y was dried out and split at the end where the hardline inserts. I had to physically touch the line as it was perfectly lined up with the Y connector making it look like it was inserted properly. So, I reinserted the hardline, zip tied the rubber Y connector around it, re-plumbed the hardline back into the 4 way, and took her out for a drive... shifts were only slightly harsher and some sluggishness returned. In other words, it was back to where it was previously.

I didn't bother with further troubleshooting on the rat's nest side. I'd read the threads on the "pressure activated waste gate swap" many months ago... re-read them over the weekend. I ordered the parts from ATP this morning. I'm going to eliminate the rat's nest and it's associated vacuum headaches.

tyl604 · #9 04-08-2019, 09:02 AM

It sounds to me like your vac is not bleeding off fast enough when you hit the throttle. Your 300D is designed to upshift with no vacuum (not technically 100% accurate but good enough for now) and it needs vac for smooth downshifting only. You should have about 12 inHg of vac behind the VCV (between it and the tranny) at idle. When you hit the throttle the VCV or maybe the trans modulator bleeds off the vac almost immediately to give you a smooth upshift.

So I am not sure whether the trans modulator or the VCV bleeds off the vac but I think one of them is a problem.

To test just pull the plastic line to the trans modulator off the VCV. Drive around a while and you should get smooth upshifts but a clunky downshift when you come to a stop. Or as I have done hook up the Mityvac to that open plastic line and pump it up just before coming to a stop; it should downshift smoothly it you have vac pumped up. Release the vac and drive and you should get smooth upshifts. (This was on my 300SD but I think yours works the same).

87tdwagen · #10 09-26-2020, 03:32 PM

Quote:

Originally Posted by Bengoshi2000

Still chasing down vacuum leaks, but slowly getting better. I do have one question I can't seem to find the answer to... does the cap on the vac modulator hold any vacuum? Mine is dry and split.

If the cap needs to hold a vacuum then I've got to replace/repair it before doing much more.

The cap does indeed hold vacuum, and a split cap will negate the module completely due to the vacuum loss.

Just had similar hard shifting and some flaring issues on my 300TD and after tracing down vac leaks with my mity vac it came down to the cap was leaking. Ordering a few spares now.

In a pinch, for diagnostic purposes or until your replacement cap comes in, cut yourself a clear plastic wrap square or blank larger than the cap and place it on your modulator holding it in place with your cracked/old rubber cap, this closed up the leak on mine and the modulator now holds vacuum and the car shifts correctly again.

The black modulators tend to leak from either the cap or less often the o-ring on the module itself. Both are replaceable. Just because the modulator doesn't hold vacuum does not mean it's bad, it could be leaking from one of these two areas.

The replacement part, New, modulator with the external white plastic adjustment ring and no cap, apparently is of a different, sealed, design negating the need of a cap altogether.

If you cap is still cracked, replace it, your shifts will improve.

pawoSD · #11 09-27-2020, 12:11 AM

Yup, the cap is very important, needs to be there, and in good shape. I had an incident on my 300SD back in the day where the cap fell off and shifting turned to garbage overnight. Found the missing cap after lengthy inspection, replaced, back to normal.

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