Trans. Modulator Vacuum Question - PeachParts Mercedes-Benz Forum

Maroon 300D · #1 04-30-2006, 09:12 PM

I have a really hard first shift and have been looking at my vacuum system. I can't for the life of me figure out where the line that goes to the modulator is supposed to connect to. That white plastic thing, sometimes called the transducer, that sits on the IP? It seems all the lines leading out of that go to the firewall. I haven't been able to locate a line that seems to be going to the tranny modulator.

I have looked over the vac diagrams supplied by phantom s, but none of them seem to coincide exactly with what I'm seeing under the hood.

I've done a lot of searching and my head is spinning with vacuum info. right now.

It is just my first upshift that it really clunky, BTW. Sometimes the next shift is also rough. All other shifts, including downshifts, are smooth and barely noticeable. It never flares and the clunkiness of the first two upshifts differ based on throttle and outside temp. Less throttle=less clunky for the first shift, with the second seeming to like a little more throttle, but not too much. It's less clunky when warm.

I have a leak in my brake booster line too, I think. When I tested the check valve it fluctuated betwen 20 and 22 and wouldn't stay steady. Areas down the line from there have low vacuum but give a steady reading.

If this was my only problem, though, wouldn't all my shifts have some trouble?

Biodiesel300TD · #2 04-30-2006, 09:39 PM

The vac line that goes to the tranny modulator is black, and it starts at a three way that is connected to the vacuum control valve on the rear of the injection pump. The line drops back behind the rear of the engine and the firewall.

It sounds more like you may be needed a B2 piston and maybe a K-1 spring, instead of it being a vac problem. But before you tear into that make sure you vac system is all sealed and tight. Try plugging off the lines that go to the magic black bow on the top of the valve cover. That may help the situation too. Also check your tranny fluid and maybe do a fluid and filter change.

20-22" on the vac pump is in the good range for the pump. If it is jumpy you may not have had a good seal with the gauge. If there was a leak down the line somewhere it would be a constant on and the reading would be consistantly low.

Diesel Giant · #3 04-30-2006, 09:41 PM

You need to look at your whole vacuum system, maybe this will help.

http://dieselgiant.com/mercedesvacuumtroubleshooting.htm

85chedeng300D · #4 04-30-2006, 10:47 PM

try the 300 SD California by phantom

Maroon 300D · #5 05-01-2006, 12:25 AM

Andrew - When I tested the vacuum on the check valve on the brake booster line I know I had a good seal on both ends, so I'm going to go ahead and replace the line. What also makes me think I have a leak there is the fact that the reading for the part of the booster line going into the brake cylinder is only 17 hg, and the reading going into the green dashpot before the transducer is only 8 hg, but my vac pump is evidently producing 20 hg at least.

I think I saw that black line you're talking about, but I thought I saw it connected to the exhaust manifold somewhere. I don't have any vacuum components on my valve cover but I do have vac lines going to my EGR which I haven't tested yet. I'm not sure if a PO modified this or if the car came without the vaccuum elements on the valve cover, I think some of them did but I'm not sure. If it used to be there, all traces have disappeared.

I will have to read about the B2 piston. I've heard of it but know nothing about it. I've read a little about the K-1 kit but I thought the symptom for needing that fix was flairing.

I've been trying to check my tranny fluid level, but I keep failing! First I read in the owner's manual that if the fluid is cold, the level will be 12 mm below where it would normally be. Well I checked it and it appeared to be WAY overfilled. I did some research and a lot of people on here said that's what the dipstick will read when the fluid is cold. Makes me wonder why the owner's manual says what it says. I checked it again today after just getting the engine up to temp., and it still read way high. I did some more research and it turns out I have to drive at least 10-15 miles before I can get an accurate reading. So I'll check it after work tomorrow. I don't suspect a fluid problem because I don't have any slipping, but I'll check to be sure.

DG, your trouble-shooting page is great and has been invaluable to me, thanks a lot.

85, I have looked over the 85 SD diagram as I figured it's the one that will relate to my car the most. I guess it is, I will just have to look a little hrder for that line.

Thanks for the help.

tangofox007 · #6 05-01-2006, 12:29 AM

Quote:

Originally Posted by Maroon 300D

I've been trying to check my tranny fluid level, but I keep failing!

Is the engine idling when you check the transmission fluid?

Maroon 300D · #7 05-01-2006, 12:35 AM

Quote:

Originally Posted by tangofox007

Is the engine idling when you check the transmission fluid?

Yes.

MonsieurBon · #8 05-01-2006, 01:44 AM

Quote:

Originally Posted by Maroon 300D

Andrew - When I tested the vacuum on the check valve on the brake booster line I know I had a good seal on both ends, so I'm going to go ahead and replace the line. What also makes me think I have a leak there is the fact that the reading for the part of the booster line going into the brake cylinder is only 17 hg, and the reading going into the green dashpot before the transducer is only 8 hg, but my vac pump is evidently producing 20 hg at least.

You should be getting way less than 20 hg at the green dashpot. I believe DG's site says 10-15 is normal, due to the orifice.

Samuel M. Ross · #9 05-01-2006, 02:40 AM

I did not search your thread to see if you mentioned whether you have the vacuum diagram for your engine/transmission... and if you don't, check at the following website under "Vacuum Schematics" in the left side of the opening page... see if you can find one for your year/model/engine.
http://www.peterschmid.com/
There are other transmission related reference material there also!
Sam

Maroon 300D · #10 05-01-2006, 01:23 PM

Well I'll be getting a new brake bosoter hose this week. I guess I'll put that in and then see what happens. Hopefully the vac modulator will get the proper vacuum then. It's definitely not getting it now as the reading both before and after the green dashpot in front of the transducer/vac. control valve is 8 hg.

MonsieurBon · #11 05-01-2006, 05:25 PM

Quote:

Originally Posted by Maroon 300D

Well I'll be getting a new brake bosoter hose this week. I guess I'll put that in and then see what happens. Hopefully the vac modulator will get the proper vacuum then. It's definitely not getting it now as the reading both before and after the green dashpot in front of the transducer/vac. control valve is 8 hg.

Again, go through Diesel Giant's DIY Vacuum guide. You should NOT being getting anything close to 20" Hg at the little green dashpot. I believe he says it should be something like 10-15" Hg, and should drop to zero the more throttle you give it.

Samuel M. Ross · #12 05-02-2006, 11:44 AM

Ralph,
I know the concept is a bit abstract but these vacuum systems in automobiles can function as "static" and/or "dynamic" systems... and this system used to control both shifting and EGR in these vintage M-B(s) are definitely "dynamic" by virtue of the bleeding action of the "VCV" [vacuum control valve] on the side of IP [injection pump]. And in a "dynamic" system such as this I believe the restricted orifices [ Ts, in-line connectors, dash-pots, dampers etc], all of these can become critical to setting up the proper balance. Clean out any/all such orifices. IMHO this vacuum balance probably needs to be re-balanced as the tranny ages. I have discovered that M-B has six different in-line restricted orifices to help accomplish such vacuum balancing !

Yes " abstract ", and too much of such theory can sometimes make one's head spin... but that's why we like these relatively simple vintage machines. Try being a DIY mechanic on a car with 3 separate computers and 500 sensors under the hood and elsewhere. It's nearly impossible... even for the full-time professional mechanics. We can count our blessings for all we need for this common problem is a gauge and a few vacuum line fittings to help us solve such problems.
Hang in there,
Sam

Maroon 300D · #13 05-02-2006, 02:45 PM

Quote:

Originally Posted by MonsieurBon

Again, go through Diesel Giant's DIY Vacuum guide. You should NOT being getting anything close to 20" Hg at the little green dashpot. I believe he says it should be something like 10-15" Hg, and should drop to zero the more throttle you give it.

I know I shouldn't be getting 20 hg at the dashpot, but I should be getting more than 8hg with no throttle. Yet 8 hg is what I've got both before and after the dashpot. My theory is that this is due to the leak in the brake booster line. I think the fact that I have 17 hg at the brake cylinder is also due to this leak, my recollection is it should be 20hg or slightly more.

Hopefully, when I replace the leaky booster line the vacuum will increase from 8 hg to 10-15 hg at the dashpot and the modulator will also receive more vacuum. If not I may have to replace the elements/line between the booster line and the dashpot.

Thanks for your help, BTW.

Maroon 300D · #14 05-02-2006, 02:48 PM

Sam, thanks for the encouragement. I'm a big fan of your posts, by the way. You are quite thorough about explaining your problems and solutions, and I have a feeling a lot of people will benefit.

Maroon 300D · #15 05-03-2006, 01:35 PM

FYI - I just realized that the part I was calling the transducer on this thread is actually the vacuum control valve. On my car the trasnsducer is the blue plastic thing on the on the driver's side fender.

Sorry for any confusion! If I have more time later I will edit out those references.