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  #1  
Old 07-27-2015, 12:49 PM
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1984 300CD VCV Valve Broken, options?

I sure hate these vacuum controlled MB transmissions. I've spent a lot of time looking for the source of my vacuum leak and I believe that it's the VCV Valve.

What are you guys doing when you need to replace these? I hate to pay $80-$100 for a 30 year old used part that will probably not have long before it breaks too. MB doesn't make this part anymore and it's critical to the transmission shifting. What are we going to do with our cars as these parts eventually become scarce? Is there a way to repair the internal diaphram that anyone is aware of? Is there a way to get the transmission to shift well without it? Should I just risk buying one sight unseen online and hope that it works?

Just for clarity's sake, I'm talking about this:
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1984 300CD VCV Valve Broken, options?-d203215990oes.jpg  

Last edited by mrbison; 07-27-2015 at 01:28 PM.
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  #2  
Old 07-27-2015, 01:04 PM
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The VCV is actually a bleed-off valve. It is supposed to leak vacuum. That is what the outlet on the side is for.

As I have no idea what you were troubleshooting exactly, I can't offer other advice.

If you feel you need to replace it, the answer is yes just buy a used one. I suggest you put a want to buy (WTB) ad in the used parts for sale and wanted subforum and confirm whomever replies that their transmission was shifting correctly on the car the unit came from. Chances are very good that will mean the unit is working properly.

These are 30 year old cars. The replacement parts availability is, overall, excellent.

There are some items that are bound to fall off the parts grid with each passing year, but when I can order small fittings and other obscure stuff and have them at the dealer overnight (or 3-4 days from Germany) I don't find much to be upset about.
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2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22
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  #3  
Old 07-27-2015, 01:18 PM
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How are you determining that it is faulty? The Vacuum Control Valve is designed to vent vacuum in relation to throttle position. This is done by a Y-connection to the input side of the valve.

The last time I saw a report of a suspect VCV, it resulted in the discovery of a leaking vacuum cap on the modulator.

1984 Mercedes-Benz 300SD Base Sedan - Automatic Transmission - Page 4
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84 300SD 350K+ miles ( Blue Belle )
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  #4  
Old 07-27-2015, 01:18 PM
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Thanks for the reply, let me ask you this question.

I'm aware that the VCV should leak off vacuum but I'm not aware of how much. My transmission was upshifting very hard and clunking on the downshifts. It was pretty unbearable. I've extensively checked all other vacuum systems and the door locks and climate control both work. My VCV is the type that cannot be adjusted sadly. I keep reading about how people are adjusting their VCV valves to certain vacuum specs and it solves their shifting issues. I removed mine completely in a moment of frustration and have driven it without the VCV. The shifting is very smooth except that there is a little flaring and occasional slippage.

My understanding is that these parts cannot be "bench tested" reliably to determine if they are functioning properly but the car seems completely different without it and I feel like it either needs to be adjusted or it is not functioning properly. I'm not really sure which direction to go from here... I think the part is broken but I'm not sure how to confirm this. If the part is broken I would like to replace it. If it is not broken I'm thinking it should be adjusted but I cannot see a way to adjust this style of VCV Valve. I'm open to suggestions that anyone might have.
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  #5  
Old 07-27-2015, 01:26 PM
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Quote:
Originally Posted by SD Blue View Post
How are you determining that it is faulty? The Vacuum Control Valve is designed to vent vacuum in relation to throttle position. This is done by a Y-connection to the input side of the valve.

The last time I saw a report of a suspect VCV, it resulted in the discovery of a leaking vacuum cap on the modulator.

1984 Mercedes-Benz 300SD Base Sedan - Automatic Transmission - Page 4

I've pulled vacuum on the modulator alone and there aren't any leaks there. I've checked for vacuum leaks everywhere and come up with nothing. Still there are rough shifts and every once in a while the engine won't shut off or my power brakes randomly stop working. To test a hunch I removed the VCV and plugged the vacuum lines going to it. Since then I have not had any of these issues at all. Engine shuts off every time, haven't lost power braking, and shifting is smooth (except now there is a flare and slipping as I already mentioned).

This result (combined with the fact that I can't seem to locate any other vacuum leaks) makes me highly suspicious that it is the VCV Valve. If you know a way to scientifically test the VCV with a gauge I'd definitely be interested in the method.
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  #6  
Old 07-27-2015, 01:58 PM
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This sounds more like a marginal vacuum pump. It could be either the pump's internal check valves or the main external check valve. I'm thinking the former is more suspect.

There are a couple of restrictors that prevent a transmission vacuum or climate control vacuum problem from seriously affecting the power brakes.

When I came across this intermittent type of problem on mine, several years ago, it was an internal check valve on the vacuum pump. There is a rebuild kit available for the pump. It didn't find the kit here at PeachParts but they can probably obtain it if you give them a call. Be sure and ask for the newer style of rebuild kit to get the correct parts.

It isn't a difficult job to rebuild the pump but there are quite a few small bolts you don't want to round off.
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  #7  
Old 07-27-2015, 02:15 PM
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Quote:
Originally Posted by SD Blue View Post
This sounds more like a marginal vacuum pump. It could be either the pump's internal check valves or the main external check valve. I'm thinking the former is more suspect.

There are a couple of restrictors that prevent a transmission vacuum or climate control vacuum problem from seriously affecting the power brakes.

When I came across this intermittent type of problem on mine, several years ago, it was an internal check valve on the vacuum pump. There is a rebuild kit available for the pump. It didn't find the kit here at PeachParts but they can probably obtain it if you give them a call. Be sure and ask for the newer style of rebuild kit to get the correct parts.

It isn't a difficult job to rebuild the pump but there are quite a few small bolts you don't want to round off.
Interesting thought. I have considered this possibility too. With the engine running I have tested the reading off the main vacuum line that goes to the brake booster and get ~ 21 mmHg. I believe that this is a healthy vacuum reading? Please correct me if I am wrong. I have thought about rebuilding the pump just to rule out this possibility but haven't yet since the pump seems to be producing enough vacuum. Is it possible that the pump is not producing consistently good vacuum? If I can't figure out how to test the VCV perhaps I'll rebuild the pump and see if that fixes my issue. I have a new in box modulator as well but I have yet to put it on. The original modulator seems to hold vacuum so I don't think that it really is needing replacement.
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Old 07-27-2015, 02:36 PM
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The problem is that it is intermittent. It fooled me with good readings as well. After the pump rebuild cured mine, I attributed the problem to the check valves beginning to wear and not fully function.

I would avoid the modulatot. I thought this was the problem on mine as well. From what I've read an experienced, the main failure of the modulator is leaking. It requires pressure guages and fittings to adjust it correctly.
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  #9  
Old 07-27-2015, 02:52 PM
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Quote:
Originally Posted by SD Blue View Post
The problem is that it is intermittent. It fooled me with good readings as well. After the pump rebuild cured mine, I attributed the problem to the check valves beginning to wear and not fully function.

I would avoid the modulatot. I thought this was the problem on mine as well. From what I've read an experienced, the main failure of the modulator is leaking. It requires pressure guages and fittings to adjust it correctly.
Sounds like you've had a pretty similar experience as me. I think I'll take your advice and focus on a pump rebuild. Even if it doesn't fix the problem it'll still need to be done one of these days sooner or later. Hopefully it'll do the trick.

I've actually already adjusted the modulator. When I first got the car I had more enthusiasm than good information and read that adjusting the modulator could fix hard shifts. While this is true technically of course it rarely is the main problem. So anyway, I spent a lot of time changing the modulator pressure and didn't notice any difference... Of course the stupid thing is that if the vacuum pump is failing, doing a rebuild may require that I have to readjust the modulator settings if I'm thinking about it correctly. Anyway, I should have just left it alone.
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  #10  
Old 07-27-2015, 03:09 PM
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To check whether your issue is/was the VCV, you would need to get hold of a vacuum gauge and plumb it into the line going to the transmission modulator, with enough line to run it into the cabin and test it while driving. With the VCV installed, obviously.

If the VCV is behaving properly you should have (IIRC) ~10 hg of vacuum while driving, but this should drop to zero when the transmission shifts.

There are posts here and there on how to actually calibrate a VCV (they may be on BW) but I was not able to understand their instructions so haven't tried.

If you have been troubleshooting I assume you have renewed the rubber joints for the various vacuum connections. These puff up with age and can add up to a major loss of vacuum, cumulatively.
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2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22
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  #11  
Old 07-27-2015, 03:49 PM
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If adjusting the modulator made no difference it could be leaky. I thought a simple vacuum check could suffice on them for this but am not sure. I suspect it has a diaphragm but if ruptured transmission oil should appear in the line perhaps.

Something is probably going on if adjusting it made no difference. I guess a test might be to tee in a vacuum gauge between the spaceship and the modulator as already suggested.

If vacuum does not vary it is probably the spaceship. If vacuum remains flat and low perhaps the modulator is leaking. Personally I probably would grab both parts off of a junker and substitute them one at a time.

Since some suspect it is an intermittent vacuum pump the transmission should change shifting sensations. My suspicion is if the transmission issue is constant it may not be the vacuum pump itself.

On the other hand loss of the power brakes periodically and random shut off issues does sound like general vacuum issues as well. Makes me suspect you may possibly have more than one issue in the vacuum system.
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  #12  
Old 07-27-2015, 04:06 PM
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Quote:
Originally Posted by SD Blue View Post
... There are a couple of restrictors that prevent a transmission vacuum or climate control vacuum problem from seriously affecting the power brakes.
...
Are you guys reading this?
This says that even if you unplug the VCV hose (max leak possible), it would not affect the main vacuum (like power brakes).

A main vacuum problem sounds most likely. Why not monitor that line with a vacuum gage inside the car while driving? If you see it drop, you could try capping off the VCV hose and see if it still does so. That is the only way to find an intermittent problem. Don't jump to conclusions from one null result.

Re parts like a VCV, I have plenty such components and climate boxes I pulled from the JY. Easy to try swaps. That is how you maintain older vehicles. I even do that w/ my newer vehicles. Some here pay $200 for a new "engine shock top", but I consider that an IQ test.
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  #13  
Old 07-27-2015, 05:04 PM
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Quote:
Originally Posted by mrbison View Post
I've pulled vacuum on the modulator alone and there aren't any leaks there. I've checked for vacuum leaks everywhere and come up with nothing. Still there are rough shifts and every once in a while the engine won't shut off or my power brakes randomly stop working. To test a hunch I removed the VCV and plugged the vacuum lines going to it. Since then I have not had any of these issues at all. Engine shuts off every time, haven't lost power braking, and shifting is smooth (except now there is a flare and slipping as I already mentioned).

This result (combined with the fact that I can't seem to locate any other vacuum leaks) makes me highly suspicious that it is the VCV Valve. If you know a way to scientifically test the VCV with a gauge I'd definitely be interested in the method.
My bad, I did not read this before my last reply.

The amount of vacuum going through the VCV should not affect your brake booster to that extent. When my vacuum pump diaphragm ruptured (old style pump) on my wagon I was only getting about 10 hg in the main line and my brakes worked fine... the shutdown worked but was very sloooow.

Examine the main vacuum line very carefully (including underneath) for cracks at all the joints and the fittings before writing off the pump. More than one person has rebuilt or replaced the pump only to find their line had an issue.

Have you confirmed that your vacuum hoses are routed correctly? There are diagrams online and there should be a sticker under the hood.

Someone jump in here if I am wrong, but my understanding is that adjusting the modulator (within reason) won't make any difference if the VCV is not working and the vacuum never gets cut to allow the transmission to shift properly.
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2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22
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Old 07-27-2015, 06:12 PM
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https://www.youtube.com/watch?v=Yn1fj9_OTm0

That shows a VCV working, for the most part.

You mention ~21 mmHg, that should be inHg. The two orifices may have different readings, you want the low value to go to the VCV.

Measure at the output of the VCV, what do you get? Should be ~12 inHg.

I wonder why an 84 has a non adjustable unit. My 83 SD has one, as does the 84 CD.
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Old 07-27-2015, 07:11 PM
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Quote:
Originally Posted by Zacharias View Post
To check whether your issue is/was the VCV, you would need to get hold of a vacuum gauge and plumb it into the line going to the transmission modulator, with enough line to run it into the cabin and test it while driving. With the VCV installed, obviously.

If the VCV is behaving properly you should have (IIRC) ~10 hg of vacuum while driving, but this should drop to zero when the transmission shifts.

There are posts here and there on how to actually calibrate a VCV (they may be on BW) but I was not able to understand their instructions so haven't tried.
You mean to get a "T" connector and run a line with a gauge attached off of the line that comes from the VCV and goes to the modulator? I'll try this and report back. As for calibrating the VCV, as I already mentioned, my VCV is not the kind you can calibrate. Some have nuts that you can turn to change the spring tension while other types (like mine) just have a non adjustable sleeve.
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