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:

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.

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

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. :(

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.

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.

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.

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.

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.

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.

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.

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.

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.

When you say "spaceship" you mean the VCV? Also, I'll check the vacuum lines again. Maybe I do have another issue as you suggest. Thanks.

Another good idea. I'll monitor the main vacuum line as well as the VCV / modulator line from inside the cabin with a gauge.
What's an "engine shock top"?

The hoses are routed correctly with the exception the that lines related to the EGR are capped since I've removed mine.

I've also done common engine stuff to like the banjo bolt and line, timing adjustment, tried turning up the ALDA some, etc. Car is actually pretty peppy, turbo seems to work fine.

You are correct, I meant 21 inHg, thanks for the clarification. I'll put my VCV back on and measure its output as I drive. As for the non adjustable unit I'm not sure. Mines a Cali car, maybe that explains the difference? Or possibly someone else before me changed it out? Not sure.

I bench tested mine exactly like this guy in the video. Mine leaks vacuum when his does not. If I actuate the linkage rod it lets off vacuum very quickly of course, but at no point will my VCV hold vacuum like his does in the video.

Yes, but this will not affect the braking and shut off with the severity you mentioned in a previous post.

True. Still, this means my VCV is bad - no? The intermittent brake/ shutoff failures probably point to a separate issue I'm thinking.

Yes I have seen the non-adjustable type. I had a few VCVs in my accumulated parts... when I found out about adjusting, I opened them up and found two of the three were not adjustable. So much for cornering the market.

So if you have the VCV reinstalled into the system, yes you T-connect the vacuum gauge into the line going to the modulator. That line doesn't necessarily come from the VCV directly (doesn't on my wagon), but I guess on your car it does. Should ~10 Hg (or whatever that measure is abbreviated as) at idle and when driving, the vacuum is reduced from the main line reading.

Then you drive normally and watch to see if the vacuum drops at shift points to zero. If it does, your VCV is fine. If there is no drop, you need to get another VCV and possibly then need to set it up.

google peter schmidt
he has the proceedure for transmissions on his website
Do you have the bfs? blue flying saucer?
disconnect the two lines and attach them to each other...test
have you checked your vacuum modulator in your tranny?

OK, let's think about this. In the video, maximum vacuum is at idle. When the vacuum drops with throttle lever movement, the vacuum is diminished. This is also when the transmission will be shifting.

It will really depend on how badly it is leaking. Is it holding vacuum at all? Also, is the non-adjustable VCV supposed to hold vacuum without any bleed-off? Does your system have the metered orifice in-line with the vacuum input? If not, it may have been a method of compensating for this. It is hard to condemn the VCV without knowing absolutely what values are expected.

However, if your main vacuum is dropping off due to a faulty internal checkvalve, it will appear to the VCV, and transmission, as nearly zero. This will give you harsh shifts. Since the main vacuum is the source, it wouldn't be a separate issue. It would be very frustrating to go to the trouble of replacing the VCV and have no change to the shift quality.

Sorry, I don't recall the name. The "engine shock damper" is a mini-shock absorber on both sides of the engine. People speculate its main function is to avoid a slight shake when the engine turns off, though it might help slightly in dampening shaking when running. Thus, it is not an essential component and probably 99.8% of cars do not have one. The top piece is a little metal/rubber bushing that costs ~$200 new. A few people actually pay that much. I wouldn't. I fixed mine by scraping off the old rubber and squirting in polyurethane caulk. Similar for the VCV and other high-priced components. The junkyard is your friend. Grab some parts before they crush the cars. If you buy all this stuff new, you will soon have $20K into a $3K vehicle.

BTW, the reason for the VCV is so the diesel can simulate the intake manifold vacuum of a gas engine for which these transmissions were originally designed. In a gas engine, manifold vacuum drops as you open the throttle (i.e. pressure rises). The VCV produces a similar vacuum signal that drops as the fuel flow increases (via "accelerator"). The "blue saucer" on 1985+ cars is a 1:1 vacuum amplifier between the VCV and transmission. It makes the VCV work better. It also adds a few other inputs related to turbo boost.

I would assume that means the engine shock mount. They are metal frames moulded with bushings and they deteriorate with age.

I missed the original post and what was being referenced to introduce them into this chat....

A Cali trans should be, as far as vacuum goes, the same as a Federal.

It does seem as if you have a failed unit, as you mention it won't hold vac at idle.

Somewhere above is a thread from someone who has a manual trans, that unit (VCV) will be "virgin", I sold the one from my 82 240, manual trans, haven't heard back from the buyer.

If possible, post pics, I still don't understand the non adjustable unit on your year car. To be sure, pull the plastic "dome" or bullet shaped item, you'll see a nut with a spring, or should.

On the non-adjustable ones I had, instead of a nut there was what I would describe as a rivet or a 'blind bolt' with no way to adjust it, securing the spring.

Okay, fortunately mine were/are adjustable.

Hello mrbrian,

Send me a PM and I can forward you a document with lots of pictures that I wrote about adjusting the vacuum and transmission pressure. Bear in mind that it is based on a 1981 300SD with an adjustable VCV. If you end up with another non-adjustable VCV at least you'll be able to adjust all the other related components. If you find an adjustable one, you'll be able to adjust everything.

The document is too large to attach (1.8 Mb) otherwise I would.

Hope it helps.

So I finally got around to getting the VCV back on the car so I could test it while it was hooked up to the car's vacuum pump.

The gauge read 15 at idle and as I actuated the linkage lever the reading quickly dropped to 0. Now I'm thinking the VCV is probably ok.

It got dark before I was able to get the linkage attached and take the car for a drive. I'll try and test drive it tomorrow and monitor the readings.

I gave the main vacuum line a good inspection and can't see any potential leak sources. I'm wondering if maybe the brake booster is leaking and this could explain both the intermittent failure of the power brakes I have experienced in the past as well as the shifting harshness. Anybody know how to test the brake booster for leaks?

I don't think you should be able to get the VCV to drop vacuum to zero by just moving the linkage while the car is running, stationary. It should only drop the vac to zero when the transmission should be shifting. Linkage movement with the car sitting still doesn't mimic driving conditions.

Really? When you press the accelerator, the linkage moves the arm in the same way - why would it be different? On a gas engine the vacuum would be close to zero at full throttle so the VCV should mimic the same kind of situation. Anyway, I'll do a test drive and see what the car is shifting at. Could be the linkage arm needs adjusting. Anyone know if the VC is supposed to be at 0 when the car shifts?

The entire purpose of the VCV is to bleed off vacuum, so as to have the vacuum at 0 at the point when the car should shift. That is why it is there. If it is out of adjustment you will never get the transmission to shift as it should.

What you wrote suggested you had the car running, stationary, when you tested the VCV. If so, whatever movement you made of the linkage was nothing like driving conditions. An unloaded engine sitting still with the car in P or N would severely over-rev if you moved the linkage as far as it would go under driving conditions.

Guys, here are some manuals you'll find useful.

126 FSM, a couple of transmission manuals, trailer hitch plans, 01 DodgeRam FSM

I'll leave the link active until tomorrow then deactivate the sharing.

link down. send me a pm if you can't find the manuals online.

To those dealing with trans vacuum issues, there is a section in the manual that lists modulator pressure. MY 84SD has a 722.303 trans and green modulator for example. The trans ID is above the pan on the passenger side. The exact measured pressure suggested in the manuals varies by trans and modulator color but all trans with the same color modulator use similar pressure readings.

Set the modulator then make a gauge by drilling a 1/8" hole in a nickle (US). Follow the FSM and set the VCV to 0 with the gauge installed. Then adjust the VCV up or down until you have the firm shifts you want without flare. All of the needed info is in the manuals.

Also, 15" vacuum is a little low. Try plugging all of the vacuum systems and find out what the pump is putting out. IIRC, both of my SDs are above 18".

Like I said, I'll deactivate the link tomorrow. Professor Google turned all of this up so it isn't secret. Happy reading.

Harsh shifts are a symptom of too little vacuum given the wear and settings on your particular trans. Details are in the trans manuals above. Just go through the process. Make sure the vac pump is putting out, then adjust the modulator & VCV.

Go back and fix any ancillary vacuum leaks that you have to plug in order to get the shift right. Usually the climate vac pods and possibly the door vacuum locks need attention - and can affect the trans.

So today I drove around and watched a vacuum gauge as it was plumbed into the line going to the modulator. Looks like the vacuum reading at idle was around 9-10. As I accelerated, the reading quickly dropped to around 2-3, then stabilized around 5-6. Most shifts were happening around 4-6 or so. Still hard shifting of course. The only thing I can figure is that I must have a vacuum leak somewhere else that I haven't found yet. Does this seem like a likely cause to you folks given my readings?

Also, thanks to Mahone for you most excellent document. I intend to read through it thoroughly, thanks very much.

Junkman, I wasn't able to use the links... I'll PM you. Thanks.

The FSM calls for the VCV linkage to be disconnected for the test. There is a 10mm gauge that measures where the vac is supposed to drop. I made a gauge by drilling a 1/8 hole approximately in the middle of a nickle. Disconnect the linkage, open the VCV 10mm based on your fancy custom gauge which approximates what the pedal would do without reving the engine and adjust the VCV. Leave the cap off the VCV, hook up the linkage and go drive. Harsh shifts and you need to increase vacuum (or plug some leaks which cause low vacuum). Soft shifts and you decrease vacuum.

The modulator pressure needs to be correct and vacuum leaks need to be under control or at least consistent before you mess with the VCV.

Zach - the SD is designed so that vac drops immediately to zero as soon as you hit the accelerator pedal; car does not need to be in motion. I assume his CD is just like the SD. The car should upshift quite smoothly with no vac and you can test it by driving around with the vac line unhooked. The only thing you should notice is clunky downshifts.

You only need vac for a downshift. If you unhook the vac line from the vcv and tee in a Mityvac to the tranny, you can drive with zero vac and test upshifts and you can pump up the Mityvac (ie provide vac to the tranny) before coming to a stop to test for smooth downshifts.

In short:

upshift - smooth with zero vac
downshift - clunky with zero vac but smooth with vac

Of course I am assuming the SD rules apply to the CD.