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Automatic Transmission Vacuum Modulator valve

27K views 30 replies 5 participants last post by  Gregs300CD 
#1 ·


I conducted some tests this morning. I removed the line going to the trans and pulled good vacuum on it. This means that the transmission modulator valve is holding.

The car shifts hard when I disconnect the vacuum line from the vac source and cap the source. This means that my worries over a damaged transmission are over! It was slipping on 1-2 shift before.

So.. now I'm trying to diagnose if it's the 3/2 Way valve on top of the valve cover (which does have a worn cam) or the "transmission Vacuum Modulator valve" the one on the side of the engine that's controlled by the throttle, pictured above. I tried to pull vacuum on it and it doesn't hold.

Any thoughts? The pictured part is $178 and the 3/2 way valve is 30 or so.

I wanted to get a picture from someone with an early diesel to see how the vacuum hoses are plumbed to the 3/2 valve also if possible. I'm getting close!


Thanks

Alex
 
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#2 ·
Pictures? what pictures?;)
 
#3 ·
That valve should be tested installed and with the engine running... it should hold vacuum at idle, and drop to zero towards full throttle. I haven't tried to test it un-installed, so I wouldn't know exactly what you'd expect from it, but I'm guessing it should hold vacuum in that instance...
 
#4 ·
STILL no pictures:eek:
 
#5 ·
Can someone take a picture of the early cam style 3/2 valve for me? I think the hoses on mine were wrong.

Eric I'm not sure what you're referring to with the pictures but I posted one above. I'm not sure if you're having trouble seeing it.

I'll test it on the car. Which port should be tested, the top or the side?
 
#6 ·
Thats the whole point, in your first post you refer to pictures and I cant see them;)
 
#7 ·
Admittedly, I went lazy on this one, because the replacement part is $200 or so, and my car seemed to shift perfectly when I had gone over everything else...

I'd test it from the top port... apply vacuum, then modulate the throttle from the engine bay mechanical linkage... vacuum should start to bleed as throttle is increased.

I don't have an early style wagon, or I'd throw you a pic... but that 3-2 switch should be connected to some things all the way around... you should be getting cam action as well... look for something with a 'nipple' near the turbo or the air cleaner... there should be, I believe, a hard vacuum line that is missing from that rubber vacuum line with the screw in it (from your video on Youtube)...

I've read that you can do away with this system somehow on the later models, but the later model generally have two of those 3-2 valves on top of the valve cover... I'm not entirely certain which of the two does what... I kind of just hit up the vacuum system in a brute force way, and am discovering the specifics of the functions as I go along...
 
#8 ·
There's a picture in the first post. I'm not sure why myself and others can see it and not you. Perhaps flickr is blocked where you are?

It's just a picture of the Auto trans vacuum modulator so the question should be answerable pic or not. The picture shows a vac hose from the top port of the valve to a tester. It doesn't hold vac.
 
#9 ·
Sorry... maybe you're not understanding my post, or where I'm coming from. My wagon differs from yours, therefore a picture of the 3-2 valves (as there are two on my car as opposed to the one on yours) will not help you out. Yes, I can see your picture. That is the main vacuum 'switch', if you will, for the transmission. It is mounted on the back of your injection pump... as I wrote before, it's easier to test it while it's still on the car. If the rest position on the valve is the same as it is when the valve is mounted in the car, YES, it should hold vacuum... but as I have never tested it outside of it's mounted position, I can't tell you for certain that the valve is set in the proper position to hold vacuum. (and my car is elsewhere right now, so I can't go and look)

Perhaps if you posted some more detailed pics of your engine compartment, someone could be of help to you.( by detailed, I mean a full layout of the engine compartment, and then some detailed pics to go along with it)

Certainly, if it never holds vacuum, position dependent, it is faulty, and must be replaced. Dealer prices will be high, and most likely you are looking at a 2-3 business day turnaround if the part has to come from NJ, or longer if it comes from Torrance, CA. The part itself is in the $200 range. If you need more specifics, I suggest you search 'vacuum' in this forum, and read on... there will be some easy answers to your question in the many posts your search will bring up.

You can also try AutohausAZ.com for parts... they're pretty good. Sorry you don't know how to fix your car, and that it appears we do not understand each other/or that I have been completely unhelpful, and as such, have added to your anxiety... good luck on the sale of your vehicle... the body on it looks great.
 
#10 ·
this is what an '82 looks like...
 

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#12 ·
Thanks, finally got a pic and if you look at the kick down cable, its the one with the red rubber cover, its wrongly adjusted and this will cause shift problem. Especially 2/3, try adjusting this first:thumbsup: and then we will see where to go next.
 
#11 ·
this as well...
 

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#13 ·
Eric... it's not that simple... the picture of the engine is not HIS engine, it's MY engine... His is a 1980 wagon, which is much different than mine, which is a 1982 wagon.
 
#14 ·
OH i see but i still need a picture to be able to help thanks
 
#15 ·
If you read my post above, I've asked him for one... let's see what he comes up with...
 
#16 ·


Note that the side port on the modulator is unused. Should it be?



Should the red/black hose be going to the top port on the modulator? It is now. The 3/2 valve doesn't depress at all with throttle actuation. It's worn but it looks like the entire mechanism isn't causing the 3/2 to compress. The "white" cable is missing a cable clip at the end, behind the throttle linkage.



I have no idea if the routing of these hoses is right.
 
#17 ·
I think that hose set up looks pretty good... I think there should be a hose to the turbo, but as your car is an older version of mine, perhaps they rerouted the turbo lines elsewhere... and I think that capped end should be there, but again... not sure (there's a vacuum control system that prevents overboost of the turbo)... For a rough schematic, you can go to Performance Products to get a rather rough idea of where things should be in the engine compartment (type in your car type, and then try the 'shop by schematic' option, and look for 'engine vacuum'...

As for that white valve and your lack of cam action, do this to see what I mean in the following explanation: get a metric hex wrench and remove the valve from the valve cover... in the place of where the valve should be, you'll see the hole for the hex bolt you removed, and then towards the driver's side of the car in the same area, you should see a little hole. On the 3-2, the flapper thing is attached through two ears with a pin. When that valve sits atop the valve cover, the bottom of that pin should fit into that little hole, and the top of it should be just about flush with the top ear of the vacuum valve... you see, you only getting one point of attachment of the vacuum valve to the valve cover, which allows the vacuum valve to pendulum away from the cam as it exerts pressure on the vacuum valve... you need two points of anchor.

Also, go to Dieselgiant.com and look into the 'mercedes maintenance section'... there should be something there covering EGR, and you might find some pictures more pertinent to your plight...
 
#19 ·
I think you may be able to do without it if you block off the vacuum line with a golf tee... but again, I am not sure on you car... That 3-2 valve generally has to do with EGR, and not shifting... But I'm not sure if you've understood that any vacuum leak anywhere in the system will effect the vacuum for shifting if the leak is large enough...

For now, why don't you just isolate the trans vacuum from the rest of the system so you can figure out what parts of the vacuum system may not be working (e.g. the main trans vacuum valve, the one on the trans, or the hard line and/or rubber hose connectors.

Related, but aside: does your car shut off immediately when everything is connected and you turn the ignition switch to off? if not, you've just found a general source for a vacuum leak, if not the specific source...

And I do believe that you're main vacuum valve... the one on the first picture you posted on this thread, is most likely bad... too bad I can't advise you with certainty how to be sure wthout getting your hands on a working one you can swap in...

So, I'm getting bored of this: check the line from the main trans vacuum valve to the vacuum valve on the transmission to see if it holds vacuum. You do this by disconecting the line from both valves, plugging on end and applying vacuum to the other... then check each valve individually. With certainty the one on the trans will hold vacuum... the other one should be as described. At idle, it will hold vacuum, and when throttle action is applied the vacuum should go to zero at full throttle. Anything wrong with any of those things will cause trans problems. I've never had those problems, and don't want to simulate them, so I can't tell the specifics of what the exact problems associated with vacuum failure in the trans vacuum would be...

Really, if you haven't done yet, search this issues on this forum, and if you have searched and have not found the info you need, I'm sorry, but that's all I know that I can share with you.
 
#21 · (Edited)
http://www.autohausaz.com/secure/PartImages/1230700046.jpg

Your main valve should look exactly like this. When you move the metal arm up and down, at one of max/min, it will hold vacuum, and at the other it won't. In the middle, it should bleed vacuum.

The one down by the tranny, I believe, you tried to adjust. AND THAT ONE WILL HOLD VACUUM.
 
#23 ·
I want 12.5% of the profit...
 
#25 · (Edited)
thanks Greg-

I have a tough time keeping things technical because everybody uses slightly different terminology, and I can't seem to learn me those acronyms...

Is the number 12 (mm Hg) pulled from the FSM, or does it come from another source. I just applied vacuum, careful not to overdo it, and then I watched it drop under throttle after the needle remained steady for a few minutes... again, I'm learning, and I admittedly didn't look for perfection in the VCV because I knew how much it cost... so with a part like that, I thought I'd let it die a graceful death as long as the car shifted well enough when I was done.
 
#26 ·
Well, Alex?

Was it a vacuum problem, or a transmission problem?
 
#27 · (Edited)
It still seems to be a vacuum problem. With no vacuum going to the transmission it holds gear and shifts hard, albeit too hard.

I've owned this car for a week. I bought it sight unseen from Florida, paid shipping up here and it arrived nothing as advertised. I really don't have much interest in spending more time on the car. If I can fix this transmission issue It will add value toward the sale. I drove it home for the first time today. The steering rack, or box has so much play in it that I can't go over 40 or the steering wheel shakes uncontrollably. The car loses power and shuts off every 15 minutes for no reason and can't shift properly. If someone's interested in an otherwise (insert an attempt at something positive here) car they can have it for 2k. I have 3500 into it and 10 miles on it.



Alex
 
#28 ·
I'm close by...perhaps we can meet (and I can meet your car)... PM me... someone from CT wanted some help with his car this weekend, Sunday to be exact... I'm just curious... I don't have 2K or the need for another fixer (although it would have been very tempting if it were a turbo...)... three cars is enough!

That shut off could be a fuel delivery problem. Though I have a feeling your car will be a little like my W123... lots of niggling problems here and there to sum to a large one...

Looks pretty good, though.
 
#29 ·
I have not seen a 3/2 valve hooked up like yours, but there are several variations of the vacuum circuit. No turbo or EGR valve makes this easier.

The VCV is a variable pressure leak, not as simple as a variable hole size. It can be cleaned out if needed but usually only needed if inconsistent. The open port on it should connect to a hose running into the dash(no connection just clean-er air) I have cleaned them by taking the side off and rinsing brake cleaner through the top part several times. They should open at a vacuum related to he position of the lever. Don't worry if it won't completely seal, but it should be smooth. I have never had one completely seal, yet have never tested a new one.

The 3/2 way valve: the side with one port should connect to the tranny. On the side with 2 ports one should hook to full vacuum and the other to a Y fitting connected to the top of the VCV. The other end of the Y should connect to vacuum (directly or not to the brake booster hose) There should be a .06" orifice between the vacuum and the Y.

The 3/2 should have a positive seal to one port or the other, not leaking in any way. If not replace.

The 3/2 on this car serves to deliver full vacuum to the tranny at zero throttle.

The modulator should hold vacuum, they can leak intermittently and be a pain to figure out. Unhook the line from the 3/2 valve and tranny modulator valve. Blow the line out and see if you find tranny fluid in the line. The valve can be adjusted without a gage, but I haven't had any luck doing so. A gage isn't real hard to make.

If all the joints are good in the steering, the steering box can be adjusted. But great care must be taken when doing so.

If it is dieing and/or loosing power, 99.9% chance it is a filter or the screen in the tank. Change the prefilter. Blow on the fuel line to try and flush the crap out of the tank screen. If that fixes it(temporally) you know where the problem is.
 
#30 ·
Awesome advice cytle...
 
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