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  #1  
Old 09-24-2008, 02:14 AM
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Does anyone know how to test a Vacuum Control Valve?

I looking to replace my Vacuum Control Valve (otherwise known as the "proportioning valve" I believe in the Brotherton article) due to the fact that is leaks slowly from the top port, but I'm not sure exactly how to test a new(or used rather) one to see if it works. I understand that they are supposed to control the amount of leaking in the vac system, but I don't know the proper testing procedure to see if one is good or not. I do have a mityvac but not sure how to test it due to the attached lever that controls the leak.

If anyone can please shine some light on this subject I would greatly appreciate it. I'm trying to smooth out my transmission shifting and have hunted down my last leak (after fixing about 10 other ones all throughout the system) at the VCV.

Thanks,
Dan


Last edited by Dan83-300D; 09-24-2008 at 02:25 AM.
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  #2  
Old 09-24-2008, 08:07 AM
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Well, have you tested that vacuum levels are appropriate, and decreases with fuel application?
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  #3  
Old 09-24-2008, 11:17 AM
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As 94mgm said, you'll want to test it in the car first. T you mity vac inline on the transmission side of the VCV. Then disconnect the linkage that goes to the injection pump so you can go full throttle without the engine actually being at full throttle. With the car running read the vac on the mity vac. You should have in the teens at idle and zero at full throttle.

But with the VCV off the car you should be able to hook your mity vac up to the port that goes to the vac pump and it should hold vac. If it doesn't than the valve is bad.
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  #4  
Old 09-24-2008, 01:43 PM
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Originally Posted by Biodiesel300TD View Post
But with the VCV off the car you should be able to hook your mity vac up to the port that goes to the vac pump and it should hold vac. If it doesn't than the valve is bad.
Why should a valve that leaks by design hold vacuum?
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  #5  
Old 09-25-2008, 03:01 AM
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Yeah, that was my question Tango, I imagine to test it properly you'd need to know when it's supposed to leak and when it's not. That's what I'm unsure of.
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  #6  
Old 09-25-2008, 09:28 AM
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Since you failed to provide a vehicle year and model, I'm going to assume that the vehicle is the '83 W-123 in your profile.

With this VCV, you can do a quick check of the valve by connecting the Mityvac to the top port. Pump up the Mityvac and read the gauge. If the valve stops at some point between 8" and 15", you're off to a good start.

Now, watch the gauge and slowly open the rack. The gauge should fall off proportionally to the amount of rack opening until it finally reaches zero near the point of maximum fuel.

While you're checking with the Mityvac, connect it up to the modulator line to the transmission. Pump it up and see if the modulator holds vacuum. A slow leak (15 seconds) is acceptable. Any greater leak is not acceptable.
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  #7  
Old 09-25-2008, 11:37 AM
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Quote:
Originally Posted by tangofox007 View Post
Why should a valve that leaks by design hold vacuum?
Becuase it's not suppose to leak down at idle. It will hold vac. It only leaks down when you use the throttle.
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  #8  
Old 09-25-2008, 11:45 AM
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Originally Posted by Biodiesel300TD View Post
Becuase it's not suppose to leak down at idle. It will hold vac. It only leaks down when you use the throttle.
Of course it leaks at idle. It dumps all vacuum below the setpoint. Inlet vacuum runs about 20". Output to the modulator is 10-14" typically.
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Old 09-25-2008, 11:27 PM
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First off, thanks everyone for pitching in their thoughts and expertise here. I've been struggling with this damn transmission for 2 years now and nothing's helping so far, so any suggestions from fellow MB DIYers is much appreciated.

Ok, so I tested the VCV and it held at 6" at idle and then dropped to zero when I opened the "rack". Is that sufficient, do you think? It wouldn't even let me pump any farther than 6" to begin with though. Also, I tested the transmission line and that did not leak at all.

My next question for anyone who'd like to humor me would be: If my VCV is working properly, what could possibly be causing my (slightly) hard 1-2 shift and my obnoxious 2-3 flare up? I've fixed all leaks throughout my vac system; I've replaced the K1 spring kit; I've tried changing the little plastic orifices (leading to the surge damper--I bought every size orifice that MB still makes); I've adjusted the white plastic screw connected to the throttle linkage; replaced both switchover valves that sit on top of the valve cover; changed trans fluid and filter...I'm running out of options here I think. Is there a specific procedure to follow to adjust these transmissions or is it all guess work, tweeking this and that and then test driving? Also, I've tried adjusting the VCV by the screw under the "conehead cover" but that messed up my shifts way worse.

I read the Steve Brotherton article, but it wasn't much help for me because I'm still just a novice and he was discussing some procedures that I didn't quite understand.
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  #10  
Old 09-25-2008, 11:35 PM
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Quote:
Originally Posted by Dan83-300D View Post
First off, thanks everyone for pitching in their thoughts and expertise here. I've been struggling with this damn transmission for 2 years now and nothing's helping so far, so any suggestions from fellow MB DIYers is much appreciated.

Ok, so I tested the VCV and it held at 6" at idle and then dropped to zero when I opened the "rack". Is that sufficient, do you think? It wouldn't even let me pump any farther than 6" to begin with though. Also, I tested the transmission line and that did not leak at all.

My next question for anyone who'd like to humor me would be: If my VCV is working properly, what could possibly be causing my (slightly) hard 1-2 shift and my obnoxious 2-3 flare up? I've fixed all leaks throughout my vac system; I've replaced the K1 spring kit; I've tried changing the little plastic orifices (leading to the surge damper--I bought every size orifice that MB still makes); I've adjusted the white plastic screw connected to the throttle linkage; replaced both switchover valves that sit on top of the valve cover; changed trans fluid and filter...I'm running out of options here I think. Is there a specific procedure to follow to adjust these transmissions or is it all guess work, tweeking this and that and then test driving? Also, I've tried adjusting the VCV by the screw under the "conehead cover" but that messed up my shifts way worse.

I read the Steve Brotherton article, but it wasn't much help for me because I'm still just a novice and he was discussing some procedures that I didn't quite understand.

At 6" vacuum to the modulator, the shifts should be very firm. The fact that you have a flare on 2-3 indicates that the vacuum system doesn't have the capability of modulating the shifts anymore.........the transmission is worn internally.

However, there are still two possibilities:

1) Disconnect all vacuum to the transmission and see what you get on a test drive. Hopefully, the 2-3 flare is eliminated. Notice how firm the 1-2 gets..........it might be tolerable..........might not be.

Report back and we'll go from here.
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Old 09-26-2008, 12:59 AM
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Quote:
Originally Posted by Brian Carlton View Post
Of course it leaks at idle. It dumps all vacuum below the setpoint. Inlet vacuum runs about 20". Output to the modulator is 10-14" typically.
Good to know. Learn something everyday.
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  #12  
Old 09-27-2008, 09:24 PM
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Ok, I disconnected the black line that runs to the transmission and gave it a test drive. The results are that the 2-3 flare is gone but now (as you mentioned it would) the 1-2 and even the 2-3 shifts real hard. Also, just as I'm coming to a stop, the trans makes a clunking noise. I'm guessing that's the 2-1 downshift?
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  #13  
Old 09-27-2008, 09:33 PM
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Quote:
Originally Posted by Dan83-300D View Post
Ok, I disconnected the black line that runs to the transmission and gave it a test drive. The results are that the 2-3 flare is gone but now (as you mentioned it would) the 1-2 and even the 2-3 shifts real hard. Also, just as I'm coming to a stop, the trans makes a clunking noise. I'm guessing that's the 2-1 downshift?
OK, you're between a rock and a hard place. You've got two options:

1) Try to dial in the vacuum at idle to about 4" or so..........hopefully that will eliminate the 2-3 flare and still provide a decent 1-2. It might not be exactly 4"........might be 3".........might be 2".........you'll need to tune it.

2) Get the Superior Shift kit and hope that the new springs will eliminate the 2-3 flare. No assurances, but it's probably $60. well spent.
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  #14  
Old 09-28-2008, 10:51 PM
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Ok. So does it still sound like trans might be worn internally to you? And also, (novice question here) how would I go about tuning the vacuum to the specified quantity as you mentioned?
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  #15  
Old 09-28-2008, 11:30 PM
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Originally Posted by Dan83-300D View Post
Ok. So does it still sound like trans might be worn internally to you? And also, (novice question here) how would I go about tuning the vacuum to the specified quantity as you mentioned?
On the side of the VCV is a small dome. Carefully pry off the dome. There will be a tiny slotted screw under the dome. This is the adjustment screw, however, it's VERY SENSITIVE. Movement of 5 DEGREES is sufficient to make a change to the vacuum level. Movement of 1/2 turn WILL DESTROY THE UNIT.

So, connect the Mityvac to the top port and pump it up. Turn the screw very slightly and see if the needle drops. If it does not drop, pump it up and confirm that you adjusted it higher. Now turn it the opposite way and watch the needle fall.

Adjust the vacuum to something between 2" and 6" and see if you can find a decent position where it won't flare but you don't get a 2-1 downshift clunk.

You'll have to live with whatever you get on the 1-2 upshift.........unless you buy the Superior shift kit and install it.

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