Failed Vacuum Control Valve? - PeachParts Mercedes-Benz Forum

Rocketrider · #1 10-28-2008, 07:53 AM

I noticed on the '82 300 SD that the rubber boot coming off the top of the VCV had worn through. I'm still working on my vacuum issues, so my tranny isn't shifting like it should. I replaced the rubber hose coming off the top of the VCV with a Y connector and plugged in the MityVac. It was reading 17 and increased with RPM. From what I have read, the VCV bleeds off vacuum from the bottom tube that goes into the cabin. If I disconnect the vent line and connect the MityVac to it then I get no vacuum when I pump it up, indicating that my vent line is not plugged. When I connect the MityVac to the vent connection, I get 0 vacuum regardless of RPM. I'm assuming that my VCV has failed. Is my logic flawed or are my assumptions correct?

Diesel911 · #2 10-28-2008, 11:43 PM

Don't know much about that part except that the Vacuum is supposed to start of high when your foot is not on the pedal and the closer you get to full throttle the closer to zero the vacuum gets; here are some threads:

http://www.peachparts.com/shopforum/showthread.php?p=869411

http://www.peachparts.com/shopforum/showthread.php?p=834716

Brian Carlton · #3 10-29-2008, 12:02 AM

Your entire testing procedure is flawed.

The first thing to do is to connect the Mityvac directly to the transmission modulator line. Pump it up and see if it holds vacuum. A slow bleed is fine..........say 15" in 15 seconds..........but, anything more requires attention at the modulator.

Then, disconnect supply vaccum from the T that is above the VCV and connect your mityvac to this port. You'll have the Mityvac on one side and the line to the modulator on the opposite side and the line down to the VCV on the bull of the T.

Pump up the Mityvac and get a reading. It should be somewhere between 10" and 15".

Now, gradually open the rack and watch the gauge. It should slowly fall as the rack is opened. You might need to pump up the Mityvac during the test if the modulator is leaking slightly. When the rack is fully open, you should not be able to pump up the Mityvac much at all..........very close to zero vacuum.

That's a properly functioning VCV and it can be tuned to move the idle setpoint up or down depending on shift quality.

I'll be away for a few days and will see how you're doing when I return.

Rocketrider · #4 10-29-2008, 11:04 AM

I trust your expertise Brian, so I'll do exactly as you say. However, I'm curious as to which part of my original procedure is flawed. As I understand, the purpose of the VCV is to varibly bleed off vacuum between the supply and the modulator to the vent that runs into the cabin. As the VCV opens with RPM, cabin air should be sucked into the vent of the VCV thus creating a vacuum at the vent at that location. When connecting the MityVac to the vent port, I get no vacuum at any RPM. This may not be the best way to test the issue, but please let me know so that I can have a better understanding of the system.

Quote:

Originally Posted by Brian Carlton

Your entire testing procedure is flawed.

The first thing to do is to connect the Mityvac directly to the transmission modulator line. Pump it up and see if it holds vacuum. A slow bleed is fine..........say 15" in 15 seconds..........but, anything more requires attention at the modulator.

I did this first and if there is any bleed at all it is less than 15" in 15 seconds.

Then, disconnect supply vaccum from the T that is above the VCV and connect your mityvac to this port. You'll have the Mityvac on one side and the line to the modulator on the opposite side and the line down to the VCV on the bull of the T.

Pump up the Mityvac and get a reading. It should be somewhere between 10" and 15".

Now, gradually open the rack and watch the gauge. It should slowly fall as the rack is opened. You might need to pump up the Mityvac during the test if the modulator is leaking slightly. When the rack is fully open, you should not be able to pump up the Mityvac much at all..........very close to zero vacuum.

Ahh, I see! This way, I the car isn't running and I can open the rack completely.

That's a properly functioning VCV and it can be tuned to move the idle setpoint up or down depending on shift quality.

I'll be away for a few days and will see how you're doing when I return.

Rocketrider · #5 10-29-2008, 11:13 AM

I also noticed that there is a procedure to test the VCV on PeterSchmidt.com, but it doesn't provide alot of detail. Plus, the pictures are barely ledgible. I will run the test Brian recommended first. If I still have no success, I will try the procedure on PeterSchmidt.com. Has anyone tried it? It says to disconnect the push rod and advance the accelerator linkage to take up free travel. I'm not really sure I understand. Maybe it'll make sense when I look at my car. Thanks a bunch for your help Brian and 911.

Rocketrider · #6 10-31-2008, 08:42 AM

Although I feel like such an idiot, I have resolved the issue with my tranny! Yesterday, I hooked up the MityVac directly to the top of the VCV and pumped it up to 10-15". I disconnected the actuator rod from the side and opened the valve. To my surprise, it bled the vacuum to 0". I was perplexed that when I hooked everything back up and put the MityVac in with a Y connector and saw no bleed. Then I removed the Y connector and looked inside to find a BB. No wonder the VCV wouldn't bleed, it wasn't getting any vacuum to begin with. Now my transmission shifts as smooth as silk! Thanks for everyone's help. I've learned alot from you guys once again. I'll be sure to pay it forward.

Brian Carlton · #7 10-31-2008, 09:48 AM

Good job.

Never underestimate the power of a moron PO to challenge everything that you know about the vehicle. Sometimes, you can struggle for weeks attempting to figure out what he did.