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  #1  
Old 09-03-2014, 09:38 PM
Waste_Gate's Avatar
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Location: Chapin, SC
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VCV Won't Suck

Hey guys, its been a while but I need some advice. I was tracking down a sudden vac leak (my locks suddenly wouldn't hold vac overnight) and traced it to the diesel door vac element.

While testing everything under the hood with my new mityvac, I believe I may have over pumped the VCV. I know there is a new problem with the vac system now since the trans is now slamming into gear. I know slamming into gear on a 1985 means no or inadequate vac, I've had that problem before.

To keep from slamming into gear on my 25 mile trip home today, I pulled the line that goes to the VCV and plugged it into the spot for the coupe seats on the 5-way distributor. It didn't slam into gear anymore but it did slightly flare on hard acceleration as I expected. So take it from me, if your vcv goes out, use a vac supply to keep the slamming down.

How do I remove the VCV? How is VAC supplied to the VCV? It will hold vac with engine off and it will leak down as it should when I pull the throttle linkage. How can it have zero vac when engine is running but hold vac when engine is off?

Thanks and sorry if I included too much detail.

Willson
85 300CD
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  #2  
Old 09-03-2014, 10:20 PM
Silber Adler's Avatar
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I just tested my VCV. I pulled my pump to 20" and as moved the throttle linkage it went down proportionally. I put my pump directly on the fitting on the vcv. Your VCV seems good if you have a non leaking vacuum and a gradual reduction as you open the throttle by hand.
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86 VW TD
Mahindra Diesel
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Old 09-04-2014, 06:35 AM
Waste_Gate's Avatar
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vac supply to VCV

Thanks Silber, I will hook the mityvac to the main line and see if it still leaks down when I pull the throttle. My problem is I have 0 Hg at idle from my VCV. Does the VCV create vacuum from the injection pump or what supplies the VCV with vac?
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Old 09-04-2014, 08:21 AM
Silber Adler's Avatar
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The vacuum pump on the front of the engine makes the vacuum. THe main vacuum line goes from the pump to the master cylinder brake booster.The small lines off the main line have jets in them, they are in series with eachother with each one having a smaller jet, so the vacuum goes down the further you are from the main line.
The vcv has a variable jet. The transmission hooks up between the last fixed jet and the vcv.

A leak down could be from several sources Those plastic connectors in various colors are the jets.

Other posters are more experienced.
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85 Merc 300D - Unwinding 31 years of wear
86 VW TD
Mahindra Diesel
Iseki Diesel

In 2007 I didn't own a diesel.
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  #5  
Old 09-04-2014, 09:53 AM
Zacharias's Avatar
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Location: West Quebec
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Everything Silber has said is correct, but let me add that you missed the first obvious troubleshooting step on any vacuum issue: check the output from the vacuum pump by putting a vacuum gauge onto the main vac line where it enters the brake booster. That assures you that you have a baseline of proper vacuum going into the system, then you can check to see if/what other components are bad. See:
Mercedes Vacuum Troubleshooting

The VCV in theory is always bleeding off some vacuum, when the car is running, but it bleeds off more as throttle opening increases to where it drops to zero to allow the transmission to shift properly.

To the question that was not answered, the vacuum control valve is held on the top of the IP by two or three small flat-head screws. It is a stand alone item and is not connected to the internals of the IP in any way. Do not remove it unless you have to, and it doesn't sound like you have to in this case.
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  #6  
Old 09-04-2014, 10:04 PM
Waste_Gate's Avatar
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Fixed

Zacharias and Silber, I wanted to let you know the problem with inadequate vac to trans was not a fault of the VCV. To start out, yes I tested the main line and have 22 hg on it. My main vac line was rebuilt and for one, there was a leak in the 3way splitter coming off the main line that fed the trans lines with the restrictors or jets. I fixed that with a new 3way and got 15 hg but it was slowwww to build up to that. I took it for a drive and the slamming was ever so slightly less but I knew it still wasn't getting enough vac. So I took the white with blue striped line feeding the egr switchover off and ran a line from its place on the main line distributor to the junction with the jets in it and now it's shifting like butter and no more flaring so I know the VCV is doing its job too.

I had the wrong understanding of the VCV's impact on vac. As soon as i realized it was dumping from an already supplied supply of vac, I decided to increase that amount of vac, I could probably have removed a jet and keep the original plumbing but for now its shifting too smooth to mess with.

Thanks guys,
Willson
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