vcv problem???????
 

Hi I have a 1987 hymer with a 4 speed auto box and full vacuum control but has started changing gear at high revs so put a vacuum gauge on it and from feed I'm getting 24" pressure but when I put on other side of vcv I hoped for around 14" but only got 6" Max so cleaned the vcv with brake cleaner and reset it but still the same so is the vcv in need of replacing as cannot find 1 anywhere in UK so can any body tell me if it does need a new one and were I can get good secondhand 1 or a new 1 thanks Dave will look at replys when I get up as 1am over here again thanks Dave

Hi again not sure but might be looking wrong item, just checked vacuum pump and is reading 24" pressure, a good reading from what I can see,the problem is the gauge took 20 seconds to get there. 1 is it meant to be this slow on a merc as when I put same gauge on my car a Peugeot it went to same but was instant. 2 if it meant to be instant like my car, then do you think I need to be looking at repairing the pump first in case that's the problem thanks Dave

What's a hymer, Dave?

Hi Scott, a hymer is a German motorhome, the best were given merc running gear, the rest had to put up with fiat running gear but don't last as long as a good merc, look in Google 1987 Mercedes hymer 660, old but I love them, Dave.

Did you check the vacuum directly at the pump? If so, then you might have a problem with the check valves inside the pump. Mine would read instantly.

The VCV is adjustable. If it is all the way closed, you should get full vacuum with the handheld pump. There is a thin rubber diaphram inside the vcv. Spraying it with brake cleaner is not a good idea. That is just my opinion. I believe the VCV is NLA.

You will need to find the vacuum spec for your particular Engine.

On My 84 300D there is the Main Vacuum and between that and the Vacuum Valve on the Fuel Injection Pump is a restricted Orifice that reduces the Vacuum to the Vacuum Valve.

As previously stated the Vacuum Valve itself has an adjustment.
If the Vacuum Valve is working the Vacuum will be high when the Accelerator Pedal is not depressed and as you depress the Accelerator Pedal the Vacuum gets lower.

It also is not unknow for the Vacuum Modulator Cap to be cracked or missing and the Modulator itself to be not holding Vacuum properly.

I did not give any vacuum specs because yours are likely different.

This article is made more for the Cars but the same idea applys.
Transmission Vacuum and adjustments
Mercedes-Benz Transmission, Steve Brotherton, ImportCar, February 2002

Hi well a quick up date, because of the body round the engine in hymer , its hard to get to the vacuum pump so managed to take the pipe of it and bingo, reading 24" but most importantly it was instant so with it been a bank holiday cannot get any parts for it, so repaired it with a glue that takes 12hours to set, so will start her up tomorrow and hopefully will run OK, will let you know if all OK, as if is OK will of saved 100s in garage costs, Dave

Hi , just to thank you for the help with this matter, and telling me of the links that have helped a lot to understand how the vacuum system works, well I have just taken my last set of reading's and , minter 24" and it was instant so checked the other side of the vcv 14" and again instant, so I tuck her for a run and she was in top gear at 35mph with a smooth gear change, so ran her a little harder and she changed mint according to pressure on throttle also kick down working OK, just to mention the cut off for the ignition works a lot better, so once again thank you all on this page for your input. Dave.:D:D:D

24" is too high for the input to the VCV.

Usual pressure is around 12-14 I find.

The 'idle' position is normally around 9-11 falling to near zero when throttle opened full.

There are No VCV's left in Europe according to Mark Cosovich....

--Providing the VCV is modulating pressure with throttle-position--Its OK, and the fault is elsewhere.

Hi Al, thanks for that will check it all over again next weekend, but when took her out she ran best she has for 2 years, running like she did when I got her in 2007, but will check vacuum again, Dave

My VCV receives full 21+ as it is connected directly to the booster line.
IIRC, I read 12-0 after the VCV bleed.

Hi cheers for that, guess its down to each motor and how they are set up, as mine is reading 24 out of pump going into vcv and 14 at the other side, she is running mint at this setting, Dave