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venting system control valve

818 Views 5 Replies 4 Participants Last post by  Govert70227
This is my first post on this forum, although I've done countless repairs just by reading archived threads. I can't even begin to say how valuable this resource has been to me.

My car is an '82 300TD. I have some questions about a plastic component that's vacuum controlled-- one vacuum line going into the top and one into the side near the bottom. From pictures and charts that I've seen, it's called the Venting System Control Valve.

My first question is, what does this part do?

Secondly, I've tested the compression on it, and the top is fine, but the plug on the side doesn't hold any vacuum at all. Is it supposed to be like this, or is it completely blown out?

My final question is, if it is blown out, does anybody know where I can get a replacement, because I can't seem to find it anywhere.

I'm attaching a picture to help clarify.

Thanks so much.


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It is called the Vacuum Control Valve, or VCV for short. It is there to mimic the vacuum in the intake manifold of gas engines. Gas engines have a throttle in the intake, which is controlled by the accelerator pedal. If the throttle is closed, there is a vacuum in the intake manifold, the more you press the pedal, the more the throttle is opened and the vacuum decreases. This vacuum is used as a engine-load signal for the ignition timing, automatic transmission and the EGR.

Most W123 diesels don't have a throttle, so there is no vacuum in the intake manifold. Vacuum is produced by a vacuum pump. To get a engine-load signal, this vacuum control valve was installed, the more you press the accelerator pedal, the more a vacuum line is vented, so that there is no vacuum at full load.

The VCV is used to control the oil pressure inside the automatic transmission and the EGR. If you drive at full-load, the oil pressure inside the transmission has to be high, when driving at low-load, oil pressure can be lower.

The line on the side is the vent line, that should not hold vacuum.

If the engine is running, the vacuum measured at the top should be about 10 inch Hg. (-0.35 bar).
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There is no ignition timing on a diesel.
I was talking first about gas engines and how vacuum is used on those engines.

I'm not sure how the VCV can control oil pressure in the transmission.
The variable vacuum is "fed" to the vacuum modulator on the transmission. The modulator transforms the vacuum signal into a movement of a pin, which controls the modulation pressure inside the transmission.
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