solenoid inline w/ air hose, intake manifold to injector pump?

I have an 82 300sd turbo diesel. There is a solenoid mounted near the driver side firewall that my local mechaniic said was safe to just always bypass. One mechanic said it is electrically activated by a
signal from a pressure sensor on the intake manifold that senses intake air pressure or vacuum. With this solenoid bypassed I have an air hose always running from the intake manifold air to the injectors pump. What is this solenoid called? What does it do and is it safe to leave it bypassed?
He said he had to bypass this solenoid that is always clogging up so that my transmission will shift smoothly and it sure does shift a lot smoother now but I want to know what the solenoid is for. They have 2 mechanics and each told me a different story of what the solenoid did.
thanks, George
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The valve you cite is called the switchover valve and is indeed a safety valve that's triggered by a 16 psi sensor on the intake manifold. It should NOT be by-passed.
The line to this valve comes from the intake manifold where it does sometimes get plugged up with oil and soot, but at that end, not at the by-pass valve. The purpose of this line and valve etc. is to transmit the turbo's boost pressure to the injection pump so the IP "knows" to add additional fuel to the engine as the boost rises. The safety valve ensures that the engine won't run away if the turbo's wastegate (11 psi pressure relief valve) were ever to stick shut. In that rare event too much boost would be sent to the injection pump which would add even more fuel and the engine would be uncontrolled. So don't by-pass it.
Finally, the transmission's shift smoothness is controlled by a VACUUM powered modulator whose vacuum is controlled by a white plastic vacuum valve attached to the rear of the injection pump.
Find someone who knows the difference between pressure and vacuum, some one who knows what he's talking about, not someone who's illogically stringing together a bunch of handy phrases.
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They actually fixed at least 2 problems but the one mechanic described the fix as if they were related. The car was accelerating very slow and also shifting hard. I think they must have fixed only the slow acceleration problem by bypassing the valve. I was told the fuel supply was getting limited too much so the sensor for the valve must have been activating the valve all the time as if it thought the intake always had more than 16psi pressure. Else, the valve was actually stuck in the limiting position.
But also they did something that made the vacuum correct going to the transmission and they talked like this bypass fix is what did it but I'm not sure how the 2 are related. Also my cruise control has started working now.
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