Thanks everyone for the help with the switchover valve.

Yeah, these cars have numerous switchover valves, depending.

The SOV concerning the ALDA is a redundant turbo safety device. I've been driving a car with one bypassed for over a decade.

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83 SD

84 CD

The ALDA line carries intake manifold PRESSURE, never vacuum.
Turbo boost pressure pushes the ALDA diaphragm = adding more fuel.

.

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so does the valve hold pressure or not? I just want to know if it is good or not. thanks

The SOV concerning the turbo handels pressure, as whunter mentions. If too much pressure is occurring, the SOV will vent off any excess.

The pressure surrounds a capsule inside the ALDA.

The more the pressure, or turbo boost, the more fuel is allowed to the IP.

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83 SD

84 CD

It's a little confusing to describe. In it's normal configuration (turbo pressure below 15 pounds or so) it would be open to the ALDA allowing turbo pressure to be seen by the ALDA. If the pressure exceeds the max set point then the valve "switches" dumping pressure to atmosphere allowing the ALDA to see atmospheric pressure, not boost pressure and thereby reducing fuel to keep the engine from grenading.

So if you were to try to test it on the bench you would not see any pressure at least if you were measuring it on the side you were applying pressure. As you apply pressure with the valve unpowered the pressure vents to the ALDA port and with it powered it vents to the atmosphere port. If you were to put your gauge on the ALDA side, you would then see pressure if the switchover valve were "open" and no pressure if it were closed (power applied).

The reality is that probably 98% of ALDA switchover valves have never been activated as they are only there as a failsafe device should the turbo get into an overboost situation...very rare indeed.

Maybe we're talking about a different switchover valve??

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To answer the op question... No. The switchover itself does not hold pressure or vacuum. It is an air flow director. If you take the hoses off it, and subject either port to vacuum or pressure, one spot will hold, the other two will vent. However, if you leave the sov connected to the Alda, and apply vacuum to the manifold side ov the sov, it will hold vacuum, or pressure unless powered, then it will dump to atmosphere.

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