Can anyone explain exactly how the overboost protection is supposed to work? I have figured out that the change-over valve between the intake manifold banjo bolt and the ALDA can vent the boost so the ALDA cuts back on fuel. But what detects the overboost condition and sends a signal to the change-over valve to make it dump the boost?

In fiddling with my wastegate adjustment, I hit 17 psi and didn't get de-fueled, so I'm trying to learn enough about the system to diagnose it. This is an '82 300D but somehow ended up with the KKK turbo in it. The change-over valve is connected but obviously the system doesn't seem to work.

The switchover valve is supposed to bleed off pressure from the intake once a certain psi is reached.

There is a procedure to check the switchover valve, but I can't find it right now.

17 psi won't trip the switchover valve.

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Pressure is vented from the ALDA capsule, not the intake manifold. (Except when the switchover valve is connected backwards.)

Grounding the lead to the switch would make for a good start.


17 psi should close the 1.1 bar overboost protection switch, thereby actuating the switchover valve.

The overboost protection switch on the intake manifold. When closed, it provides a ground path for the switchover valve solenoid.

The switchover valve does not "dump the boost." It just vents the line from the valve to the ALDA. The intake-to-switchover valve line is blocked.

You might try removing the switch from the intake manifold and cleaning the port. It's pretty common for the switch to get gunked up with manifold residue.

You're correct, I'm mathematically challenged at times.

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