1985 Mercedes 300D Turbo Diesel - Boost/Turbo Question

[marco5]

1985 Mercedes 300D Turbo Diesel
500,000KM
Non Cali version

Hello,

I have been doing quite a bit of testing with my turbo system and boost. I've replaced all boost cables that go from my intake to the switch-over valve and the switch-over valve to the ALDA. I've also replaced the cable from the waste-gate to the turbo which was cracked badly.

I've come up with the following prognosis for the vehicle:

The turbo is spooling but not kicking in at all. Even after replacing all of the cracked boost cables.

The turbo boost starts boosting just after 2000RPM and maxes around 3750RPM. The max boost is 9PSI - this is really good considering original turbo.

When I bypass the switch over valve, its night and day difference. This means directly connecting a boost cable from the intake to the ALDA and not connecting the switch-over valve. I don't just want to replace the switch-over valve without confirming it's bad. Yes I know I can leave it bypassed but I want the original setup for protection in the case of an over boost.

Here are my questions:

Is this valve known to cause issues?

Does leaving this disconnected/bypassed cause any other problems other then the over boost protection possibly with transmission shifting?

How can I now diagnose the switch being bad and not a ECU/computer issue?

What is the ECU what gives this voltage and what should the voltage be? How can I test the voltage going to this valve?

Thanks in advance - you guys are the reason I'm able to drive this beauty!

https://www.photobox.co.uk/my/photo/full?photo_id=21829094341