There is a boost control switchover valve on the firewall kinda behind the oil filter housing, there is also a bango bolt (hollow bolt) in the back of the intake holding the supply line for the switchover valve, pull the bolt and see if the hole, bolt or line is clogged with carbon. Clear it and see if it boosts, if you have boost pressure to the switchover valve, bypass it. If that fixes the boost issue, it'll be wiring or some other eletrical problem, if there is no boost from the hole in the intake (just freewheel it to 3k with the bolt out while cleaning) then pop the elbow leading to the turbo and run it up one time, see if the pinwheel is spooling up.
99% of these are nothing more than bad switchover valves or clogged control lines.
Project Smoker, '87 603 powered wagon
Hauler, 96 CTD can you say torque?
Toy 73 Cougar xr7 convertible
Acme Automotive Inc.