You have a switchover valve connected to the ALDA; it prevents overboost to the turbo. If the plastic switchover valve hoses are disconnected, connected to the wrong nipples, or if the switchover valve has gone bad, you will get no power. I could not get my '81 300SD over 30mph at full throttle when mine went bad. New one was maybe $25. Pictured below at center right. It looks like a metal cube with a nipple on the top. Attached to the firewall on my SD.