i have a 1985 300sd turbo. The turbo has worked fine since I bought i 6 months ago but a week ago I started hearing a belt screeching type noise. I thought it was a loose belt. Now all of the sudden The turbo is not working. The car struggles to reach 55 mph. Can finally get to that speed after driving at 3000+ rpm then shifting hard. Need any advice I can get. I am unfamiliar with these cars and engines since its the first one Ive owned. Thanks a lot