That must be a common thing with 85 5000's because our does the same thing.
The Haynes manual doesn't say much about the autocheck system except that falt finding and repair must be left to the dealer shop because of the need for special equipment. Still clear as mud isn't it.
I did find that you can purchase the factory owners manual from Bentley Publishing
http://www.bentleypublishers.com/product.htm?code=rat5
They also have the factory repair manual in book or cd rom form. They are a bit spendy though.