If you don't have the Autopilot maintenance manual, separate from the standard maintenance manual, it's really hard to trouble shoot. The next problem is most avionics shop will refuse to work on these units as they have been burned by working on them and finding more problems than they started with.
Some of the issues are connections being corroded from the computer and all the components in the computer are aging. There is one shop that I know will still work on them in Tulsa but most shops I have talked to are really hesitant to dig in. I'm not saying you can't find a shop to help you so do ask. But Tulsa might be worth a call to discuss.
Ultimately we will need to replace these antique boxes for the new low cost Garmin or Tru Trac units. I know a couple guys with 182's that are having the Garmin unit installed with two G5 for AI and HSI. $18 to $20k installed. Sounds like a lot of money but considering the price of an Stec it's a bargin.
http://www.autopilotcentral.com/