If the audio is still good, that eliminates the chuff switch. It's rare for the Smart Smoke itself to go bad, usually that regulator is the culprit.
If you're in there working on it, consider installing the Super-Chuffer II, it manages the smoke fan and gives you smoke at idle as well. You can also take advantage of the optional features of Rule-17 lighting and automatic cab light control. With the Super-Chuffer, you won't be using any of the smoke unit logic for controlling the fan, problem solved! You'll also get a lot better chuffing performance.
