Mike,
Which TruTrak autopilot do you have? I looked through the Sorcerer documentation and could find nothing on an internal magnetometer. I've also done a Sorcerer installation in a Legacy and don't recall anything about a magnetometer. TruTrak does have internal gyros. The Sorcerer operating manual states that if GPS data is lost the track function switches to bank;
i.e. it uses its internal gyros to maintain bank. The autopilot function of the ADI Pilot also switches to a gyro backup mode if GPS data is lost.
Tom Gourley