Marc - If the fuel pump affects the ADAHRS, then its magnetic field is interfering. The card compass may not be as sensitive (or as accurate) as the ADAHRS's magnetic sensors for a variety of reasons. As a rule of thumb, you can use a card/whiskey compass to rule out a bad location (if that other compass also deviates), but even if it passes basic test you may not have a sound location for the ADAHRS. Without going in to too much technical detail - the ADAHRS can see the full 3D magnetic vector. This lets it be correct in all attitudes, but that also means that its sensitive to variations that a card/whiskey compass may not always show in a static test. Though the card compass may look good on the ground, once aircraft dynamics (pitch, bank, yaw) come into play, you'd likely get poor performance from a card compass in that same location as well.