Due to the nature of the magnetic detection devices in the module, the compass may occasionally show false directions even when operating properly. Since the circumstances that give rise to these false readings may vary, some general knowledge of how the compass works is helpful in determining why false readings occur. Such perceived malfunctions may be the result of special circumstances, or a true system malfunction. Regardless of the problem, the first step in diagnosing the compass system is recalibration. If the problem persists after recalibration, review the following information regarding compass system functions for help in determining if the system is operating properly.

Recalibration of the compass may be necessary if the vehicle is operated a long distance away from the point of the initial compass calibration. There are many reasons for this, including the following:

When these circumstances arise, a malfunction may be suspected when, in fact, the system is operating normally.

An approximate 15 degree deviation (declination) from true NORTH occurs between the east and west coasts. This declination is the result of the earth's curvature. Various other deviations throughout the country are the result of differences in the magnetic fields.

The compass system may require recalibration if the vehicle's structure is altered sufficiently. This occurs because any significant alterations in the vehicle's structure will cause corresponding alterations in the vehicle's inherent magnetic field. Thus, when the vehicle's structure is altered, as in a collision, the compass readings may reflect the offset in the vehicle's magnetic field. The vehicle operator may then discover differences in compass readings from those previously observed along familiar routes.

The electronic compass, like any compass, is influenced by external magnetic fields. Filters that are programmed into the compass function to counteract much of this external magnetic interference. These filters vary with the vehicle speed. They function by relating the vehicle speed to the time in which the vehicle could conceivably change directions. The filters correspond to the following speeds:

The filters function by taking into account that vehicle directional changes ordinarily do not occur as quickly at higher speeds as at lower speeds. The filters help avoid false readings caused by large steel structures such as the following objects:

Sometimes, the field projected by these structures leads to a reading that represents too drastic a change of direction for the prevailing driving conditions. When this happens, the filters intervene in order to prevent changes in the directional display. However, under borderline conditions, as in large cities with slow driving, the compass may give false readings and appear to be malfunctioning. If this occurs, check the compass in an open area, free from magnetic disturbances.

When travelling on winding roads, the compass readings may oscillate between 2 neighboring directions, such as NORTH and NORTHWEST. This occurs when the road coordinates coincidentally border the coordinates used by the system for the 2 different directions.