Compass: Testing and Inspection

The Compass Mini-Trip Computer is capable of performing a diagnostic self check on many of its internal functions. Compass Mini-Trip Computer diagnostics may be performed using a DRB III scan tool and the proper Body Diagnostic Procedures Manual or by the following procedure.
1. With the ignition switch in the OFF position, press both the US/M and STEP button.
2. Turn ignition switch to the ON position.

The Compass Mini-Trip Computer will perform internal checks while lighting all segments of the vacuum florescent display Upon completion of the internal check, the Compass Mini-Trip Computer will display one of five messages;
- PASS - Passed Self Test
- FAIL - Failed Self Test
- BUS - Not receiving J1850 messages from Single Board Engine Controller (SBEC) and Body Control Module (BCM)
- bus1 - Not receiving J1850 messages from Single Board Engine Controller (SBEC)
- bus2 - Not receiving J1850 messages from Body Control Module (BCM)

If any segment of the Compass Mini-Trip Computer fails to light replace the computer.

If FAIL is displayed, repeat the test. If it still fails, replace the Compass Mini-Trip Computer module.

If a BUS(X) message is displayed, check the corresponding J1850 PCI Data Bus Circuit for proper operation. Refer to the appropriate diagnostic manual for additional diagnostics.

COMPASS CALIBRATION
If the "CAL" indicator is lit, the compass should be calibrated. The vehicle will be shipped with the "CAL" light on. If during normal driving, the compass appears to be inaccurate, calibration may be required.

To Calibrate the Compass
1. Put the CMTC into COMPASS/TEMP mode using the STEP button.
2. Depress the STEP and US/M buttons simultaneously until CAL appears in the display.

Fig.1 Compass Calibration:

3. Drive the vehicle in about two circles (Fig. 2) on a level surface in an area free of large metal objects, such as other vehicles, large buildings, bridges, underground cables, railroad tracks, etc.

NOTE: The compass will usually calibrate after the vehicle has been driven in a circle comprising five quadrants or one and a quarter turns.

When the "CAL" indicator goes OFF, the compass is calibrated and should display correct headings (for all conditions). Verify proper calibration by checking North (N), South (S), East (E), and West (W). If the compass does not appear accurate, repeat the calibration procedure in another area.

VARIANCE SETTING PROCEDURE
Variance is the difference between magnetic North and geographic North.
1. Depress the STEP and US/M buttons simultaneously until VAR appears in the display, then release the buttons.

Setting The Variance:

2. The CMTC will display the current variance zone (1-15). If the variance needs to be changed use the STEP button to select the correct zone number from the figure below (Fig. 3). After selecting the desired variance zone, the CMTC will return to normal operation after 5 seconds or by pressing the STEP and US/M buttons simultaneously.