Trip Computer: Testing and Inspection

If the problem with the compass mini-trip computer module is an inaccurate or scrambled display refer to Self-Diagnostic Test below. If the problem with the compass mini-trip computer module is incorrect Vacuum Fluorescent Display (VFD) dimming levels, use a DRB scan tool and the proper Diagnostic Procedures to test for the correct dimming message inputs being received from the instrument cluster over the J1850 data bus. If the problem is a no-display condition, use the following procedures. For complete circuit diagrams, refer to Overhead Console.
1. Check the fused ignition switch output (run/ start) fuse in the junction block. If OK, go to Step 2. If not OK, repair the shorted circuit or component as required and replace the faulty fuse.
2. Turn the ignition switch to the ON position. Check for battery voltage at the fused ignition switch output (run/start) fuse in the junction block. If OK, go to Step 3. If not OK, repair the open fused ignition switch output (run/start) circuit to the ignition switch as required.
3. Turn the ignition switch to the OFF position. Disconnect and isolate the battery negative cable.
Remove the overhead console. Check for continuity between the ground circuit cavities of the roof wire harness connector for the overhead console and a good ground. There should be continuity. If OK, go to Step 4. If not OK, repair the open ground circuit to ground as required.
4. Connect the battery negative cable. Turn the ignition switch to the ON position. Check for battery voltage at the fused ignition switch output (run/start) circuit cavity of the roof wire harness connector for the overhead console. If OK, refer to Self-Diagnostic Test below, for further diagnosis of the compass mini-trip computer module and the J1850 data bus. If not OK, repair the open fused ignition switch output (run/start) circuit to the junction block as required.

Self-Diagnostic Test
A self-diagnostic test is used to determine that the compass mini-trip computer module is operating properly electrically. Initiate the self-diagnostic test as follows:

NOTE: The compass mini-trip computer self - test can also be performed using a DRB III. Refer to Auto Self Test under the System Tests.

1. With the ignition switch in the OFF position, simultaneously depress and hold the STEP and RESET buttons while rotating the ignition switch to the Run/ON position.
2. Continue to hold both buttons depressed until the compass mini-trip computer module enters the display segment test. In this test, all of the Vacuum Fluorescent Display (VFD) segments are lighted while the compass mini-trip computer module performs the following checks:
- Non-Volatile Memory Status (NVM)
- RAM Status
- ROM Status
- ASIC Communication Test Status
- Compass Test Status
- DC Status
- J1850 Communications Test

NOTE: If module is equipped with the Universal Transmitter, the module also checks "Universal Transmitter Communication Test Status".

3. Following completion of these tests, the compass mini-trip computer will display one of two messages: "FAIL" or "PASS." Press RESET or STEP to exit and enter normal mode. Respond to these test results as follows:
- If no test result message is displayed, but compass mini-trip computer operation is still improper, the use of a DRB III scan tool and the proper Diagnostic Procedures manual are required for further diagnosis.
- If the "FAIL" message is displayed, the compass mini-trip computer module is faulty and must be replaced.
- If the "PASS" message is displayed, the compass mini-trip computer module is OK, no faults are present.
- If any VFD segment should fail to light during the display segment test, the compass mini-trip computer module is faulty and must be replaced.
4. If the first seven tests pass, the Compass mini-trip computer shall verify that all the required J1850 messages are present on the J1850 bus. If all required J1850 messages are present on the J1850 bus, or if the ignition switch is turned to the OFF position, the compass mini-trip computer module will automatically return to normal operation. The required J1850 messages are:
- Dimming message
- Fuel Used message
- Distance Pulses message
- Percentage Fuel Tank Full message
- Fuel Type message
- VIN (Vehicle Identification Number)
- Vehicle Speed message
- Last Engine Off Duration message
- Ambient Air Temperature message

NOTE:
- The Fuel Used J1850 message shall be ignored if the engine type is Diesel or Compressed Natural Gas.
- Pressing the STEP or RESET switches during any portion of the testing procedure will cause the compass mini-trip computer to exit diagnostics and return to compass/temperature mode.
- If the compass functions, but accuracy is suspect, it may be necessary to perform a variation adjustment. This procedure allows the compass unit to accommodate variations in the earth's magnetic field strength, based on geographic location. Refer to Compass Variation Adjustment.
- If the compass reading has blanked out, and only "CAL" appears in the display, demagnetizing may be necessary to remove excessive residual magnetic fields from the vehicle. Refer to Compass Demagnetizing.