Compass And Mini-Trip Computer Module - Description
DESCRIPTION
NOTE: The Compass Mini-Trip Computer (CMTC) module is sometimes referred to as Electronic Overhead Module (EOM) for the purposes of scan tool and diagnostic information naming. Please treat the CMTC and EOM as one and the same when diagnosing the vehicle.
The Compass Mini-Trip Computer (CMTC) is located in the overhead console and consists of the following components:
- HomeLink(R) transceiver (3) - if equipped
- Vacuum-Fluorescent Display (VFD) (6)
- Push button function switches (1, 2, 3, 4 and 5)
- Replaceable light bulbs to illuminate the push button function switches
The CMTC contains a central processing unit and interfaces with other electronic modules in the vehicle on the Controller Area Network (CAN) data bus. The CAN data bus allows the sharing of sensor information. This helps to reduce wire harness complexity, reduce internal controller hardware, and reduce component sensor current loads. At the same time, this system provides increased reliability, enhanced diagnostics, and allows the addition of many new feature capabilities. The individual components of the CMTC module are not serviced separately and if damaged or inoperative, must be replaced as a unit.
The CMTC provides several electronic functions and features. Some of the functions and features that the CMTC supports and/or controls, include the following display options:
- Compass and Temperature - provides the outside temperature and one of eight compass readings to indicate the direction the vehicle is facing.
- Trip Odometer (ODO) - shows the distance travelled since the last trip computer reset.
- Average Fuel Economy (AVG ECO) - shows the average fuel economy since the last reset. Average fuel economy is a running average of the amount of fuel used and the distance the vehicle has traveled. When the fuel economy is reset, the display will momentarily blank. Then, the display will show the same value as before the rest. The reset value is based on a minimal amount of fuel used and the distance traveled from the previous drive cycle. The display may take several miles for this value to change dependent upon driving habits.
- Distance To Empty (DTE) - shows the estimated distance that can be traveled with the fuel remaining in the tank. This estimated distance is determined by weighted average of the instantaneous and average fuel economy, according to the current fuel tank level. This is not resettable. Significant changes in driving style or vehicle loading will greatly affect the actual drivable distance of the vehicle, regardless of the DTE displayed value. When the DTE value is less than 30 miles estimated driving distance, the DTE display will change to an alternating test display of "LO" and "FUEL". This display will continue until the vehicle runs out of fuel. Adding a significant amount of fuel to the vehicle will turn off the "LO FUEL" text and a new DTE value will be displayed, based on the current values in the DTE calculation and the current fuel tank level.
- Elapsed time (ET) - shows the accumulated ignition-on time since the last trip computer reset.
- Blank screen - the compass mini-trip computer VFD is turned off or in Blank Screen Mode.
COMPASS
NOTE: If the compass functions, but accuracy is suspect, it may be necessary to perform a variance adjustment. This procedure allows the compass unit to accommodate variances in the earth's magnetic field strength, based on geographic location.
While in the compass/temperature mode, the compass will display the direction in which the vehicle is pointed using the eight major compass headings (Examples: north is N, northeast is NE). The auto-calibrating compass unit requires no manual adjusting in normal use. The only calibration that may prove necessary is to drive the vehicle in one or two complete circles, on level ground, not exceeding 8 km/h (5 mph). This will reorient the compass unit to its vehicle.
The compass unit also will compensate for vehicle magnetism acquired during normal use. However, avoid placing anything magnetic directly on the roof of the vehicle. Magnetic mounts for an antenna, a repair order hat, or a funeral procession flag can exceed the compensating ability of the compass unit if placed on the roof panel. Magnetic bit drivers used on the fasteners that hold the overhead console assembly to the roof header can also affect compass operation.
TEMPERATURE
NOTE: The displayed temperature is not an instant reading of conditions, but an average temperature. It may take the temperature display several minutes to respond to a major temperature change, such as driving out of a heated garage into winter temperatures.
The temperature displays the outside ambient temperature in whole degrees. The temperature display can be toggled from Fahrenheit to Celsius using the U.S./Metric push button.
Displayed temperature reading stays in the temperature unit memory. When the ignition switch is turned to the On position, the temperature will display the memory temperature if the engine OFF time is less than 255 minutes. If the engine OFF time is more than 255 minutes, the temperature will display the actual temperature sensed by the ambient temperature sensor. The temperature display update interval varies with the vehicle speed.
The temperature function is supported by an ambient temperature sensor. This sensor is mounted outside the passenger compartment near the front and center of the vehicle, and is hard wired to the Front Control Module (FCM). The instrument cluster, also known as the Cab Compartment Node (CCN) averages and filters the ambient temperature signal and supplies it to other modules on the CAN data bus. Data input for all other compass mini-trip computer functions, including VFD dimming level, is received through CAN data bus messages. The maximum temperature displayed is 66° C (140° F). It could take several miles for the accurate temperature to be displayed as the CCN filters out heat from the engine compartment.
The CMTC uses its internal programming and all of these inputs to calculate and display the requested data. If the data displayed is incorrect, perform the self-diagnostic tests. If these tests prove inconclusive, the use of a scan tool and the proper Diagnostic Procedures Information are recommended for further testing of the CMTC and the CAN data bus.
NOTE: The CMTC will display min and max temperatures of -40° C (-40° F) and 66° C (140° F).