Compass/Mini-Trip Computer (CMTC)
COMPASS/MINI-TRIP COMPUTER (CMTC)When equipped, the Compass/Mini Trip Computer (CMTC) is located in the overhead console. The CMTC supplements the standard vehicle instrumentation. The CMTC uses a vacuum fluorescent (VF) display to supply the vehicle operator with a compass heading, outdoor temperature, average fuel economy, distance to empty, trip odometer, and elapsed ignition on time. If equipped, the CMTC is also available with an integrated Universal Garage Door Opener (UGDO) known as HomeLink.
The CMTC function buttons are labeled C/T, RESET STEP and USIM. The three UGDO buttons are labeled with dots to indicate the channel number.
Most of the CMTC display information is received over the PCI bus. The CMTC sends and receives data over the PCI bus, communicating with the NGC (3.7L and 4.7L), or JTEC (5.9L), and the Instrument Cluster.
VEHICLE INFORMATION DISPLAY
The CMTC provides the following functions:
- Compass direction
- Outside temperature
- Elapsed ignition on time
- Distance to empty
- Average fuel economy
- Trip Odometer
The CMTC will not display information for any of the screens for which it did not receive the proper PCI bus data. Refer to the symptom list in the Overhead Console section for problems related to the CMTC.
The CMTC receives the following messages from the Instrument Cluster:
- Verification of US/Metric status
- VF display dimming brightness and exterior lamp status
- Trip Odometer data
- Elapsed Ignition On Time data
- Average Fuel Economy
- Distance to Empty
The CMTC receives the following message from the NGC or JTEC:
- Vehicle Speed
Compass and Temperature Display
This display provides the outside temperature and one of eight compass readings to indicate the direction the vehicle is facing.
Trip Odometer (ODO)
Shows the distance traveled since the last reset.
Average Fuel Economy (AVE ECO)
Shows the average fuel economy in MPG (miles per gallon) or L/100 Km since the last reset.
Instant Fuel Economy
Shows the fuel economy for the last few seconds in MPG or L/100 Km.
Distance to Empty (DTE)
Shows the estimated distance that can be traveled with the fuel remaining in the tank. This is calibrated using the MPG for the last few minutes.
Elapsed Time (ET)
Shows the accumulated ignition ON time since the last reset.
US/M Button
The US/M button is used to toggle the display between English and Metric measurement units.
STEP Button
The STEP Button can be used in one of the following ways:
1. To sequentially select one of 4 displays or blank display in the following order:
- Average Fuel Economy
- Distance to Empty
- Trip Odometer
- Time Elapsed
- Off (Blank)
2. To set the magnetic variance zone when VARIANCE = X (X=1-15) is indicated in the VF display.
Reset Button
The RESET Button has two different functions:
1. To clear the trip functions that may be reset
2. To enter and exit the diagnostic mode
Pressing the RESET button once will clear the trip function that is currently being displayed and the CMTC will send a PCI bus beep request to the Instrument Cluster. If the RESET button is pressed again within 3 seconds, the CMTC will reset ALL of the trip functions and an additional beep request is sent to the Instrument Cluster. The trip functions that may be reset are:
- Average Fuel Economy
- Trip Odometer
- Elapsed Time
A reset will only occur if one of the trip functions that may be reset is currently being display. The CMTC module will send a beep request to the Instrument Cluster.
Simultaneously pressing the RESET button and the RESET button while turning the ignition from Off to On will enter the CMTC into the self- diagnostic mode.
Compass/Temperature (C/T) Button
Actuating the Compass/Temperature Button (C/T) will cause the CMTC to display the compass and temperature information. This function will operate from another traveler display. The CMTC simultaneously displays the compass reading and the outside temperature. Vehicles equipped with the 3.7L or 4.7L engine receive outside temperature information via the PCI bus from the NGC. JTEC equipped vehicles receive outside temperature information via the PCI bus from the HVAC module. The CMTC module internally senses and calculates the compass direction.
Traveler Display Functions
Using the STEP button will change the CMTC between modes of operation and display the appropriate information according to data received from the PCI Bus.
Compass Orientation
Upon ignition on, if the calibration information stored in the CMTC memory is within the normal range, the CMTC will perform in slow Auto-Cal mode. In slow Auto-Cal mode, the CMTC continuously compensates for the slowly changing magnetic field of the vehicle, The compass module detects changes in the vehicle magnetism and makes appropriate internal corrections to ensure proper displayed direction.
However, if the calibration information stored in the CMTC memory is not within the normal range at ignition on, the CMTC will enter fast Auto-Cal. CAL is displayed along with the temperature.
Auto activation of the fast Auto-Cal mode will also occur when the CMTC is subjected to high magnetic field strength levels, which cause all compass readings to be erroneous for a continuous period of five (5) minutes. During fast Auto-Cal, CAL will be displayed along with the temperature.
Fast Auto-Cal can also be performed manually, by pressing and holding the RESET button for 10 seconds during the Compass/Temperature display mode.
Setting Magnetic Zone Variance
Variance is the difference between Magnetic North and Geographic North. In some areas of the country, the difference between Magnetic and Geographic North is great enough to cause the compass to give false readings. If this occurs, the compass variance must be set. Refer to the Zone Variance map for the correct zone. Follow these steps to check or change the variance zone:
- The ignition switch must be in the On position and the CMTC display must not be blank.
- If the compass/temperature data is not currently being displayed, momentarily press and release the C/T button to display compass/temp information.
- Press and hold the RESET button until VARIANCE = XX is displayed. The CMTC will display the variance zone stored in memory and the word VARIANCE.
- Using the STEP button to select the proper variance zone number, 1 through 15.
- After selecting the proper zone number, momentarily press and release the RESET button. The variance zone is then stored in the memory and the CMTC returns to normal operation.
Compass Calibration
The compass module has 2 types of auto-calibration; slow-cal and fast-cal. Slow-cal ensures that during normal vehicle operation the compass performs auto-calibration functions to keep the compass sensors in their proper operating range. Whenever the ignition is On and the CMTC receives PCI bus data indicating that engine RPM is greater than zero, auto-calibration is performed continuously.
If the calibration information stored in the compass module memory is not within the normal range after a power-up cycle, the compass will display CAL. The CMTC will enter into the fast-cal mode until calibration is complete.
To enter the compass into Manual Calibration mode, perform the following steps:
- Drive the vehicle to an area away from any large metal objects or overhead power lines.
- Ensure that the proper variance zone is selected. See "Setting Magnetic Zone Variance."
- The ignition switch must be in the On position and the CMTC display must not be blank.
- Press the C/T button to view the Compass/Temperature display.
- Press and hold the RESET button until CAL is displayed, then release the button.
- Drive slowly, less than 5 mph (8 kph) in at least 1 complete 360 degree circle.
- CAL will remain illuminated to alert the driver that the compass is in the calibration mode.
- After calibration is complete, CAL will turn off.
If the compass appears blank, unable to be calibrated, or the compass displays false indications, The vehicle must be demagnetized. Refer to Compass Demagnetizing Procedure.
SELF-CHECK DIAGNOSTICS
The CMTC is capable of performing a diagnostic ,self cheek on its internal functions. CMTC diagnostics may be performed using a DRBIII or by using the following procedure:
1. With the ignition switch in the OFF position, depress and hold the RESET and the STEP buttons.
2. Turn the ignition switch to the ON position.
3. Continue to hold both buttons until the software versions are displayed, then release the buttons.
4. All of the VFD segments will illuminate for 2-4 seconds. Check for segments that do not illuminate or illuminate all the time.
5. When the self-check is complete the CMTC will display one of the following messages:
- PASS SELF TEST
- FAILED SELF TEST
- NOT RECEIVING J1850 MESSAGE
6. To exit the self-check mode, depress the RESET button or cycle the ignition switch and the CMTC will return to normal operation.
If a Communication fault is displayed, refer to the symptom list. If a FAILED SELF TEST is displayed, the CMTC must be replaced.
AMBIENT TEMPERATURE SENSOR
The ambient air temperature is monitored by the NGC in 3.7L and 4.7L vehicles, or by the HVAC module in JTEC equipped vehicles, and is displayed by the CMTC. The NGC or HVAC module receives a hardwire input from the ambient temperature sensor (ATS).
The ATS is a variable resistor that operates on a 5 volt reference signal circuit hardwired from the NGC or HVAC module. The resistance in the ATS changes as the outside temperature rises or falls. The NGC or HVAC module senses the change in reference voltage through the ATS resistor. Based on the resistance of the ATS, the NGC or HVAC module is programmed to correspond to a specific temperature. The NGC or HVAC module stores and filters the ambient temperature data and transmits this data to the CMTC via the PCI Bus. The ATS cannot be adjusted or repaired and, if faulty or damaged, it must be replaced.
AMBIENT TEMPERATURE SENSOR FAULT CODES
The outside temperature function is supported by the ambient temperature sensor (ATS), a signal and ground circuit hardwired to the NGC or HVAC module, and the CMTC display.
If the CMTC display indicates 54 °C (130 °F) or the ATS sense circuit is shorted to ground, the temp display will be 54 °C (130 °F) to indicate a SHORT circuit condition.
If the CMTC display indicates -40 °C (-40 °F) or the ATS sense circuits is open, the temp display will be -40 °C (-40 °F) to indicate an OPEN circuit condition.
If there is an OPEN or SHORT circuit condition, it must be repaired before the CMTC VFD can be tested.
The ATS is supported by the NGC or HVAC module. Ambient Temperature Sensor DTCs will be recorded in the NGC or HVAC module. The ATS can be diagnosed using the following Sensor Test. Test the ATS circuits using the diagnostics in the Body Diagnostic Procedures Manual. If the CMTC passes the self test, and the ATS, the circuits, and PCI bus communications are confirmed to be OK, but the CMTC temperature display is inoperative or incorrect, replace NGC or HVAC module.
AMBIENT TEMPERATURE SENSOR TEST
1. Turn the ignition OFF.
2. Disconnect the ATS harness connector.
3. Measure the resistance of the ATS using the following min/max values:
TEMPERATURE RESISTANCE
0 °C (32 °F) 29.33 - 35.99 Kilohms
10 °C (50 °F) 17.99 - 21.81 Kilohms
20 °C (68 °F) 11.37 - 13.61 Kilohms
25 °C (77 °F) 9.12 - 10.86 Kilohms
30 °C (86 °F) 7.37 - 8.75 Kilohms
40 °C (104 °F) 4.90 - 5.75 Kilohms
The sensor resistance should read between these min/max values. If the resistance values are not OK, replace the Sensor.