Low Tire Pressure Indicator: Description and Operation
The Tire Pressure Monitor (TPM) indicator gives an indication to the vehicle operator of the status of the TPM system. This indicator is controlled by a
transistor on the instrument cluster circuit board based upon cluster programming and electronic messages received by the cluster from the Wireless
Ignition Node (WIN) (also known as the Sentry Key Immobilizer Module/SKIM, the Sentry Key REmote Entry Module/SKREEM or the Wireless
Control Module/WCM) over the Controller Area Network (CAN) data bus.
The TPM indicator Light Emitting Diode (LED) is completely controlled by the instrument cluster logic circuit, and that logic will only allow this
indicator to operate when the instrument cluster receives a battery current input on the fused ignition switch output (run-start) circuit. Therefore, the LED
will always be OFF when the ignition switch is in any position except ON or START. The LED only illuminates when it is provide a path to ground by
the instrument cluster transistor. The instrument cluster will turn ON the TPM indicator for the following reasons:
Bulb Test - Each time the ignition switch is turned to the ON position, the WIN sends an electronic TPM indicator lamp-ON message to the
cluster to illuminate the TPM indicator for about three seconds as a bulb test.
TPM Indicator Lamp-On Message - Each time the cluster receives an electronic TPM indicator lamp-ON message from the WIN, the TPM
indicator or the TIRE LOW PRESSURE textual message will be illuminated. The indicator can be flashed ON and OFF, or illuminated solid, as
dictated by the WIN message. The indicator or textual message remains illuminated until the cluster receives a lamp-OFF message from the WIN,
or until the ignition switch is turned to the OFF position, whichever occurs first.
TPM Textual Warning Message - When the WIN detects certain monitored fault conditions, it will send electronic messages requesting the
cluster display a textual warning message. Each time the cluster receives an electronic textual warning request message from the WIN, a
SERVICE TIRE PRESS. SYSTEM textual message will appear within the cluster reconfigurable dot matrix display to accompany a flashing
TPM indicator or to replace the TPM TIRE LOW PRESSURE textual message. This message indicates that the WIN has detected a problem in
one of the TPM system components.
TPM Low Pressure Message - On vehicles equipped with the optional premium version of the TPM system, each time the cluster receives an
electronic low pressure message from the WIN, a LEFT FRONT , RIGHT FRONT , LEFT REAR or RIGHT REAR LOW PRESSURE
textual message will appear within the cluster reconfigurable dot matrix display to accompany a solid TPM indicator. If the vehicle is equipped
with a full-sized matching spare tire and wheel assembly, an electronic spare low pressure message from the WIN to the cluster will cause a
SPARE LOW PRESSURE textual message will appear in the reconfigurable display, but the TPM indicator will not illuminate. This message
indicates that the WIN has received a low tire pressure input from the tire pressure sensor in the identified location on the vehicle. The premium
TPM system is not offered in markets where rear fog lamps are an available option.
Communication Error - If the cluster receives no TPM lamp-ON or lamp-OFF messages from the WIN for six consecutive seconds, the TPM
indicator is illuminated by the instrument cluster. The indicator remains controlled and illuminated by the cluster until a valid lamp-ON or
lamp-OFF message is received from the WIN.
Actuator Test - Each time the cluster is put through the actuator test, the TPM indicator will be turned ON, then OFF again during the bulb check
portion of the test to confirm the functionality of the LED and the cluster control circuitry.
The WIN performs a self-test each time the ignition switch is turned to the ON position to decide whether the TPM system is in good operating condition
and whether the tire inflation pressures are too high or too low. The WIN then sends the proper lamp-ON or lamp-OFF message to the
ElectroMechanical Instrument Cluster (EMIC) (also known as the Cab Compartment Node/CCN). If the instrument cluster turns ON the TPM indicator
after the bulb test, it indicates that the inflation pressure of a tire is too low or that a malfunction has occurred and the TPM system is ineffective.
For proper diagnosis of the TPM system, the WIN, the EMIC, the CAN data bus or the electronic communication related to TPM indicator operation a
diagnostic scan tool is required. Refer to the appropriate diagnostic information.