Trans Temp Indicator: Operation

The transmission over-temperature indicator gives an indication to the vehicle operator when the transmission fluid temperature is excessive, which may lead to accelerated transmission component wear or failure. This indicator is controlled by a transistor on the instrument cluster circuit board based upon cluster programming and electronic messages received by the cluster over the Programmable Communications Interface (PCI) data bus. These messages are sent by the Powertrain Control Module (PCM) or by the Transmission Control Module (TCM), depending upon the model of the automatic transmission. The transmission over-temperature indicator bulb 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 bulb will always be off when the ignition switch is in any position except On or Start. The bulb only illuminates when it is provided a path to ground by the instrument cluster transistor. The instrument cluster will turn on the transmission over-temperature indicator for the following reasons:

• Bulb Test  - Each time the ignition switch is turned to the On position the transmission over-temperature indicator is illuminated for about three seconds as a bulb test.
• Trans Over-Temp Lamp-On Message  - Each time the cluster receives a trans over-temp lamp-on message from the PCM or TCM indicating that the transmission fluid temperature is 135° C (275° F) or higher, the indicator will be illuminated. The indicator remains illuminated until the cluster receives a trans over-temp lamp-off message from the PCM or TCM, or until the ignition switch is turned to the Off position, whichever occurs first.
• Actuator Test  - Each time the cluster is put through the actuator test, the transmission over-temperature indicator will be turned on for the duration of the test to confirm the functionality of the bulb and the cluster control circuitry.

The PCM or TCM continually monitors the transmission temperature sensor to determine the transmission operating condition. The PCM or TCM then sends the proper trans over-temp lamp-on or lamp-off messages to the instrument cluster. If the instrument cluster turns on the transmission over-temperature indicator due to a high transmission oil temperature condition, it may indicate that the transmission or the transmission cooling system are being overloaded or that they require service. If the transmission over-temperature indicator fails to light during the bulb test, replace the bulb with a known good unit. For further diagnosis of the transmission over-temperature indicator or the instrument cluster circuitry that controls the indicator, refer to DIAGNOSIS AND TESTING - INSTRUMENT CLUSTER  . For proper diagnosis of the transmission temperature sensor, the PCM, the TCM, the PCI data bus, or the electronic message inputs to the instrument cluster that control the transmission over-temperature indicator, a DRBIII(R) scan tool is required. Refer to the appropriate diagnostic information.