Circuit Description
The Automatic Transmission Fluid Temperature (TFT) Sensor is a thermistor that controls the signal voltage to the Transmission Control Module (TCM). The TCM supplies a 5 volt reference signal to the sensor on circuit 1227. When the transmission fluid is cold, the sensor resistance is high and the controller will sense high signal voltage.
As the transmission fluid temperature warms to normal transmission operating temperature, the sensor resistance becomes less and the voltage decreases. With a DTC 79 also set, check the transmission cooling system.
If the TCM detects a continuous short to ground in the TFT signal circuit, then DTC 58 sets.
Conditions for Setting the DTC
| • | The TFT signal voltage indicates TFT greater than 151°C (306°F). |
| • | All conditions met for 1 second. |
Action Taken When the DTC Sets
| • | The TCM does not illuminate the Malfunction Indicator Lamp (MIL). |
| • | The TCM will default to 140°C for a Trans. Fluid Temperature (causing a hot mode shift pattern to occur). |
| • | DTC 58 is stored in TCM history. |
Conditions for Clearing the DTC
| • | The TCM clears the DTC from Current to History when the fault condition(s) no longer exist and the ignition switch is cycled OFF and then ON. |
| • | A scan tool can clear the DTC from Current and History status. The TCM clears the DTC from the TCM history if the vehicle completes 40 ignition cycles without a failure reported. |
| • | The TCM cancels the DTC default actions when the fault no longer exists and the ignition is cycled OFF and then ON. |
Diagnostic Aids
| • | With a transmission fluid overtemperature DTC 79 also set, inspect the transmission cooling system. |
| • | Inspect the harness routing for a potential short to ground in circuit 1227. The scan tool Transmission Fluid Temperature display should rise steadily, then stabilize. |
| • | Inspect the wiring at the TCM, the transmission connector and all other circuit connecting points for the following conditions: |
| - | A bent terminal |
| - | A backed out terminal |
| - | A damaged terminal |
| - | Poor terminal tension |
| - | A chafed wire |
| - | A broken wire inside the insulation |
| - | Moisture intrusion |
| - | Corrosion |
| • | When diagnosing for an intermittent short or open, massage the wiring harness while watching the test equipment for a change. |
| • | Test the TFT Sensor at the various temperatures in order to evaluate the possibility of a skewed (mis-scaled) sensor. Use the Temperature vs Resistance table. A skewed sensor may cause firm shifts or TCC complaints, extended shift patterns, and early TCC applies to occur. |
| • | Verify the customer driving habits, trailer towing, weight or towing in Overdrive. |
| • | First diagnose and clear any engine DTCs that are present. Then inspect for any transmission DTCs that may have reset. |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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This step tests for a short to ground or a skewed sensor by verifying the fault still exists.
-
This step tests for an internal fault within the transmission by creating an open.
-
This step tests circuit 1227, of the Interior Wiring Harness, to ground.
-
The TFT Sensor is part of the Automatic Transmission Wiring Harness Assembly.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | ||
2 | Perform the Transmission Fluid Checking Procedure. Refer to Transmission Fluid Check . Did you perform the fluid checking procedure? | -- | Perform the Transmission Fluid Check | |
Important:: Before clearing the DTC's, record the Current and History DTC's information for reference. The Clear Info function erases the stored Current and History DTC from the TCM. Does the Scan Tool display transmission fluid temperature greater than the specified value? | 151° C (306° F) | Go to Diagnostic Aids | ||
Does the Scan Tool display a transmission fluid temperature greater than the specified value? | 150° C (305° F) | |||
5 |
Is the resistance within specified values? | 16 kohms at 10° C (50° F) to 133ohms at 110° C (230° F) | ||
Measure the resistance between terminal L and a good ground on the transmission case. Is the resistance less than the specified value? | 50 kohms | Go to Diagnostic Aids | ||
Replace the Automatic Transmission Wiring Harness Assembly. Refer to Interior Wiring Harness Replacement. Is the replacement complete? | -- | -- | ||
8 | Inspect circuit 1227 for a short to ground. Refer to General Electrical Diagnosis . Was the condition found and corrected? | -- | ||
9 | Inspect the TCM for faulty connections. Was the condition found and corrected? | -- | ||
10 | Replace the TCM. Refer to DTC 51 PROM Error Replacement. Is the replacement complete? | -- | -- | |
11 | In order to verify your repair, perform the following procedure:
Is the transmission fluid temperature less than 150° C (305° F) for 10 seconds? | -- | System OK | Begin diagnosis again |
