TCM Engine Speed Output Circuit Low
P0738 indicates the transmission control module (TCM) detected a low electrical signal in the engine speed output circuit. This circuit is critical for transmission shifting and fuel economy calculations, so the TCM cannot communicate properly with the engine control module about engine RPM.
What You Might Notice
- Hard or delayed gear shifts
- Reduced fuel economy
- Engine stalling or bucking at low speeds
- Inaccurate or erratic speedometer reading
- Reduced throttle response or poor acceleration
Most Common Causes
- 1
Defective Engine Speed Sensor
The engine speed (RPM) sensor may be failing or producing a weak signal that the TCM interprets as low voltage. This is the most common cause of P0738.
- 2
Wiring Harness or Connector Issues
Corroded, loose, or damaged wiring in the engine speed sensor circuit can reduce signal voltage to the TCM, triggering this code.
- 3
TCM Electrical Fault
Internal TCM issues or faulty input circuits within the transmission control module itself may fail to properly receive the engine speed signal.
- 4
Engine Control Module Communication Failure
Communication problems between the ECM and TCM can prevent proper signal transmission, causing the TCM to detect abnormally low values.
How to Diagnose It
- 1
Check Defective Engine Speed Sensor — The engine speed (RPM) sensor may be failing or producing a weak signal that the TCM interprets as low voltage. This is the most common cause of P0738.
- 2
Check Wiring Harness or Connector Issues — Corroded, loose, or damaged wiring in the engine speed sensor circuit can reduce signal voltage to the TCM, triggering this code.
- 3
Check TCM Electrical Fault — Internal TCM issues or faulty input circuits within the transmission control module itself may fail to properly receive the engine speed signal.
- 4
Check Engine Control Module Communication Failure — Communication problems between the ECM and TCM can prevent proper signal transmission, causing the TCM to detect abnormally low values.
How to Fix It
- 1
Test and Replace Engine Speed Sensor
Use a multimeter or scope to test the engine speed sensor output voltage while cranking the engine. If voltage is low or absent, replace the sensor with an OEM or quality aftermarket unit.
- 2
Inspect and Repair Wiring and Connectors
Visually inspect the engine speed sensor harness and connectors for corrosion, cracks, or loose connections. Clean corroded connectors with contact cleaner and reseat connections firmly.
- 3
Check TCM Power and Ground
Verify the transmission control module has stable 12V power supply and a clean ground connection. Poor TCM power or ground can cause signal reception issues.
- 4
Reprogram or Replace TCM
If wiring and sensors test normal, the TCM may need reprogramming or replacement. Have a dealership or qualified shop reprogram the module or install a remanufactured TCM.
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