Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Theory Of Operation
The diagnostics are able to detect circuit failures on any of the output stages in the Transmission Control Module (TCM). All output stages are controlled by a current controller that can operate an output stage in two different modes:
- Current controlled mode (pressure regulators)
- Switched mode or Pulse Width Modulated (PWM) controlled (solenoids)
Pressure Regulator Monitoring: The current regulator in the Transmission Control Module (TCM) indicates the following circuit failures to the Central Processing Unit (CPU):
- Short to battery
- Bypass to ground (high resistance short to ground)
- Short to ground or open circuit (can only be distinguished when the output stage is in idle mode, i.e. commanded current is 0 mA)
In addition to the circuit failure information, the TCM monitoring software uses a PWM feedback signal generated by the current regulator in the output stage to identify failures. To perform rationality monitoring of the output stage current, the TCM software calculates the actual current dependent on the PWM feedback signal, the output stage power supply voltage and the actual transmission oil temperature. The TCM can only distinguish between a short to ground and an open circuit failure if the output stage is deactivated. Depending on the required time to bring the transmission into a limp home mode, a further evaluation of the failure is determined after switching off the output stage in order to distinguish between both failure types.
Solenoid Monitoring: The TCM measures the actual solenoid voltages and compares them to calibrated values to identify the following electrical failures:
- Short to battery
- Open circuit
- Short to ground
When the solenoid is switched on (i.e. commanded control value is 100%), the diagnostic can not distinguish between short to ground and open circuit. In that case, the Application-Specific Integrated Circuit (ASIC) status information is used to detect the failure.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.