BACKGROUND

A diagnostic system for verifying a fail safe control path to a control relay in an hybrid electric vehicle is vulnerable to varying vehicle operational conditions (e.g., a battery voltage drop during engine crank) which can cause false diagnostic alarms. Further, a vehicle controller may undesirably disable the primary function of a hybrid electric vehicle in response to the false diagnostic alarms. A fail safe control path includes components and electrical lines that are utilized to detect a fault condition in the hybrid electric vehicle.

The inventors herein have recognized a need for an improved diagnostic system that eliminates the above-identified problem.

SUMMARY

A diagnostic system for a hybrid electric vehicle in accordance with an exemplary embodiment is provided. The diagnostic system includes a vehicle controller that is operably coupled to a battery management controller. The battery management controller is operably coupled to a monitoring circuit. The vehicle controller sends a first message to the battery management controller indicating an engine crank occurred. The battery management controller sends a second message to the monitoring circuit in response to the first message. The second message indicates that a diagnostic test is to be initiated. The monitoring circuit outputs a first signal which induces a transistor to turn off which de-energizes a voltage coil of an electrical relay in response to the second message to initiate the diagnostic test. The diagnostic system further includes a voltage sensor that outputs a voltage signal indicative of a voltage being output by the transistor to the voltage coil. The analog-to-digital converter receives the voltage signal and outputs a voltage value based on the voltage signal. The battery management controller receives the voltage value and sets a diagnostic flag to a first value if the voltage value is less than a threshold voltage value. The first value indicates that the diagnostic system is operating as desired to detect a fault condition.

