Circuit Description

The powertrain relay is a normally open relay. The relay armature is held in the open position by spring tension. Battery positive voltage is supplied directly to the relay coil and the armature contact at all times. The engine control module (ECM) supplies the ground path to the relay coil control circuit via an internal integrated circuit called an output driver module (ODM). One ODM output control is configured to operate as a low side driver for the powertrain relay. The ODM for the powertrain relay also incorporates a fault detection circuit, which is continuously monitored by the ECM. When the ECM commands the ODM for the powertrain relay to turn OFF, the expected voltage on the relay coil control circuit should be a high voltage condition. When the ECM commands the ODM to turn ON, the expected voltage on the relay coil control circuit should be a low voltage condition. DTC P0685 indicates that the powertrain relay control circuit voltage does not match the commanded state of the ODM. DTC P0685 sets when an open, high resistance, short to ground, or a short to battery positive voltage is detected on the powertrain relay coil control circuit.