What does it mean?
OBD II fault code P0887 is a generic code that is defined as “Transmission control module (TCM) power relay control circuit high”, and is set when the PCM (Powertrain Control Module) detects an abnormally high voltage (or any voltage at all) from the relay that supplies the shift solenoids with power when the transmission is operating normally.
Note that when the ignition is switched to the “ON” position, the relay control circuit should either have no voltage, or a voltage that agrees with a predetermined value. On applications that use a relay to supply voltage to the shift solenoids, the PCM checks that no voltage reaches the shift solenoids when the ignition is switched “ON”, by verifying that the relay’s contact points are open.
When the relay is open, and does not pass current to the shift solenoids, the transmission is effectively in “limp mode”, but before the relay is activated, the PCM checks for voltage at various pressure switches in the transmission. In a fully functional transmission and its control system, there should be no voltage at any pressure switches at this time and the PCM will supply power to the power control relay only after this is verified.
In practice, the shift solenoid control relay’s output is fed back to the PCM via a dedicated circuit known as the Transmission Control Relay Output Circuit, or sometimes as the Switched Battery Circuit on some applications. The image below shows a typical transmission control module (TCM) power relay, such as might be found on most modern applications.