When is the code detected?When the Powertrain Control Module (PCM) detects a fault (circuit shorted to ground, or circuit shorted to a voltage), the P1637 code will set.
P1637 Chevrolet DescriptionThe L-terminal circuit from the generator is a discrete circuit (a discrete circuit has no splices and only one source and one destination) into the Powertrain Control Module (PCM). The PCM applies ignition voltage to the generator L terminal circuit. A small amount of current flows from this circuit through the generator windings to create a magnetic field which starts the generator process. When the generator is at operating speed and producing voltage, a solid state switch for the L terminal circuit in the generator opens and the PCM detects that the initial startup current flow has stopped.
The PCM expects to detect low voltage on the L terminal circuit prior to the generator rotating at operating speed and conversely expects the circuit to be at ignition voltage potential when the generator is operational.