DTC P1643: Wait To Start Indicator Control Circuit: Description
WARNING: This page does not describe the selected car, but rather 6 other vehicles, including the 2002 GMC Savana Special, 2002 GMC Savana Camper Special, 2002 GMC Savana, 2002 Chevrolet RV Cutaway, and 2002 Chevrolet Cutaway. However, it is still accessible from the selected car via links, so may be relevant.
A dash light, wait to start, is illuminated by the Powertrain Control Module (PCM) when the glow plugs are commanded ON. When the PCM is commanding the wait to start lamp ON, the voltage potential of the circuit will be low, near 0 volts. When the PCM is commanding the wait to start lamp OFF, the voltage potential of the circuit will be high, near battery volts. The primary function of the PCM is to supply the ground for the wait to start lamp circuit.