DTC P1635: 5-Volt Reference 1 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.
The Powertrain Control Module (PCM) provides a 5-volt supply for use in powering up the sensors. This test monitors the voltage present at terminals BRD13, shared by boost and Crankshaft Position (CKP) sensors, and BRD14, the optical/fuel temperature sensor (Cam/HI. Res).