BCM Code B420, Relay Circuits: Notes
WARNING: This page is about a different car, the 1990 Cadillac Seville and 1990 Cadillac Eldorado. However, it is still accessible from the selected car via links, so may be relevant.
The BCM operates twilight, retained accessory power, high/low beam and courtesy light relays by providing a ground for the coils. When relay or relays are on, BCM output will be low. When relays are off, BCM output will be high. Code B420 will set if ignition is on and output voltage at BCM is low when high has been commanded.
NOTE:
Test number refers to test number on diagnostic chart.
Test Number 1) To determine which system has failed, select BCM output function in diagnostics. As individual outputs cycle on and off, observe their actual operation.