I have a 84 I'm not sure if the wiring is the same as an 88 but my oxygen
light and battery light would come on periodically. I checked the voltage
regulator ( it's bolted to the back of the altenator )and found the brushes
worn ( wear limit as per Bentley 5mm on 90 amp alt. ) mine were 2mm and 4mm.
good luck.

Rafael
84 Westy