I have a 2001 1500HD LS that the ABS Light is on all the time. Also the ABS system does not work when the light is on, but when it rains and the undercarriage gets wet the ABS light goes out. I suspect that it is a wiring issue, but I don't know anything about the ABS system

 

Try cleaning or replacing the ABS sensors on the front wheels. This will work sometimes.

 

you're just guessing without codes