ABS light on

Are you saying the light goes out with a trailer connected? I don't think 97 had Trailer ABS lights yet so I will assume you are talking about the truck's ABS light. Chances are you have a Mertior Wabco version C ABS system. Look around the dash, usually around the diagnostic plug for a switch that says "ABS CODES" or diagnostics or something like that. Turn the switch on after turning the key on. The ABS will start to blink out a code. You will have to write down the number of blinks. If it is a meritor system, the first set of blinks will tell what system configuration it is and the second and third set will be the fault code. Go to http://www.meritorwabco.com/literature/pneumaticabs.asp and find MM-28 ABS for trucks. This should be what you are looking for. I think around 99 they started using Version D. It will have all the info about blink codes and how to trouble shoot. You will have to be handy with a multimeter and test light if want to diagnose ABS. It will get expensive in a hurry if you start throwing parts at it. Also you may have a different system installed, so look at the pictures of the valves from the Meritor book and see if they match exactly to what you have on the truck. If not you will have to find the right book.