APC Back-UPS XS 800 120v Bypass Problem

I have this APC unit i'm trying to repair right now, wondering if anyone had similar issues and could shed some light on it. We all know how UPS' work, they keep a battery on charge 24x7, keep normal 120v flowing through it, and when 120v fails, the inverter clicks on and the juice starts flowing from the batteries. My unit simply will not keep 120v flowing through it when 120v is connected, keeps running off the inverter, with the relay clicking back to the charger every 5sec or so (watching battery voltage). I think I've found the relay that's responsible for this bypass up towards the back of the unit by the outlets, and am going to test it out (RY 4). In the mean time, was going to ask around and see if anyone else would know what it might be. Any experience or ideas on this one?