As long as you do not swallow any water through your mouth or nose, taking full ablution or a bath does not break the fast. As a matter of fact, Aisha (may Allah be pleased with her) and Ummu Salamah inform us that the Prophet took full ablution after imsak. ( Bukhari, Fasting, 25). Thus, being unclean (junub) when imsak begins does not harm the fast; taking full ablution does not break the fast, either.