my 1989 toyota pickup; overheats, installed new alternator because battery kept dying over night. but why is it happening?

Hello, Dave here, contributing to RepairPal and providing expert advice since 2010. It sounds like you have a couple different problems. First, the charging system -- testing is required. Start with the battery. Is it fully charged and able to sustain a load? If not, it must be replaced. If it passes, then go on to check your charging system. Is it passing with proper voltage output (likely 13.8 volts or above), and amperage (likely over 60 amps at 2,000 RPM engine speed)? If not, then even if it is "new" then it still may be faulty. It does happen. But you must test in that order, because the charging system can only be tested with a known good battery installed. Secondly - the cooling system. Can you verify engine temperature, and is it boiling over? Or is it the gauge you are concerned about? Repair the charging system first and recheck. If the gauge is still high, it could be a gauge problem, sending unit problem or even an engine cooling problem. Testing is in order. Seek local guidance at a shop near you -- https://repairpal.com/auto-repair-near-me Click the link or call us at 877-913-1554. We'll gladly find you a certified RepairPal shop in your area.