what is making my alternator gauge drop to zero. the battery is fully charged and the alternator was recently replaced. it’s fine. however i do hear a high thin squeal when i start up the car. when the squeal goes away,(usualy once it is warmed up), i can turn the car off, restart it and the voltage gauge will be fine.

When you hear the high thin squeal, a belt is slipping. If this is the belt that spins the alternator, the alternator isn’t producing power and hence the gauge will drop to zero.

The belt slips when the engine is cold, but does not slip as the car warms up is that rubber shrinks as it heats up. The belt, when the car is warmed up will then spin the alternator. You need to find out why the belt is slipping and correct that problem. The other problem will then be solved as well.

I’m not sure how the meter is tied into the electrical system. The charging system may be working ok and there may be a relay not closing as it should be that is causing the meter to not get a reading. Since the belt may be slipping check the tension and adjust it if it is too loose.