Voltage drops when engine on?

[miamiz]

I'm having a problem with my old Yanmar 2QM15. When the engine is on it drains the battery. I thought it was the altenator and bought another still has the same problem. So with a fully charged battery at 13.5 when I turn the engine on it drops a volt or two. Does anyone know where I can start looking to troubleshoot this? Thanks in advance! EZ

[noelex77]

Welcome to the forum
There is short somewhere. With that much current something will be getting hot.This is a simple way to isolate where the fault is. Trace the wires and feel the temperature at the battery switches starter etc.
Be careful that much voltage drop indicates you could melt something or start a fire if the fault continues.