I have no voltage feed to my starboard alternator. The large red wire goes to the battery isolator,but I believe the alternator is powered up by a smaller red purple wire. Where does it come from? It has no voltage on it.
On a marine engine, if an alternator has an external excite wire, it is usually purple and is basically keyed ignition power. When ignition power is supplied to this wire, the alternator basically turns on. This type of alternator works better with a battery isolator because you are actually sensing the battery voltage and not the alternator output voltage.
If your alternator is self exciting and internally sensing, then the single red/purple wire is the “idiot light” wire. This wire goes to ground when the alternator dies. It is connected in series with an indicator light. IMPORTANT: If you apply ignition power directly to this post on this type of alternator, you will burn up the internal rectifier.
Determine which type of alternators are on your boat using an internet search and the model number on them. If they are self exciting and internally sensing, your alternator is dead. If they have an external sense wire, trace down the red/purple wire and determine where it is not making connection to the ignition wire.