I have an o'day 37 with a universal 32 hp 4 cyl diesel charging 3 batteries (2house and 1 starting)with a 90 amp alternator through an isolation diode. I have the alternator sensing wire on the battery side of the diode, so it senses battery voltage. My tachometer runs off the feed from the alternator. I have an expanded scale voltmeter reading the starting battery voltage. When I fire up the engine, I watch the voltage on this meter gradually climb up to 14.5 V - when it gets there, the regulator in the alternator must shut down the charging field because the tach falls to 0, then occaisionally blips up to running RPM when the alternator kicks back in. This seems to happen even if I run a load on the batteries, like the refrigerator & lights. So the problem is when we're cruising under power, we only get this intermittant reading on the tach, which keeps swinging up & down. thanks, Marty G