I think my alternator is on drugs

[nhschneider]

I first became suspicious when the voltmeter on the engine's panel was only showing about 12.5 volts with the engine running. I took a meter to the alternator and measured across the B+ terminal and ground and found 20.1 volts. I figured the regulator had failed (it has over 4,000 hours on it) so I removed the alternator and took it home with us last June (for treatment and therapy). There's a highly-skilled auto-electric shop that I've worked with for years and I had them rebuild it - new bearings and a new brush kit (the regulator is part of the brush kit) and they reported that it was working fine. I believe them.

We got back to the boat a couple of days ago and I reinstalled the wayward alternator only to find the exact same problem: No indication of charging on the panel but 20.1 volts on the B+ terminal. Obviously, I've checked all my connections and everything checks out. The alternator is an Iskra 100 amp. The other alternator on the engine works fine; it's an Iskra 150 amp. I just hope there's no peer pressure on it.

Any ideas?

[Jon Hacking]

Originally Posted by nhschneider

... the voltmeter on the engine's panel was only showing about 12.5 volts with the engine running. I took a meter to the alternator and measured across the B+ terminal and ground and found 20.1 volts. … Any ideas?

If you're measuring 12.5v at the battery & 20.1v at the alternator (hopefully with the same measurator), it's pretty obvious you've got a wiring problem. Voltage doesn't just disappear like that. You've probably got either a broken wire (which can destroy your alternator diodes) or a badly corroded connection. Best to take your connections apart, clean them well, talk to them sternly but lovingly, put a bit of clear grease on them to prevent them oxidizing more, & reassemble them. Also check continuity between the battery & the alternator (both legs should be well under 1 ohm).

But if you're going cruising, you should really have external regulation for your alternators. Internal regulators can only "see" the voltage at the back of the alternator. External regulators should have dedicated sense leads to both your battery (to measure voltage) & your shunt (to measure current into the battery). See the Very Smart Regulator (only $125, & no, I have no financial interest in them, but they're the best I've found).

[smac999]

Sounds like the wire is broken or removed between the alt and battery. Yiu can't have different voltages if they are connected.

Or it's been wired to the house battery and is still voltage sensing off the start battery. So your start battery is 12v and house battery is 20v. Seen that before.

Measure the voltage at the alt with the engine off. If you don't have a diode isolator you should have 12v at alt. If the wire is missing You won't.