Last week I noticed the ammeter was not budging from 0 when the engine was running. On Friday I measured voltage on the battery terminals and sure enough, the voltage went down when I started the engine, not up. Strange thing is the voltage spiked way up into hundreds of volts every second or so (cheap digital multimeter so hard to quantify). So I pulled the alternator and went in search of a shop to rebuild it. 7 or eight years ago I had an alternator rebuilt but that shop was gone so I looked online and in the yellow pages but there just aren't any shops around here that rebuild alternators any more. I finally called a place that sounded promising but it turns out they no longer rebuild, just sell new alternators but their main business is now making bow thrusters http://www.sideshift.com/ (just a quick plug for a local businessman)

So it sounded like a good time to get a bit bigger alternator. The original alternator (only 36 years old) was 35 amps and I went with an alternator rated at 63 amps. I'll need to change out the charging wire but I haven't done that yet. I hooked up the new alternator and it is a noticeable difference in load on the engine; I'm glad I didn't go any bigger. The ammeter only goes to 40 amps so I guess I'll have to change that out too. I have a 1/both/2 switch on two group 24 batteries. I normally just use 1 battery at a time for both charging and discharging but I'm thinking I'll charge on both then switch to 1 or 2 when I shut the engine off.

Funny thing is I am still seeing the voltage spikes at the battery terminals when the engine is running at low speed. When I bring the revs up above high idle (no tachometer so don't know exactly) the voltage is steady around 14.4 V. The engine is an Atomic 4 (gas engine) and I made sure everything was switched off at the panel. The old alternator looks pretty black around the diodes. I'm a little worried I'm going to fry my new alternator. Any ideas where the voltage spikes could be coming from?

Also, anyone know where I can find a replacement diode pack for an old Atomic 4 alternator? I might as well fix it for a spare.