2nd alternator in 2 months??

'99 Merc 250 EFI, bought the boat in late May and the original Denso alternator crapped out within like 3-4hrs (running hrs not 3hrs after buying it), got an aftermarket one from DB Electrical and put that in in early June. It's been running fine for 2 months, then I go out today and after getting up on plane and cruising for a minute I look down and see the voltage at around 12 at about 4800RPM. It is usually low maybe like 12.8 or so at idle and then gets higher with higher RPM. Cruising at 4800 it is always at 14+.

Went in and took the cover off and instantly smelled electrical burning, put the meter on the alt, running I am getting 12.1 at idle, raise up to a fast idle around 16-1800 and NO increase at all. Put the meter on the batteries and I can watch it start sucking off the batteries, watch them drop down to just under 12, like 11.9. Shut the engine off and the batteries start to re-gain voltage back up to around 12.5.

Gonna speak with DB Electrical tomorrow and I'm sure they'll send me a new one no problem, but my real question is is there something else going on here that is causing alternators to crap out or is this just bad luck? I pulled the alt I got from DB Electrical (Made in China) and I can see a part of the windings burned up.

Re: 2nd alternator in 2 months??

What type of battery's you running and are they wing nutted or lock nut on terminals? loose connections and battery selectors switch play havoc on these. Only switch selector when not running...

Re: 2nd alternator in 2 months??

I'm running dual Powermaster 1,000 MCA deep cycles. They are fully charged and load test good. I used nylock nuts on the battery connections, I hate wing nuts not sure why they are ever used. The connections are clean, solid and tight. I only turn the switch when not running but in any case it is a make before break switch so even I was to it shouldn't matter, but in any case I never do.

With the setup I have I have to run the engine at 4800-5000 to cruise, it's a bit underpowered and it's a heavy boat but an alternator made for this engine should be able to handle running at 5K. Why didn't Merc just use a stator under the flywheel instead of this alternator setup? It actually looks like there is a stator under the flywheel though in addition to the alt....can that be? This is my first Merc, I've had OMC's for 25 years.

Thanks.

Re: 2nd alternator in 2 months??

Low batterys + low engine RPM = poor heat dissipation = burnt alternator. You should charge your batteries externally before replacing the alternator. Some re-manufacturers will not warranty an alternator if you installed it on a system with low batteries. If you must charge up your batteries from a deep discharge, keep engine at cruise speed for an hour or longer....don't put around.