alternator question


Old 08-09-06, 02:04 PM
Thread Starter
Join Date: Jul 2006
Posts: 17
alternator question

I've got a boat with two mercruisers on it. One engine died because its battery was dead. The other engine was still running but eventually the Voltage gauge dropped to zero on that one also.
After installing new alternators and charging the batteries the engines started fine and ran fine for a while. I wasn't sure if it was running fine just because the batteries were freshly charged or if the alternators are working so i turned on all accesseries for a while and the volts dropped to about 10 on the guages. when i turn the accessories off it comes back up to about 11. Revving the engine up to 3000rpm doesn't really change the gauge. I was just wondering if anyone can give me a clue if the batteries are charging. I would check the batteries with a voltmeter but i can't get to them with the engines running.
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Old 08-10-06, 07:52 AM
Join Date: Nov 2005
Location: North Carolina
Posts: 948
for whatever reason, your charging system is not working. while running you should have a voltage reading of 13.5 to maybe 14.0 VDC. do you have internal regulator alternators or is there and external regulator in the system ? check all your ground connections and curcuit breakers or fuses.
Old 08-10-06, 08:17 AM
Thread Starter
Join Date: Jul 2006
Posts: 17
not sure if it is internal or external. If it helps, its got just two wires on the back, (+) and (-). And a black box sticking out of the back
If it wasn't charging wouldn't the batteries die right away from running both engines with all accessories on?
Old 08-10-06, 05:08 PM
Join Date: Aug 2005
Posts: 2,268
Had a similar problem once.... though I kinda doubt it is what you're experiencing since you've had someone go through the system. My problem was simple - the alternator belts weren't tensioned tight enough - there was just enough slippage that you couldn't hear any squealing - yet the batteries slowly - but surely - died...

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