View Full Version : New Alternator putting out 11.8volts?

08-15-2010, 04:41 PM
2002 Moomba Outback V
310 Indmar TBI

I replaced my alternator because I kept draining getting low voltage. I installed the new alternator and it still is putting out 11.8 volts. I have tested both batteries and they are fine. I noticed that the purple and red wire that plug into the regulator on the back on the alternator shows 12volts before I turn on the ignition, but when I turn the ignition to the on position the voltage drops to nothing. I am not sure if this is wrong or not but if it is what could be the culprit and what should i check??

any advise helps

thank you.

08-15-2010, 07:28 PM
What is the voltage output of the alternator when the engine is running at idle? and at about 2000 rpm?
That info is needed.

08-15-2010, 11:37 PM
at idle it is putting out 11.8 to 11.9, and then at about 2500rpm it drops to 11.7 to 11.5

08-16-2010, 01:47 PM
Are you sure that you aren't running the boat solely off the batteries at this point which would draw more current at higher rpms and thereby reduce the voltage?

08-17-2010, 10:35 PM
I had somewhat the same issue until I put every component of my audio on the exact same ground (engine ground). I had multiCD unit, head unit, iPod box, two amps, all hooked to the same ground. Before I had them near each other and noticed I wasnt getting as much from my alternator. Just my 2 cents.

08-18-2010, 01:10 AM
check your hoses make sure the belt isn't getting wet and slipping I had a similar problem found small holes in the J hose that goes from the water pump to the thermostat

mastercraft dealer had 5 in stock so I would assume its a common problem

also check the belt its self for abnormal wear if it is getting wet

08-18-2010, 10:30 PM
I am pretty sure that it is running off of the battery. The belt is not getting wet and I am metering the larger red and black on the alternator and getting those readings. The smaller connection into the back of the alternator (red and purple wire on a plug) both the purple and red are putting out 12 volts. If i put the positive lead to the purple and the negative lead to a engine bolt I get 12 volts and the same on the smaller red wire with engine running 2500 rpm and at idle. I left the meter hooked up to the alternator with the engine running 2500rpm and at idle and still getting the same readings at the alternator. Could i have a bad new alternator? As far as the stereo goes I only have a deck and it is isolated on a switch, no amps or subs.

08-19-2010, 10:57 AM
Just the stereo person here so this isn't my area of specialization but it does seem pretty simple. I would verify that there is a good ground cable of adequate gauge (2-ga.) with good connectors from the battery to engine block (and alternator mount via block or other essential connection). That loop has to be closed for operation. Connections can appear good but fall off with a mild tug. Also, corrosion is often concealed under a connector or insulating jacket.
Next, once the engine is running, lift the alternator feed. The boat should continue to run for a good while off well-charged batteries. But if you get no change in the reading then you know the alternator is not operational. Retrace all connections and then get the alternator diagnosed at a shop independent of the boat. If the alternator checks out you'll have to enlist a mechanic or take the boat in for service.

Earmark Marine

08-19-2010, 02:07 PM
Like others have said, It sounds like your engine is running soley off of battery power and is not being recharged by the alternator. Is it possible you have a defetive new alternator, sure. Mechanical moving parts are more likely to fail before a wire. Not sure where your located, but around me there are places that check alternators, such as Autozone. There maybe a alternator/starter rebuild shop near you that you could take it and have it checked. Another source of information could be a local independant auto repair shop, if you have a good reputation with one. Alternators have the same job whether its on a auto, marine or on a diesel engine.

08-29-2010, 09:10 PM
I pulled the alternator and took it to the shop and The new one was indeed defective. I got a new one put it in and back to 14 volts!!! I was going crazy thinking something was wrong and it was just a bad new alternator.

Thanks for the replies guys.

08-29-2010, 10:32 PM
Glad it got resolved.

08-30-2010, 01:25 PM
I pulled the alternator and took it to the shop and The new one was indeed defective. I got a new one put it in and back to 14 volts!!! I was going crazy thinking something was wrong and it was just a bad new alternator.

Thanks for the replies guys.

This happens all the time with alternators. I'd bet a full third of the ones I have purchased in the last 10 years have turned out to be bad either immediately or within a week. Not sure why.