95 4.3 liter Mercruiser with good alternator not charging

[BIGZIG]

I have a 95 Mercruiser 4.3 liter with a good alternator that's not charging. When the ignition is on I am getting 12v on 3 wires and ground on the 4th. The alternator is similiar to the one Don posted on here except the ground terminal is in a different location. The guy who tested it for me told me to put a jumper from the battery terminal of the alt to the sen terminal to rule out that possibility, it didn't resolve issue. The only terminal I can get a meter lead on when the alt is in it's service position and engine running is the battery terminal. Any suggestions on how to verify culprit without replacing wires one at a time?

 

[Fishermark]

Re: 95 4.3 liter Mercruiser with good alternator not charging

Measure your voltage at the battery without the engine running - then measure it again with the engine running. Post your results.

12v with the engine running doesn't sound like the alternator is working to me.

Then take the alternator off and have it tested at a shop that knows alternators.

 

[BIGZIG]

Re: 95 4.3 liter Mercruiser with good alternator not charging

Voltage at the battery is .2vdc less when the engine is running than when it's not.

 

[Bt Doctur]

Re: 95 4.3 liter Mercruiser with good alternator not charging

 

[Leardriver]

Re: 95 4.3 liter Mercruiser with good alternator not charging

That is an excellent trouble shooting chart.

My test is as follows:

Voltmeter on the battery when ignition is off, note the voltage. Should be 12.6 or so.

Voltmeter on the battery when the engine is running, if the same as when not running, then measure at the output lug on the alternator to rule out a bad wire or connection.

If still about the same, as in not the 13.7 to 14.2 or so that it should be, then test the exciter (small wire also on the back of the alternator)for 12 volts.
That wire "turns on" the alternator. If you are getting 12 volts to the small exciter wire on the back of the alternator, and only 12.6 battery voltage at the alternator output lug, then your alternator is not nearly as operational as you would hope.

These tests take 30 seconds total once the engine is running. you are testing with the voltmeter at the battery, alternator output, and purple exciter wire.