Simple Test[]
Using the panel voltmeter, the reading should be around 12 with the ignition on, not running, and no other loads. When cranking it may drop down to 8 or 9 volts. Once started, it should almost immediately jump up by 2 volts or better to around 14 volts (you might need to rev to 2000 RPMS or so). If you don't see a jump of 2 volts or better when it starts, you likely have an alt or regulator failure.
Test 1: Check Voltage at the Battery[]
Use a digital voltmeter to check the battery terminals after the engine has reached operating temp. The battery should be fully charged and the meter should read approx. 14.0 VDC. w/the engine at 2000 RPMs. Write down the reading for reference; it will be used in the next step. If it is more than 14.4 VDC, the alternator regulator is not working.
Test 2: Check Voltage at the Alternator[]
The next test is easier w/gator clip voltmeter leads. You must not wear a long sleeved shirt or anything that will get caught in the pulleys. The back of the alternator has several wires connected to it, unless it is a single wire system. The orange wire is B+ and the black wire is B-. Connect the positive meter test lead to the B+ and the negative test lead to B- connections. With the engine at 2000 RPMs, the meter should read between 14.0 and 14.4 VDC and only be 0.04 volts less than the reading recorded at the battery. If the difference in voltage readings is less than 0.02 volts, the battery cables are good.
If you remove the alternator, make sure you tag each wire with the correct label for reinstallation.
The boys explained the important point; the alternator has to respond to a drop in battery voltage and turn itself off when the battery has reached 14.4 volts.
Note: If the alt. is putting out 14.04 volts at the B+ terminal, then the battery terminals MUST read at least 14.0 volts. If it reads 13.9 volts then the battery cable or the alt. cable has high resistance and need to be changed.
To determine which is the culprit, follow the voltage path from the alt. to the starter then to the battery. If the alt. is putting out 14.04 volts and the voltage at the starter and the battery is 13.9 volts, then the bad cable is the alternator output wire. If the starter post reads the same as the alt. B+ volts. such as 14.04 volts and the battery reads 13.9 volts, then the battery cable is bad.
Another Response[]
Watch your volt meter on your dash. 13 to 15 volts at ~ 2000 rpm then all is good for the alternator. With the key in the on position and NO motor running the meter should read ~ 12 1/2 volts.
If you have a digital volt meter, measure the battery with no load on it. it should read 12.6 volts fully charged. that is 2.1 volts per cell.
If it reads 10-11 volts then you have a dead cell; this is a very common problem with marine batteries.
If you have a voltmeter, with the motor running, put the leads across the battery terminals and rev the engine to 2000 rpm and check the voltage. Again it should be between 13 and 15 volts DC. If it is, then charging system is working correctly.
Appendix[]
A four wire alternator has a black ground wire, orange output wire, purple sensor wire and a red with purple stripe excitation wire. These last two wires tell the alternator what the battery voltage is and when to start charging the battery.