I recently bought a '75 Catalina 27 that came with a rather old Mercury 9.9 outboard with an alternator. The alternator wires are white and black. Does anyone know which is positive and which is negative? I don't want to hook it to the battery in reverse. Many thanks.
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Black & White
Bob Why not just hook up a multimeter to the wires? Just check to see if the voltage is positivie or negative depending on which wires you connect to, then you'll know. You should have a multimeter on board. Good luck. Stu
Bob Why not just hook up a multimeter to the wires? Just check to see if the voltage is positivie or negative depending on which wires you connect to, then you'll know. You should have a multimeter on board. Good luck. Stu
Checking alternator
Yes, use a meter, but a digital one on the voltage setting. You should get a plus or minus sign in front of the voltage that will then, again depending upon how you have placed the meters wires, tell you which wire is positive and which is negative. For example, lets say the meter has a red and black wire correctedly attached to the respective red and black "inputs" of the meter. Let's say you attach the meter's black wire to the black wire from the engine's alternator and the red wire to the white wire from the engine's alternator. Crank over the engine for a brief period and see if there is a plus or negative sign in front of the voltage. If a plus sign, then the white wire from the alternator is positive and the black wire from the alternator is negative. If a negative sign, then the white wire from the alternator is negative and the black wire from the alternator is positive. When done, make sure you properly mark these wires with pieces of tape because five minutes later you will forget which is which. As the positive and negative stud terminals of a marine battery are different sizes, you can then crimp on the respective ring size terminal onto each wire so that in the future they will be placed properly onto their respective negative and positive terminals. When you do this, buy four ring terminals, two of the size for the smaller stud and two of the size for the larger stud because it is very easy to accidentally place the first two on backwards as I have done. Also, only let the engine turn over for a very short period as the alternator really needs a battery load to dump its output and you could damage the alternator without a battery attached. Also, the alternator probably will not produce much voltage at low speed, so don't be concerned if you see low voltage when doing this test. You check that later with the wires properly attached to the battery by just running the meter leads across the battery terminals with the engine at a medium or medium high speed.
Yes, use a meter, but a digital one on the voltage setting. You should get a plus or minus sign in front of the voltage that will then, again depending upon how you have placed the meters wires, tell you which wire is positive and which is negative. For example, lets say the meter has a red and black wire correctedly attached to the respective red and black "inputs" of the meter. Let's say you attach the meter's black wire to the black wire from the engine's alternator and the red wire to the white wire from the engine's alternator. Crank over the engine for a brief period and see if there is a plus or negative sign in front of the voltage. If a plus sign, then the white wire from the alternator is positive and the black wire from the alternator is negative. If a negative sign, then the white wire from the alternator is negative and the black wire from the alternator is positive. When done, make sure you properly mark these wires with pieces of tape because five minutes later you will forget which is which. As the positive and negative stud terminals of a marine battery are different sizes, you can then crimp on the respective ring size terminal onto each wire so that in the future they will be placed properly onto their respective negative and positive terminals. When you do this, buy four ring terminals, two of the size for the smaller stud and two of the size for the larger stud because it is very easy to accidentally place the first two on backwards as I have done. Also, only let the engine turn over for a very short period as the alternator really needs a battery load to dump its output and you could damage the alternator without a battery attached. Also, the alternator probably will not produce much voltage at low speed, so don't be concerned if you see low voltage when doing this test. You check that later with the wires properly attached to the battery by just running the meter leads across the battery terminals with the engine at a medium or medium high speed.
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