Alternator output on 3.0 Mercruiser

[JohnKorte]

The orange wire and red wire is reading 12.5v on the alternator same as battery. My question is the purple wire (switched?) does that output vary depending on the voltage state of the battery?

 

[matth121]

i think the purple is switched 12v from the key. Orange charges the battery- since it's connected to the battery it will read whatever the battery is reading, the orange wire is where the voltage would increase to around 14V when alternator is spinning. if you're only getting 12.5 with it running, your alternator is not charging. a good starter repair shop can rebuild it with better parts for about 1/2 the cost of new if it's not too far gone

 

[alldodge]

+1 with matth and will add; the purple wire is switched on by the key

 

[JohnKorte]

ok so the key switched on to give power to altern.? I don't get it. Either way it looks like I need to have altern. rebuilt. One other thing. Power is always on to my dash switches and radio. Shouldn't they only come on when I turn the key to acc.?
Used boats I love them.

 

[matth121]

when running the alternator supplies power to the coil for ignition, not the battery, it monitors the key switch for 12v- if thats there- it will fire the coil and allow the boat to spark and run. no 12v from key means no power to coil. so turning off your key kills the engine. make sense?

 

[alldodge]

ok so the key switched on to give power to altern.? I don't get it. Either way it looks like I need to have altern. rebuilt. One other thing. Power is always on to my dash switches and radio. Shouldn't they only come on when I turn the key to acc.?
Used boats I love them.

Besides the orange wire there are two other wires going to the alt, one of which is purple. When you turn the key to run the purple wire supplies 12V to the field windings of the alt. Once the engine starts, the armature spins and starts producing 14.5V which sends it to the battery for recharging. The supply voltage coming back from the battery is sensed by the alts internal regulator and controls the output of the alt to keep the battery at proper charge.

If you have 12V on the purple wire going to the alt and the other wire which is black should go to ground. If the 12V is present, and the engine is running there should be 14V or so at the alt orange wire. Measure the voltage at the alt not at the battery or starter terminal. If you have 14V at the alt and not at the battery then check the 90amp fuse on the starter post

 

[JohnKorte]

I have 12.5v at the orange and red wire of the alternator. 10.5-11.3V at the purple wire. the boat does run. Per the previous post I had gear lube overflow on the alternator. meter on the dash read 10v at higher speeds 11v. so I have disassembled, cleaned but still get same reading as stated.

 

[JohnKorte]

I have 12.5v at the orange and red wire of the alternator. 10.5-11.3V at the purple wire. the boat does run. Per the previous post I had gear lube overflow on the alternator. meter on the dash read 10v at higher speeds 11v. so I have disassembled, cleaned but still get same reading as stated. Thanks Matth .