My alternator is giving 12.9 volts which I think is a bit low with headlights on. On looking at it there are 3 wires in the harness that join it, 2 thickish ones to the plug at the back and a thin one not connected to anything. Ignition light goes out when you start it. The wiring diagram in my Haynes seems to indicate 4 wires for an alternator but 2 wires for a dynamo. My alternator says FORD on its label 14v 45 amp, fitted by a previous owner. Is two wires usual, is 12.9 volts too low??
Alternator Wires 1975 P6 2200SC
- Thread starter Steve
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sdibbers
Well-Known Member
It may have been upgraded in the past. Later alternators use just two wires. One to battery (I used the + terminal on the solenoid as I have a SI four cylinder car. One to alternator light (if bulb blows alternator will stop charging). It could be that the one fitted needs to be rebuilt.
All three should be connected usually. The two thick wires are for charging and the third energizes the alternator. However if it is non-original and a self-exciting (oo-er) type it won't need the 3rd wire but you won't have an ignition light.
Remember, an alternator has no permanent magnet. It uses field coils. Obviously it is also connected to the battery but the fields should only be energized when the car is running, this is how it knowns when to operate. The ignition-on energizes the coils initially. As the alternator runs up to speed there is 12V either side of the bulb and you see no light. If the alternator output drops then the light comes on or flickers as one side still has the voltage from the battery - so usefully it is a "charge failing " indicator too.
Self-excited types has some kind of switching mechanism which responds to the movement (as you already have 12v from the battery), most likely a very small magnet or some kind of hall effect switch.
Remember, an alternator has no permanent magnet. It uses field coils. Obviously it is also connected to the battery but the fields should only be energized when the car is running, this is how it knowns when to operate. The ignition-on energizes the coils initially. As the alternator runs up to speed there is 12V either side of the bulb and you see no light. If the alternator output drops then the light comes on or flickers as one side still has the voltage from the battery - so usefully it is a "charge failing " indicator too.
Self-excited types has some kind of switching mechanism which responds to the movement (as you already have 12v from the battery), most likely a very small magnet or some kind of hall effect switch.
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