Lucas 11AC problem
Discussion
you must remember that an alternator its self is a very clever bit of kit (in the way it regulates the charge and so on), but the way it works is very simple.
you will have 2 fatter terminals (field and neutral) and one thinner one.
the thinner wire will be the one that goes to the warning bulb and onto an ignition source.
an alternator will not charge unless this circuit is getting power and the light is lit as its this charge that 'excites' the insides of the alternator to produce a charge. once the alternator is 'excited' it no longer needs the charge and it switches off, which is why the light goes out when its either working or spinning fast enough to generate a current.
first you want to pull the plug of the back of the alternator and meter this circuit (in the plug, with the ignition on) to see if there is 12 volts. if there isnt either the bulb has blown or there is no 12 volt supply.
if there is a 12 volt supply you can link a wire from the plug to the alternator (or plug the plug back in if you can get access to the other two terminals with it fitted), start the car and meter the other two terminals - these should show a voltage of anything up to 14 odd volts. if there is no voltage then you must assume that the diodes in the alternator have blown up, the brushes are worn and not making contact, the windings are burnt out etc etc.
if you know what the field resistance is for you alternator should be you can put an ohm meter across the 'f' & 'n' terminals. if the ohm reading is less assume a short in the field windings, higher its possible that the brush slip ring is dirty. if its an infinty readings its the windings that have failed.
i had huge problems with the alternator on my classic commercial and it took a lot of testing and reading up on to find the problem (this was on a big 24 volt system using an alternator with a seperate diode pack) but smaller alternaters have the same problems!
good luck!
you will have 2 fatter terminals (field and neutral) and one thinner one.
the thinner wire will be the one that goes to the warning bulb and onto an ignition source.
an alternator will not charge unless this circuit is getting power and the light is lit as its this charge that 'excites' the insides of the alternator to produce a charge. once the alternator is 'excited' it no longer needs the charge and it switches off, which is why the light goes out when its either working or spinning fast enough to generate a current.
first you want to pull the plug of the back of the alternator and meter this circuit (in the plug, with the ignition on) to see if there is 12 volts. if there isnt either the bulb has blown or there is no 12 volt supply.
if there is a 12 volt supply you can link a wire from the plug to the alternator (or plug the plug back in if you can get access to the other two terminals with it fitted), start the car and meter the other two terminals - these should show a voltage of anything up to 14 odd volts. if there is no voltage then you must assume that the diodes in the alternator have blown up, the brushes are worn and not making contact, the windings are burnt out etc etc.
if you know what the field resistance is for you alternator should be you can put an ohm meter across the 'f' & 'n' terminals. if the ohm reading is less assume a short in the field windings, higher its possible that the brush slip ring is dirty. if its an infinty readings its the windings that have failed.
i had huge problems with the alternator on my classic commercial and it took a lot of testing and reading up on to find the problem (this was on a big 24 volt system using an alternator with a seperate diode pack) but smaller alternaters have the same problems!
good luck!
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