Have you tested the various parts of an alternator?

[macafee2]

Originally Posted by Synchromesh

A few months ago a family member's Fiesta charge light started flickering then moved to permanently illuminated. I stripped the alternator expecting it would need new brushes. I found one brush had a bit of crud on it so it was sticking in the brush holder. The spring was not strong enough to press it onto the slip ring. Both brushes showed minimal wear. as did the slip rings. I cleaned off the crud and reassembled the alternator using the original brushes. It has worked perfectly since. I don't think you commented on the condition of your brushes.

Brushes seemed ok but I fitted a new voltage regulator as my first action and it did not solve the problem, of course the new regulator could be faulty, that would be fun...not.

macafee2

[Synchromesh]

In the majority of cases fitting a new regulator/brushes fixes an alternator. I am trying to think of a reason why your original alternator still doesn't charge after you fitted the new regulator/brushes. After you fitted the new regulator/brushes did you push the plastic cover down over the slip rings to release the brushes allowing them to contact the slip rings?

[macafee2]

Originally Posted by Synchromesh

In the majority of cases fitting a new regulator/brushes fixes an alternator. I am trying to think of a reason why your original alternator still doesn't charge after you fitted the new regulator/brushes. After you fitted the new regulator/brushes did you push the plastic cover down over the slip rings to release the brushes allowing them to contact the slip rings?

Not sure I follow you about the plastic cover. The brushes hung down enough to come into contact with the slip ring and moved freely.

If I put the alternator back together and spin it, perhaps via a drill, should it produce "12v" DC? I'm unclear if I need to supply it with 12v to start with. I've read on web sites it does and does not need 12v supplied to it.

macafee2

[Synchromesh]

Originally Posted by macafee2

Not sure I follow you about the plastic cover. The brushes hung down enough to come into contact with the slip ring and moved freely.

If I put the alternator back together and spin it, perhaps via a drill, should it produce "12v" DC? I'm unclear if I need to supply it with 12v to start with. I've read on web sites it does and does not need 12v supplied to it.

macafee2

Some regulators have the plastic cup that covers the slip rings already fitted but it is not slid all the way down the channels. It holds the brushes back with the springs compressed. You fit the regulator, tighten the screws then press the cup so it slides to the bottom of the channels. This releases the brushes and the springs press them against the slip rings. Perhaps yours was supplied with the cap separate so you press the brushes against the slip ring compressing the springs before you locate the regulator and insert the screws. Regarding the excitation current for the rotor: Most alternators initially obtain a small excitation current through the charge light. The current flows from the battery through the charge light to the positive side of the rotor then through the rotor to earth. When the engine spins the alternator it uses the small excitation current to produce AC which is rectified by the diodes. There are six diodes forming a 3 phase full wave bridge rectifier suppling the charging current for the battery and there are three smaller diodes forming a half wave rectifier suppling the excitation current for the stator. When the half wave rectifier supplies the rotor it no longer draws current through the charge light so it extinguishes.Theoretically if the charge light was open circuit the initial excitation current would not be available so the process described above would not be possible so the alternator would not provide output current. I think the uncertainty on the internet regarding do you or don't you need to supply 12V to kick off the alternator is because sometimes there is enough permanent magnetism in the alternator to provide a small AC output current which is just enough to supply the half wave rectifier suppling the excitation current for the stator and the current ramps up to normal operation.
An alternator is designed to produce appropriate currents and voltages when connected to a battery. If a car battery is disconnected when the engine is running the alternator output may go rogue and could damage the alternator regulator and car electronics.