hey smoke, its majjjord here
As you know in other thread I have probs too.
It is normal for the charge light to go OUT when you unplug the plug on back of alternator, and you will also get no joy unplugging the plugs and trying to measure output!
the reason is this:
the charge light on your dash is wired in series with ignition switch, and the reglator /field coil(s) inside the alternator (explained later). via the 2 pin plug
( f terminal only though, the other wire is wired straight to the batter for the regulator's reference)
In addition to this, and internal to the alternator case, part of the alternator OUTPUT is tapped into this circuirt through whats known as trio diodes, or exciter diodes - at a point between the plug on the back, and the field coil/reg, allowing it to supply its own field current when up and running..
Therefore - field coil current can only come from 2 places:
1: via the ign switch, through the charge lamp. (when the motor is OFF)
2: from the alternator itself (excitor /trio diodes) . when the motor is RUNNING, and alternator working A OK.
The purpose of the field coil is essentially to provide a source of magnetism to allow the alternator to operate .. as it has NO fixed magnets inside , it relies on this curent at all times , FYI the spinning rotor contains the field coil.. actual output current comes from the coils all round the outside that you can see through the casing.
This field coil's current is varied in response to load and engine speed by the REGULATOR, to maintain 14volts output at all times.. being only a small current, this method is much much easier and smarter than regulating the very large output current.
So, to explain the the operation a bit heres 2 scenarios:
When the car is off, but ign is on, the alternator has 0 output, therefore the only source of field current to get things rolling is from the ignition switch @ 12v via the lamp (hence it's lit!)
OK, so you start the motor, the alternator is spinning, and the alternator output is at a healthy 14 volts - now the alternator is supplying it's own field current.. hence no current flows through the lamp, as it is a path of highest resistance..(thanks to the lamp being a lousy conductor).
hope that kind of made sense.
Now, for your case, with the lamp dimming off as you rev, what I'd say is happening, is the alternator output is struggling at low RPM.. most of the current flow through the field coil is therefore being provided by the charge lamp still.. when it
should be provided by the healthy alternator output.
As you rev it, output is increasing to over 12Volts, and the lamp goes off.
Essentially, the alt is stuffed.. be it a dud diode or 2 as mentioned, or a faulty reg, whatever the reason, it's not able to provide your 14v. If you want to test things, try the following:
unplug the 2pin plug, and check there is battery voltage showing between the N terminal and ground.. as this wire should be connected straight to the battery, via a fuse.
next check, that with ign on - approx 12v (may be a bit lower) shows between the F wire and ground.
if that is ok, everything SHOULD work if the Alt is OK..
however the problem I'm personally having is, after 3 alternators, I STILL have a bright as day charge lamp, and no output from alternator. even after checking not only the above, but confirming that current Is passing through the F terminal on the back..
I'm stumped.
anyway this thead is for your prob
cheers
Jimmy
but this is how I blew up 3 or 4 alternators , untill I was edu ma cated
