Alternator issues
matty h - 23/4/16 at 09:15 PM
How does a alternator earth the warning light wire and how can I check this, it is a mem's 3 rover alternator.
The charge light does not light with ignition all wiring is fine but the wire does not have a path to earth.
Any ideas.
Matty
coozer - 23/4/16 at 09:24 PM
Positive 12v to the light and wire down to the alternater....
When the engine is off, no output from the alternator it earths the light so it comes on... Starting the engine and the output from the alternator
knocks it off...
That what you want to know??
blakep82 - 23/4/16 at 09:48 PM
12v from the key -> light -> alternator
engine not running, battery 12v, alternator 0v, current flows through the lamp, lighting it
engine running, 12v from key -> light -> 12v at alternator
0v difference, as the whole circuit at both sides of the lamp is the same voltage as the alternators output, 13.8v, no current through the lamp, so it
goes off
blown bulb or wiring not connected to the alternator perhaps?
cliftyhanger - 24/4/16 at 05:57 AM
Or a problem with the actual alternator? ie duff.
matty h - 24/4/16 at 06:23 AM
The wire from the alternator must earth the light when it it not charging this is what I think is not happening does anyone know how this works.
Regards Matty
blakep82 - 24/4/16 at 08:14 AM
quote:
Originally posted by matty h
The wire from the alternator must earth the light when it it not charging this is what I think is not happening does anyone know how this works.
Regards Matty
yep, both myself and coozer know how it works, and we explained it above 
the earthing is done inside the alternator. the key supplies 12v from the battery, through the light to the alternator, the alternator is at 0v, the
difference of 12v lights the light, from the key to the alternator. remember the alternator is bolted up to the engine. with the engine running, the
alternator and battery are at the same voltage, 0v difference, no current flows. thats it! thats how its done
britishtrident - 24/4/16 at 03:16 PM
As above --- the Brown/Yellow at the alternator is the warning light wire.
If you back probe the connection it should show;
(1) Ignition On Engine off ----Zero Volts
(2) Ignition On Engine Running ---- battery voltage volts.
The Green wire on the alternator should read battery voltage when the ignition is on.
The Red/White wire will read battery voltage with the key on engine off, with the engine running at idle with no additional electrical loads it will
read an unsteady 5 to 7 volts
The White/Red wire is interesting because it is a signal wire to the MEMS Ecu that tells the ECU what electrical load the alternator is supplying.
The signal is in the form of a rougly square wave. With no load the signal has 100% duty cycle, with all electrical loads on it has 0% duty
cycle.
Normally at idle it will have 40% to 60% duty cycle.
Denso Alt Duty Cycle Idle
[Edited on 24/4/16 by britishtrident]
