Alternator bootstrap circuit

[Alan Cordeiro <cordeiro@avlna.com>]

>howdy,
>here's an interesting situation:
>1. start the car, the alternator doesn't "come up." rev the engine once to 
>1800 or better and the alternator comes up and stays up until you shut off 
>the car. has no trouble maintaining voltage under full electrical load 
>(once it's "initiated"). temp light flashes, batt and brake light are 
>illuminated.
>2. start car, no alternator. shift into reverse (rev lights on), alternator 
>comes up (at idle this time) and idiot lights go out and stay out _while 
>still in reverse_. shift out of reverse, idiots come back on, alternator 
>stays up until car is shut off.
>
>any ideas? bad ground (where), bad diode (where), bad reg, 
>fuse/link/circuit breaker.
>
>btw, '86 4000s with low budget (55A, motorola?) alternator, ~new brushes 
>and battery.
>
>tia
>james

An alternator needs a  moving MAGNETIC FIELD to generate a voltage output.
This field comes from an electro-magnet in the rotating core, which gets
fed a controlled current from the voltage regulator. Unfortunately, until
the alternator is generating a voltage, there is no power available to
feed the field current, ( like the chicken and egg... ) since it feeds
off its output coils ( armature or stator as it is called ).

There is some "residual" magnetism, but you really need to rev up the car
to 3000 or 4000 rpm to get it to "kick in" so to speak. What alternator
designers do, is to feed a low "bootstrap current" into the field to help
give it that extra help to fire it up.

The current comes from the battery, through the alternator idiot light, into
the alternator. When the ignition is on, the alternator field power supply
is at zero volts ( not kicked in yet ) and the lamp comes on, an the small
150 mA current helps excite the rotor field coil. Once the alternator
powers up, it provides 12 volts or more to the internal field power supply,
and the light "goes out"

1. Check the idiot light, does it come on when you first turn the switch.

2. There is a small wire that goes from the idiot light into the alternator.
   Tug on it where it goes into the alternator, if it comes off in your hand,
   that might be the problem.

Alan