Re: Charging delays in 4000

[cordeiro@adi.com (Alan Cordeiro)]

>Amidst the rest of the charging questions, here's another:
>87 4000S, 4-cyl, at start up, the alternator does not charge at all until 
>the revs rise to around 4000.  After the alternator "kicks in", it 
>charges perfectly until the car is shut off again.  Then the cycle 
>repeats.  A friend of mine with an 87 VW Fox has the same problem.  As 
>far as I know, the alt. is Bosch.  It was on the car when I bought it a 
>few years ago.  It has always been noisy (bearings??), and I thought it 
>would die quickly, but just keeps on goin after about 2 years and 30k 
>miles.  I replaced the battery right after I bought it, and it started 
>during the -30deg. cold spell last winter here in Ohio.   I wouldn't 
>care, but revving to 4000rpm early in the morning not only bothers the 
>neighbors, but can't be too good on bearing surfaces when it's a cold 
>morning.

>				---Bob

I will try and guess at the problem, but here is a bit of 
background on automobile alternator theory, applicable to almost
all brands of cars.

The alternator has a stator ( outside ) that has stationary
windings, and a rotor, which appears to the stator as a
multiple pole magnet due to the shape of its overlapping
magnetic material. However, it is really one coil of enamelled
copper wire called the field winding in most cases.

When the alternator is operating, the D.C. current ( actually
voltage to begin with ) is produced by a set of auxillary
diodes ( three ) off the main three phase bridge ( six main
diodes ). However, initially, as the alternator first starts
to rotate, ( 1000 rpm ) there is very little residual magnetism 
left in the core, not even enough to overcome the forward breakdown
voltage of the rectifier diodes. Hence something is needed
to "kick-start" or "bootstrap" the field to where it generates
enough voltage in the stator to feed itself.

If the RPM goes high enough, the feeble residual magnetism in 
the rotor will eventually start up the chain reaction, and once
started, it will sustain itself.

However, most manufacturers provide a method of supplying the
field winding  with about 0.1 amps, ( don't need much ) to
help it come right up at only 800 to 1000 rpm. They feed the
battery positive to the "charging lamp" when the ignition
is switched on, and the other side of the lamp goes to
ground THROUGH THE FIELD WINDING. Once the field supply cranks
up, the lamp goes off, since there is 14 volts on both sides
of it.

To keep the lamp from glowing at high charging rates, almost
everyone puts a diode in series with the lamp to keep
higher than normal rectifier voltage from reverse feeding
through the lamp circuit.

Possible causes of the effect you are seeing

1)  Burnt out indicator lamp.
2)  Burnt out series diode.
3)  Some, one or two of the auxillary diodes in the
    bridge are "open circuit"

1) and 2) will show up as a dead lamp when the ignition
is turned on, but the engine has not been started. 3) will
need a new alternator ( or re-built ). 

I cannot see how noisy bearings would create this kind of 
problem, but one remote possibility could be that the ground 
return of the field coil winding goes through the bearings,
( bad slip ring + coil shorted to rotor .. really remote ).


Alan Cordeiro