[alfa] Re: testing a regulator (alternator llamp faint glow)

[Graham&Joan Hilder <graham.joan@xxxxxxxxxxxxxxx>]

In Digest 167, Scott asked:
"Can anyone point me to the proper procedure for testing a voltage regulator
on a 71 Spider? I had the alternator rebuilt about a year ago but the ALT
light still glows faintly while the car is running."

And in Digest 168, Jeff replied:
"THEY ALL DO THAT :-)  A little unnerving I know, but they all seem to do it
and it does not cause any problems. I've never found a definitive cure. The
most common hypothesis is that there is too large a voltage drop in all the
related wiring, fuse box, and ignition switch. If anyone has found a
definitive cure, I'd love to hear it."

On this, I agree with Jeff - all of the 105, 116 and Alfasud Alfas I've had
have done this. When I first encountered this faint alternator lamp-glow on
my first Alfa many years ago, I measured the charging voltage and it was
correct, so I investigated further. On studying the circuit diagram & wiring
layout, the explanation became clear, and Jeff's comment above is certainly
on the right track. Here is further detail, and the cure.

In the typical 105, 116 or Alfasud with a Bosch alternator, one side of the
charge lamp is connected to the battery via the ignition switch and one of
the fuses, usually the fuse which also supplies the windscreen wipers and
the heater blower. This whole circuit is powered from the main charging
output of the alternator, which is connected directly to the battery. The
other side of the charge lamp is driven from the auxiliary output of the
alternator, which is also connected to the regulator and the field winding.

When the alternator is charging correctly, one side of the light should have
the battery voltage on it, and the other side should have exactly the same
voltage on it (coming from the alternator's AUX output) - and so the lamp
does not glow. But this is only the ideal situation, and in reality, the
voltage at the Battery side of the lamp can become very slightly lower than
the battery voltage itself, and hence there is a small voltage difference
across the lamp, and the lamp glows faintly, even when the alternator is
charging the battery perfectly normally.

The problem that causes the voltage at the lamp's Battery side to be
slightly lower than the battery voltage is as Jeff suggested - too large a
voltage drop in the path from the battery to that side of the lamp. As Jeff
said, this path includes wiring, ignition switch and a particular fuse in
the fusebox. Excessive resistance anywhere in this path can cause enough
voltage drop to make the light glow faintly. The culprit is almost always
the relevant fuse, and/or its fuseholder.  The glow will usually increase if
you turn on the wipers or heater blower, as more current is then drawn
through this fuse-path and the voltage drop increases. (Sometimes the glow
will increase if you turn on a heavy load such as the headlights, and I
suspect this is because the heavy charging current may cause a higher
voltage drop across the alternator's main rectifier than across its Aux
rectifier, once again causing a slight voltage-difference across the
charge-lamp.)

The cure for the faint glow is almost always to find the fuse that feeds the
charge lamp, and clean that. If that doesn't work (and if you know your
alternator really IS charging OK), then you need to track down a dirty
connector, or bulbholder, or some other cause of a small unwanted resistance
in the path.

Now, in Digest 170, John wrote -
"I have found two simple ways to stop the alternator lamp from dimly
glowing. The first method is probably the simpler of the two and requires a
higher wattage bulb/lamp to be fitted..." (so that the small current that
still flows when the alternator is up and running is insufficient to cause
it to glow).
"The second method achieves the same but in a different way..." (fitting a
resistor in parallel with the bulb to shunt some of the field current around
it and reduce the glow).

On this, I would propose that although these methods may work,  I feel it's
preferable to study the real source of the problem on the wiring diagram,
and get to the heart of the matter -  i.e. reduce the unwanted voltage drop
that's causing the problem. (But then, I can be strange about these things!)
Another "workaround" method that you sometimes read about is to add a diode
in series with the lamp - this prevents the unwanted voltage difference
(which we described above) from causing the reverse lamp current which
causes the faint glow.

As an aside, John also discussed the frequent need to blip the accelerator
after start-up, to initially make the charge lamp turn off.  John wrote -
"The alternator regulator, before the alternator is spinning fast enough to
generate its own supply, needs  a current bleed from the battery to start
the regulator working.  This it gets via the alternator light....(snip)...
Once the alternator output exceeds about 12V the regulator then runs off
this voltage and hence the light should go out... (snip)... I think that
what happens is that within the regulator circuitry is a voltage detector
that needs to see the battery voltage go above a certain trip-point before
it unlatches the lamp current."

Not quite true, I think. While it is true that the regulator and field are
initially powered by the battery via the lamp, and later by the alternator
itself, there isn't any voltage detector that unlatches the lamp current
when the alternator voltage rises enough. The lamp simply goes out as soon
as the voltages on each side become equal, which happens quite naturally as
soon as the alternator is up & running, and outputting enough voltage to
charge the battery.  Nothing fancy - no detectors etc!  This may not be
always the case, but I'm sure it applies to all the Bosch 105, 116 & Alfasud
cars I've looked into.  (I've also found the same situation on some non-Alfa
cars too.)

Best regards,
Graham Hilder,
Paekakariki, NZ
(GTV, Alfetta GT, AlfaSud)
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