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Spider alternator lamp faint glow

A long, long time ago (Digest no.830, June 18th), Clay posted the following
query -

     Ever since getting my Spider new in 1984 the alternator lamp (or is a
battery lamp- does it measure voltage or amps?) has glowed, usually only
noticeable at night, and.... brighter when I use more accessories, such as
the heater blower (in hi and low speeds, different lamp intensities),
and....the a/c..., again with different blower speeds, brighter light.
.....the backup lights.....also make the battery lamp go on really
     I have had my Alfa mechanic at local Alfa shop here check it several
times, and he says that a lot of Spiders do this:  he says he has never
known why Spiders do this.
       It also seems to me that I need a new battery every 2 years.... a
plug-in trickle charger....stretches it to 3 years. It seems to me that I
might do better if I put in a stronger alternator......
       Finally, this is academic----why has this battery indicator light
kept glowing all the time over these it possible I got a bad
alternator from Alfa all those years ago from the factory?---or is what my
mechanic says true---that  all Spiders do this and he has never been able to
figure it out?..... Perhaps the resistance of the bulb in the indicator lamp
is too low?---it should not be this because the current across the indicator
lamp is supposed to be the difference between the output of the alternator
and the battery output----is it possible the alternator is fine, and Alfa
Romeo wired the lamps incorrectly in Spiders?
Thanks for any help from an erudite bunch of guys!  (and gals.)
     Clay Leopold
     Princeton, NJ

Sorry about this VERY late reply, but here it is anyway. My answer would be
that your mechanic is probably right, because a lot of Alfas (105s, 116s and
AlfaSud at least) DO do this, even when the charging system is working
perfectly well. I've come across this feature on quite a few Alfas, and the
first thing I do is to check the charging voltage under various loads, to
see if the warning light is giving a genuine or a false alarm. Usually I
find the charging voltage is correct and the alarm is consequently
false. The glow-symptoms are exactly as Clay describes above, except that
the accessories which affect the glow may vary from model to model,
according to the fuse-allocation details.

Years ago I got tired of this false glow and decided to track it down. With
a detailed circuit diagram & some running around the circuit with a
multimeter, the explanation became clear. The alternator lamp circuit
involves the alternator field circuit, and a fuse which supplies these
alternator functions plus various accessories such as heater blowers and
windscreen wipers (the list varies between models). Without  going into
tedious detail, the circuit is such that if there's a slightly dirty
connection in certain areas of this circuit, small voltage drops across this
small resistance can appear across the warning lamp, making it glow faintly
even though the system is charging absolutely correctly. The voltage drop
(and hence the glow) typically increases when the
relevant accessories are switched on and draw more current through this
circuit's fuse.

Because the voltage drop giving this false drive to the lamp is of the
opposite polarity to that which drives the lamp in genuine circumstances, it
would seem to be possible to fix the false alarm with a suitably-placed
diode, and you sometimes do read of folks doing this. However, I've
preferred to get to the root cause on my cars, and in every case the
unwanted resistance in the circuit has turned out to be the relevant fuse &
fuseholder. Clean these, and the fault's gone (until it's time for another

I've never worked on a Spider, but all my 105, 116 and AlfaSud cars from the
70s and 80s have been wired up very similarly and all have had exactly this
"feature", so chances are the Spider would be similar.

Regarding Clay needing a new battery about every 2 years..... well maybe you
do have a genuine charging fault, so I would first check the charging
voltage under various loads.  On the other hand, maybe the car does a lot of
short trips, perhaps with heavy electrical loads, so the alternator doesn't
have time to recharge the battery fully between starts?  Anyway, a charging
voltage check will quickly tell you what's what in the charging system

Speaking of this false glow from the alternator lamp, recently I discovered
that it's not only an Alfa feature. I went to help out a non-electrically
minded fellow in the village who said his charge lamp glowed "now & then".
(He was a pensioner, with an old car, and no spare money to spend on
professional maintenance.... crikey, all that sounds just like me!).  Anyway
this car was a Holden from the late 70s, and it had a two-terminal
alternator as do the Alfas covered above (it was even a Delco-Remy, made in
France, so I didn't feel so uncomfortable over sullying my hands on a
non-Alfa car). I found that the charge light
glowed at idle, and also at higher revs if the wipers were on, but in all
cases my meter showed that the charging voltage was right up to scratch. I
had no wiring diagram but found the relevant fuse, and cleaned it.... but no
improvement. I then found a loom-connector in the lamp-circuit between the
alternator and the lamp, and cleaned this dirty connection, and hey presto,
fault gone. Very much like the Alfa problem, except the resistance causing
the voltage-drop in the lamp-circuit was in a connector rather than the

Best regards,
Graham Hilder,
Paekakariki, New Zealand.
1750 GTV, Alfetta GTV, Alfasud Sprint & tiQ.
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