Vauxhall Astra 1.4i MPi
I realise theres a possibility that I can probably fix this problem
easily by fitting an alternator, but I'm really just curious if there
is any gurus who can explain this to me. After a journey of about 30
minutes, my battery warning lamp comes on very very faintly. This has
been happening for about three years. When I first noticed it I fitted
a new battery because there were a few occassions when the starter
sounded a bit lazy. The light still came on after a 30 minute journey.
I did do an alternator swap for another second hand one and the same
thing happened exactly. The car has kept running since, so I never
really worried too much about it. A short time ago I thought I would
investigate again, since I was planning a longish trip. I studied the
wiring diagram and could see that there is a 12v supply to the
instrument panel, which supplies current to the battery warning lamp,
the lamp having its negative terminal connected to the small terminal
on the alternator. Before cranking, this terminal is more or less a
ground if you like. The warning lamp obviously lights therefore before
cranking. When the engine is started this alternator terminal becomes
the equivalent of the charging voltage, hence the supply to the warning
lamp positive terminal and negative terminal are now equal, ie theres
no P.D. across the bulb. Anyway to get to the point, I got suspicious
of what was happening, and decided to put a diode in the line between
the warning lamp negative terminal and the small connection on the
alternator. I had a hunch that the current flow was actually in the
opposite direction to what I had generally assumed to be the norm. I
was proved right. I fitted the diode with pointed end of the triangle
(the kathode) pointing up toward the instrument panel, coming from the
alternator. In other words, when the car had been run 30 minutes, there
was lets say 1 or 2 volts more on the small pin at the alternator, than
there was at the supply voltage to the instrument panel. Obviously the
general condition that illuminates the warning lamp is when alternator
output is less than battery voltage, but in this case it was the exact
opposite. Anybody ever seen this before? The bit which adds some
mystery is that even after a new battery was fitted, and another
alternator, it still behaved the same. I can imagine that the second
hand alternator I fitted maybe wasn't as good as a new one, but I would
be surprised if both the old and new alternators had exactly this same
characteristic, ie after 30 minutes driving, putting the light on
slightly. Only for that bit of confusion I would stick a new alternator
on, but I have my doubts if indeed this would sort the problem out. I
am presuming the alternator with its voltage regulator and diode pack
is a self contained unit. There is no other regulatory circuitry
anywhere else outside of the alternator is there? The alternator is the
Bosch one, where the brushes and the voltage regulator are a single
unit, held onto the rear of the alternator by two screws. I would
really like to understand whats going on if anyone can help. Thanks