[urq] Voltage regulator and idiot light question

Mark R speedracer.mark at gmail.com
Wed Nov 19 13:19:43 PST 2008

Interesting on the Sparco switch.  Of course, with the battery failing and
shorting out, a TON of current was going through that switch, so it's hard
to condemn it as a manufacturing defect.  It failed before the wiring did
(because of internal resistance versus the wire), which is probably a good

Needless to say, the fumes are corrosive as well.  Might want to do your
best to clean around the battery.  Also, if you smelled the acid, the
battery wasn't properly vented to atmosphere (outside of the vehicle).
Proper venting is critical.  Another level of protection is a gel battery
(Optima, etc.).  They will gas out at high voltages, but there's no risk of

The alternator light is a dummy light.  It only will illuminate when the
alternator output isn't high enough.  I once had to explain the concept
(semi-incorrectly) this way:
If the alternator is outputting say 14.5 volts and is supplying the lower
voltage battery (say at 13 Volts) all is well... no light.  If the current
reverses such that the battery is supplying 12 Volts to a dead (0 volts)
battery, the current is reversed and the light illuminates.  In fact, there
have been cars out there which this is an accurate description.

Modern cars have a little more fancy electronics to inspect the voltage and
direction.  Bottom line- most idiot lights won't illuminate when supplying
too much voltage.  Most "look" at the direction only.  Looking at the
potential wouldn't really help.  EX: a battery at 10 Volts can be charged.
14.7V (alt supply) minus 10V is a 4.7 volt potential.  16V minus 13.5 V
(fully charged battery) is only a 3.5V potential.  Newer cars with fancier
electronics also have to take into account the actual voltage when looking
at potential for it to be useful.  I'm afraid a mid 1980's Audi doesn't
qualify as "newer!"   ;-)

I too would consider oil temp (if nothing else, to know when it's OK to push
the engine), water temp, and oil pressure to be more critical.  Perhaps
wiring in an oil pressure buzzer (for too high AND too low) and swapping the
voltmeter with the oil pressure meter.  That way you're covered.  REALLY
easy to wire in a buzzer under the dash.

Mark Rosenkrantz (with stable voltage AND high current)

On Wed, Nov 19, 2008 at 1:15 PM, Max Baker <max at warped.org> wrote:

> Hi Louis-Alain,
> There was just a great discussion on the 24 Hours of LeMons list about
> cut-off switches.  Apparently the Sparco ones are very prone to failure,
> and one team went through a couple of them.   But I guess that's a
> little late for you.
> -m
> Louis-Alain Richard wrote:
> > Hi there,
> >
> >
> >
> > My car, an 1983 urQuattro with a Bosch 90A alternator and a battery under
> > the rear seat, had a voltage regulator problem last week. The problem
> (+16V)
> > made the battery boil and lose half of its water, and made the driver a
> bit
> > dizzy. More, when the driver stopped, the car wouldn't restart, no more
> > juice at all. But, the problem was not the dead battery : I've installed
> a
> > SPARCO safety electrical cut-off switch between the front seats, and it
> > looks that it melted internally, preventing current flow.
> >
> >
> >
> > But why the ALT idiot light never lit ?
> >
> >
> >
> > Isn't it suppose to show any problem, no charge or overcharge ? I've
> > installed a voltmeter, but to keep the dashboard stock looking, it is in
> the
> > glovebox with the air-fuel ratio meter and the diff lock rotary switch.
> The
> > 3-hole center panel is filled with oil and water temp, and oil pressure.
> I
> > was sure these meters were more important than voltmeter, but I may
> > reconsider my choice now.
> >
> >
> >
> > Louis-Alain
> >
> > 1983 Quattro with a new battery, new voltage regulator, new switch and a
> > funny smell in the cabin.
> >
> >
> >
> > _______________________________________________
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> > quattro at audifans.com
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> >
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