Dim battery light on/off

You need to test the charging system.

Get a voltmeter. Put the test leads between the battery poles. Run car and report reading. Turn car off and take battery reading immediately. Wait an hour and take reading again. Note room temperature.

Post results or google how to test your charging system and battery and compare.

Once you do that, you should clean your battery terminals and retest to see if you get different results.

 

Do a voltage check at the battery at idle - or whatever RPM the engine drops to when you get off the gas/hit the brakes. Make sure the idle is set to spec - probably 1000 RPM - but check the manual. Also, be sure before making the check that the engine, after start, has triggered the alternator cut-in - around 2200 RPM. You should be seeing something in the 13+V range or at the very least the high 12's, increasing as the RPM rises.

(I have never understood that particular Ferrari "feature," whereby the alternator has to first hit some cut-in RPM to switch to the charge mode... )

 

IMO anything less than 14.2v across the terminals at idle and you have a charging problem.

 

Nothing wrong with the OP making a sanity voltage check, but for the conditions given (headlights "on", CIS fuel pump "on", brake lights "on", probably water fans "on", etc.) with the engine at low RPM, an alternator rated at 80A (at full speed) not being able to keep up with a 40~50A current load doesn't seem too terrible to me -- they bumped up the alternator to 85A on 328...

OP = do you have any added electronics or uprated headlight bulbs?

 

Your stock alternator should be able to keep up with your stock electrical system. If original it's probably not putting out 14.2+v at idle that it should. A rebuilt alternator may be in order.
I had similar symptoms prior to a complete failure (dead car).

A rebuilt alternator fixed everything. I don't see the light glow anymore

 

Start by checking all the negative (ground) terminals, starting at the battery. A very common problem, with results such as you mention. Clean connections with boiling water and baking soda, then rinse well, with cold water. Even on the battery. The positive terminal does not build up corrosion. Scrape the ground connection to the battery clean after getting rid of corrosion.

Cheers,
Rich

 

I would test the system before attempting any repairs to know if the connections are fixing the problem or not.

 

I never knew about the alternator cut in. Interesting. Always thought I had an issue. This forum is an absolute wealth of information. I swear I read as much as I can here. Knowledge is power. So very nice to have "contact" with intelligent and unselfish people.

 

I'm not so certain about that cut in. I think it may be just for the light to start working. I think if you measure the voltage between the battery poles you will get normal charging voltage immediately regardless of the minimum RPM being achieved. Remember you have a voltage regulator on the alternator and the main charging line (path of least resistance) runs straight to the battery (via the starter). I don't know how some minimum rpm switch could trick the alternator voltage regulator to stay low until the minimum RPM is reached.

I certainly can be wrong and would love to understand how that all works but it seems too hard to achieve to me.

 

There's no designed-in alternator cut-in. Instead, it's an artifact of the alternator output at idle RPM when 12v from battery is being applied to the exciter instead of 14ish from the alternator. Plenty of other cars will do this as well; it depends on the idle speed setting and the characteristics of the alternator.

More detail: http://www.ferrarichat.com/forum/308-328/406667-328-needs-2000-rpm-start-charging.html#post142274056

 

Now I got it!

 

Wow, I needed this discussion. My 328 - when I first got it - had been sitting for a while, so the battery was discharged.When I started it, the light still glowed but I failed to rev it to turn off. I went in the house while the car warmed up.

It coughed. It died. Me thinking in highly technical terms:"Uh oh".

Turns out the discharge rate exceeded the battery's ability to keep the motor alive. Now I blip til it goes off, never another problem.

 

From Wikipedia:

http://en.wikipedia.org/wiki/Alternator_(automotive)

Automotive alternators require a voltage regulator which operates by modulating the small field current to produce a constant voltage at the battery terminals. Early designs (c.1960s-1970s) used a discrete device mounted elsewhere in the vehicle. Intermediate designs (c.1970s-1990s) incorporated the voltage regulator into the alternator housing. Modern designs do away with the voltage regulator altogether; voltage regulation is now a function of the electronic control unit (ECU). The field current is much smaller than the output current of the alternator; for example, a 70 A alternator may need only 7 A of field current. The field current is supplied to the rotor windings by slip rings. The low current and relatively smooth slip rings ensure greater reliability and longer life than that obtained by a DC generator with its commutator and higher current being passed through its brushes.

The field windings are supplied power from the battery via the ignition switch and regulator. A parallel circuit supplies the "charge" warning indicator and is earthed via the regulator.(which is why the indicator is on when the ignition is on but the engine is not running). Once the engine is running and the alternator is generating power, a diode feeds the field current from the alternator main output equalizing the voltage across the warning indicator which goes off. The wire supplying the field current is often referred to as the "exciter" wire. The drawback of this arrangement is that if the warning lamp burns out or the "exciter" wire is disconnected, no current reaches the field windings and the alternator will not generate power. Some warning indicator circuits are equipped with a resistor in parallel with the lamp that permit excitation current to flow if the warning lamp burns out. The driver should check that the warning indicator is on when the engine is stopped; otherwise, there might not be any indication of a failure of the belt which may also drive the cooling water pump. Some alternators will self-excite when the engine reaches a certain speed.