In a message dated 97-09-28 17:51:55 EDT, suhring@lancnews.infi.net writes:
> When I start the car, which is fine, the light will go off
> but with a faint glow remaining. As RPMs increase, the red
> glow will brighten. If I get RPMs above 4000, the light is
> fairly bright, but will dim back down when RPMs drop.
>
> If I use any electricals: lights, turn signal, lighter, etc,
> the warning light will glow bright and will all but go
> away when I turn off the electrical devise. By the way, all
> electricals work fine and the alternator belt seems fine
> with its tension.
Scott:
Electrically speaking, the red light is connected directly between the
battery and the output of the alternator. If the battery voltage is at the
proper value, and the alternator is producing the correct charging voltage,
the light will be out. Under normal operation, there is often a slight
difference between the voltage of the two devices, depending on RPM, charge
state of the battery, number of devices on, etc, but the difference is too
small to light the bulb enough to be seen. When you first turn the key on,
but before the engine is running, the alternator is producing no voltage, so
the lamp has the full battery voltage on one side, and zero volts on the
other, alternator, side, so the lamp glows at full brilliance. Once the
alternator comes up to speed, it's output voltage is very close to battery
voltage, and the lamp goes out.
If the battery voltage drops, due to a fault in the battery, but the
alternator is ok, the light will come on to show that the alternator is
charging the battery. The higher the RPM, the higher the charge voltage, and
the brighter the lamp will glow.
If the battery is ok, and the regulator inside the alternator is faulty, the
alternator can overcharge, producing a voltage higher than the battery
voltage, and the light will glow. Again, the higher the RPM, the brighter the
glow.
To test your car, first measure the battery voltage, with the engine and all
accessories off. If the voltage is low, less than 12.6 volts, then the
battery is bad. Turn on the lights, and you should see the voltage drop quite
a bit, if this is the case.
If the battery tests OK, then place a voltmeter probe, (positive lead) into
the connector on the back of the alternator, at the terminal with the largest
brown wire, with the voltmeter negative lead connected to ground, and rev the
engine. If the voltage reads high, over 15 volts, then the regulator is the
problem.
OTOH, probably the easist way to test it is to take the car to a battery
dealer and have them run a test of the entire system. It takes about 15
minutes, is free, and if you need a new battery, they can put one in on the
spot.
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://www.sky.net/~boballen/mg/Masters/
'74 MGBGT---3000mile/year driver, original condition
'68 MGBGT---organ donor for the '74
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