I'm posting again because the first version got lost somewhere in the mail
system.
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The warning light has two functions:
1) It tells you if the voltage generated by the alternator is too low,
therefore no charge.
2) It helps getting the alternator started while or after starting the
engine.
1)
The warning light is connected between the battery voltage and the internal
alternator voltage. If the latter voltage is too low, the voltage over the
light increases, which eventually will make it glow to even burn brightly.
If you turn the ignition on the alternator voltage is zero, the light
voltage is approximately 12 Volts, allowing you to check whether the light
still is OK.
2)
An alternator has two coils: on the "outside" the steady coil(s) in which
the power is generated, on the "inside" the rotating coil which generates
the required magnetical field. The alternator output voltage can be
regulated by modulating the magnetical field strength of the rotating coil.
The magnetical field strength depends on the current through the rotating
coil. The voltage regulator thus controls the current through this coil.
An alternator is self supporting in a sense that it generates its own power
for the rotating coil current. This is a classical chicken and egg
situation: without power no current, and without current no power output.
To make sure that the system will work the warning light is connected in
such a manner that it supplies some coil current at start-up. The generated
alternator output is immediately used to increase the coil current, thus
generating more output, until normal output is achieved.
After starting the engine the light may glow or burn, although this is a
bit unusual, but after revs over 1000/1500 rpm the light should definitely
go out. To simply test alternator output: start the engine, rev it up to
e.g. 2500 rpm, switch on lights and measure battery voltage. The reading
with lights on should be above 13 or 13.5 Volts, but never higher than 15
Volts with lights switched off.
If in doubt: bring it to a repair shop. They can test alternator /
rectifier/ regulator operation in a few minutes by connecting it to a test
set-up.
Anne van Leeuwen
The Netherlands
1972 TR6
>>> "Sean Johnson" <sean_johnson@milacron.com> 27/05/97 09:05 >>>
Among other things, my Spit's alternator warning light is burning whenever
the engine is started.
I checked my manual (Haynes). It has a diagnostic procedure that involves
a
voltmeter, ammeter, and variable resistor.
Short of making a trip to Radio Shack for the above items, does anyone have
a
diagnostic procedure that's a little more practical? I have a basic
voltmeter, and can get a digital multimeter. I didn't find anything
alternator testing from the web page. I know this comes up frequently; I
should have saved more lists!
Also, could someone who understands electrons/smoke a little better than I,
explain how/when the warning light works?
Help and opinions gratefully accepted.
Sean Johnson
'75 Spitfire/'79 engine
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