Message text written by "James A. Ruffner"
>It should measure the same, since the voltage at the battery terminals,
when
the altenator is active, is the altenator voltage. The altenator runs at
13.5-14.5 V, depending on the level of charge on the battery. If the
altenator
is not in the circuit (engine speed is low enough that the altenator's
output
voltage is generally below about 13 V,) the regulator removes the altenator
from the circuit, and the voltmeter will drop to about 11 V. The
difference
between the battery's natural voltage (12.64 V) and the altenator's 14V is
enough such that the the altenator charges the battery (14V - 12.64V =
1.36V)
is enough the keep the battery charged. The voltmeter thus will only see
the
altenator's voltage when the regulator allows the altenator to be in the
circuit.
<
On closer inspection you will find a large brown wire that runs from the
power tap (on later model TR's , the starter solenoid on early model cars)
directly to the output terminal of the alternator. The alternator (which
is actually a three phase AC generator - which is why it is called an
"alternator") contains a full wave rectifier bridge consisting of six
diodes that rectifies the AC voltage and makes it compatable with a DC
system. There are also three more diodes that supply power to the voltage
regulator so the diode pack in there has a total of nine rectifiers.
These diodes will inherently prevent current feeding back into the
alternator when it is not generating voltage so no cut-out relay is needed
with these types of electrical generating devices. The older dynamo
devices were from the days before solid state diodes were available and
rectification was accomplished by the commutator and brushes and since
these devices will pass current in either direction (one reason you can
repolarize a generator) a cut-out relay was manditory in the voltage
regulator for these devices.
The bottom line is that the questioner wanted to know whether to measure
the voltage at the battery or the alternator the answer is: It doesn't
matter but the battery is easier to access.
Dave
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