Initial Excitation Circuits; Regulator Input Test - Ford VSG-411 Service Manual

04-9
Charaina Svstem 04-9
DIAGNOSIS AND TESTING (Continued)
Initial Excitation Circuits
GENERATOR
REGULATOR
EXClTATlON
RESISTOR
I
GENERATOR
FIELD
FUSE
IGNITION
SWITCH
GENERATOR
WARNING LIGHT
I
INSTRUMENTS
(IF EQUIPPED)
tNSTRUMENT
FUSE
(IF EQUIPPED)
= BATTERY
initial excitation voltage reaches the field terminal through
an excitation lead from the ignition switch to the regulator,
and an excitation resistor inside the regulator. If the circuit
is equipped with a voltmeter, this is the only path. If the
circuit is equipped with a generator warning light, the
warning light provides an alternate path in parallel with the
excitation resistor. This excitation function of the warning
light is an incidental result of its being positioned in the
circuit so as to compare battery and output voltages, but it
is important because it can mask an open in the excitation
circuit.
Normally, initial excitation current is drawn through both
paths, but either one will pass sufficient current if the other
is open. If the excitation resistor is open, or higher in value
than it should be, there will still be excitation current to
magnetize the field at startup, but the voltage at the field
terminal will be lower because of the larger voltage drop
through the increased resistance, Likewise, if the warning
light bulb is burned out, the excitation resistor will provide
field current, but also at reduced voltage. A burned-out bulb
is easily detected if it fails to light when the key is turned
ON, and the other warning lights and gauges respond
normally.
Regulator input Test
This test measures excitation supply voltage to the
regulator. Perform this test if the field terminal voltage is
less than 1.5 volts.
1. Key ON.
2. Carefully probe the yellow wire terminal in the pigtail
connector for the regulator excitation lead. Battery
voltage should be present.
EXCITATION
RESIST
(YEI
VOLTMETER