External Circuit Tests; Field Voltage Test - Ford VSG-411 Service Manual

04-8
Charaina System
04-8
DIAGNOSIS AND TESTING (Continued)
3. With the engine running at 1000 to 1200 rpm, check
the voltage from the " R" terminal to ground.
GENERATOR
' VOLTMETER
Even if the output voltage is low, the " R" terminal
voltage should be within 0.5 volt of the " 0" terminal
voltage. If the " R" terminal voltage is higher than the
output voltage, it indicates an open diode or diodes,
and the higher " R" voltage would signal the regulator
to reduce output before normal charging voltage could
be built up. This causes an undercharging condition.
If the voltage at the " R" terminal is more than 0.5 volt
lower than the output voltage, one or more of the field
diodes is defective. In this condition, the regulator
would normally increase the generator output to bring
the sense voltage up to the regulating value, and an
overcharging condition would result.
4.
Continue with External Circuit Tests.
External Circuit Tests
This test checks the circuit between the generator and the
battery by measuring the voltage from each battery
terminal to ground.
1.
With the engine running at 1000 to 1200 rpm,
measure the voltage from the positive (+) battery
terminal to ground. Voltage at the positive (+) terminal
should be the same as the generator output voltage. If
it is lower by more than 0.2 volt, excess resistance is
present in the generator output lead or in the battery
positive cable or connections. Check for wire or cable
damage, or for loose or dirty connections.
2.
Measure the voltage from the battery negative (-)
terminal to ground. It should be zero. If it is more than
0.2 volt, check the ground circuit. The following
connections must be clean and tight:
l
Generator mounting bolts
l
Generator and engine mounting surfaces
l
Ground cable connections to the engine and
battery
Field Vdltage Test
This test checks for initial excitation voltage at the " F"
terminal. Perform this test if the output voltage did not rise
above battery voltage in the output voltage test, but did rise
with the regulator bypassed.
1. Make sure the field lead is connected.
2. Key ON.
3.
Measure the field voltage by carefully probing the " F"
terminal.
GENERATOR
VOLTMETER
CAUTION: To avoid grounding the field and causing
component damage, use a taped probe or connect the
field lead through a jumper to provide meter probe
access to the terminal.
A reading of 1.5 to 3.0 volts is normal. A zero-volt reading
indicates an open in the initial excitation circuit, either in the
regulator or in the circuit between the battery and the
regulator.
If battery voltage appears at the " F" terminal, an open is
present in the field circuit, either in the coil or in the
brushes. The meaning of voltage readings between these
extremes depends on the specific equipment being tested.