20
Electrical Tests
V Drop high current
Connect COM probe to negative battery post or portion
of circuit closest to battery ground. Connect CH1 probe
to ground connector or test point of suspected circuit
farthest away from battery negative post (NOTE: probes
may be connected in opposite fashion to measure voltage
drops in power circuits). Make sure power in circuit, so
normal current is flowing through circuit. Check amplitude
of signal.. It should generally not exceed 300mV. Clean,
repair, or replace connections or connectors as needed.
V Drop low current
Connect COM probe to negative battery post or portion
of circuit closest to battery ground. Connect CH1 probe
to ground connector or test point of suspected circuit
farthest away from battery negative post (NOTE: probes
may be connected in opposite fashion to measure voltage
drops in power circuits). Make sure power is on in circuit,
so normal current is flowing through circuit. Check
amplitude of signal. It should generally not exceed 100mV.
Clean, repair, or replace connections or connectors as
needed.
Alt Output V Test
Connect COM probe to the ground circuit, engine block,
or negative battery post. Connect the CH1 probe to the
positive battery post. Turn off all accessories, start the
engine and hold at 2500 RPM for 2-3 minutes. The
alternator’s output voltage should generally be 0.5 - 2.0
volts higher than the static battery voltage was with the
key off. Consult the vehicle manufacturer’s specifications
for specific acceptable voltage ranges.
Alt Diode Test
Connect COM probe to negative battery post, or alternator
case. Connect CH1 probe to positive battery post, or
better, B+ output terminal in back of alternator. With key
on, engine off, turn all accessories on for 3 minutes. Start
engine and let it idle. A waveform that looks like a string
of back-to-back rolling hills should appear if alternator’s
diodes are good. Bad diodes will appear as large 2-3
times normal amplitude dropouts in waveform. Some
spikes or noise may be normal in some systems.
(24 pages)
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