Cessna 150M - Over-Voltage Warning System; Troubleshooting the Alternator System

16-36. OVER-VOLTAGE WARNING SYSTEM.
16-37. DESCRIPTION. The over-voltage system
consists of an over-voltage sensor switch and a red
warning light labeled "HIGH VOLTAGE". The over-
voltage sensor is attached to the wire bundle behind
the instrument panel and the light is located on the
right hand side of the instrument panel. When an
over-voltage tripoff occurs the over-voltage sensor
turns off the alternator system and the red warning
light comes on. The ammeter will show a discharge.
Turn off both sections of the master switch to recycle
the over-voltage sensor. If the over-voltage condition
was tranSient, the normal alternator charging will
resume and no further action is necessary. If the
over-voltage tripoff recurs, then a generating system
malfunction has occurred such that the electrical
accessories must be operated from the aircraft bat-
tery only. Conservation of electrical energy must be
practiced until the flight can be terminated. The over-
voltage light filament may be tested at any time by
turning off the "Alternator" portion of the master
switch and leaving the battery portion on. This test
does not induce an over-voltage condition on the elec-
trical system. On models prior to aircraft serial
15073084, A1500313 and F15000769, should nuisance
trip-outs occur caused by voltage spiks or transient
voltage, Cessna Single-engine Service Letter SE72-15
dated April 21, 1972 should be complied with.
NOTE
The over-voltage sensor switch contains solid
state devices. Observe proper polarity before
supplying power. Grounding the orange lead
or interconnecting orange and black leads will
destroy the device. When removal is required
for replacement, identify (tag) wiring and follow
the wiring diagram in Section 20 for rewiring.
16-38. TROUBLE SHOOTING THE ALTERNATOR SYSTEM.
TROUBLE
AMMETER INDICATES HEAVY
DISCHARGE WITH ENGINE
NOT RUNNING OR ALTERNA-
TOR CIRCUIT BREAKER OPENS
WHEN MASTER SWITCH IS
TURNED ON.
PROBABLE CAUSE
Shorted radio noise filter
or shorted wire.
Shorted diodes in alternator.
REMEDY
1. Remove
cable from output
terminal of alternator. Check
resistance from end of cable
to ground (MASTER SWITCH
MUST BE OFF). If resistance
does not indicate a direct short,
proceed to step 4. If resistance
indicates a direct short, proceed
to step 2.
2.
Remove cable connections
from radio noise filter. Check
resistance from the filter input
terminal to ground. Normal in-
dication is infinite resistance.
If reading indicates a direct
short, replace filter. If no
short is evident, proceed to
step 3.
3.
Check resistance from ground
to the free ends of the wires which
were connected to the radio noise
filter (or alternator if no noise
filter is installed). Normal indica-
tion does not show a direct short.
If a short exists in Wires, repair
or replace wiring.
4. Check resistance from output
terminal of alternator to alterna-
tor case. Reverse leads and
check again. Resistance reading
may show continuity in one direc-
tion but should show an infinite
reading in the other direction.
If an infinite reading is not ob-
tained in at least one direction,
repair or replace alternator.
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