Trouble Shooting - Cessna 1969-1976 172 Series Service Manual

16-16. TROUBLE SHOOTING THE BATTERY POWER SYSTEM.
T ROUB L E
BATTERY WILL NOT SUPPLY
ABLE OF CRANKING ENGINE.
POWER TO BUS OR IS INCAP-
PROBABLE CAUSE
Battery discharged.
Battery faulty.
Faulty contactor o r wiring
between contactor o r master
switch.
Open coil on contactor.
Faulty contactor contacts.
Faulty wiring between con-
tactor and bus.
REMEDY
1. Measure voltage
at "BAT"
terminal of battery contactor
with master switch and a suit-
able load such as a taxi light
turned on. Normal battery
will indicate 11.5 volts or
more. If voltage is low, pro-
ceed t o step 2. If voltage
is
normal, proceed to step 3 .
2. Check fluid level in cells
and charge battery at 20 amps
for approximately 30 minutes
o r until the battery voltage
r i s e s to 15 volts. Check bat-
t e r y with a load type tester.
If tester indicates
a
good bat-
tery, the malfunction may be
assumed to be a discharged
battery. If the tester indicates
a faulty battery, replace the
battery
.
3 . Measure voltage at master
switch terminal (smallest) on
contactor with master switch
closed. Normal indication is
zero volts. If voltage r e a d s
zero, proceed to step 4. If
a
voltage reading is obtained
check wiring between contactor
and master switch. Also check
master switch.
4. Check continuity between
"BAT" terminal and master
switch terminal of contactor.
Normal indication is 16 to 24
ohms (Master switch open).
I f ohmmeter indicates an open
coil, replace contactor. If
ohmmeter indicates a good
coil, proceed to step 5.
5. Check voltage on "BUS"
side of contactor with master
switch closed. Meter normally
indicates battery voltage. If
voltage is zero o r intermittant,
replace contactor. If voltage
is normal, proceed to step 6.
6. Inspect wiring between con-
tactor and bus. Repair o r re-
place wiring.
16-3