Operation - Cessna 1969-1976 172 Series Service Manual

15-32. ENGINE INDICATORS.
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TROUBLE PROBABLE CAUSE
15 -33. TACHOMETER.
REMEDY
15-34. DESCRIPTION. The tachometer used on
Cessna single-engine a i r c r a f t is
a
mechanical indi-
cator driven a t half crankshaft speed by a flexible
shaft. Most tachometer difficulities will be found
in the drive-shaft. To function properly, shaft hous-
ing must be free of kinks, dents and s h a r p bends.
T h e r e should be no bend on
a
radius shorter than six
inches and no bend within three inches of either
terminal. I f a tachometer is noisy
o r pointer oscil-
lates, check cable housing for kinks, s h a r p bends
and damage. Disconnect cable a t tachometer and
pull it out of housing. Check cable for worn spots,
b r e a k s and kinks.
P r e s s u r e line clogged.
P r e s s u r e line broken.
Fractured Bourdon tube.
640 speedometer cable g r e a s e o r Lubriplate
No. 110. Insert cable in housing as far as
possible, then slowly rotate to make s u r e it
is
seated in engine fitting. Insert cable in
tachometer, making s u r e it is seated in drive
shaft, then reconnect housing and torque to
50 pound-inches (at instrument).
15-35. OIL PRESSURE GAGE.
Check line f o r obstructions. Clean
line.
Check line f o r l e a k s and damage.
Repair o r replace damaged line.
Replace instrument.
15-36. DESCRIPTION. The Bourdon tube-type oil
p r e s s u r e gage is
a
direct-reading instrument, opera-
ted by a p r e s s u r e pickup line connected to the engine
main oil gallery. The oil p r e s s u r e line from the in-
strument to the engine should be filled with kerosene,
especially during cold weather operation, to attain
a n immediate oil indication.
Gage pointer loose on staff.
Damaged gage movement.
NOTE
Replace instrument.
Replace instrument.
Before replacing a tachometer cable in hous-
ing, coat lower two thirds with AC Type ST-
Foreign matter in line.
Foreign matter in Bourdon
tube.
Bourdon tube stretched.
15-37. TROUBLE SHOOTING.
Check line f o r obstructions.
Clean line.
Replace instrument.
Replace instrument.
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Foreign matter in Bourdon
tube.
Dirty or corroded movement.
GAGE DOES NOT REGISTER.
Replace instrument.
Replace instrument.
GAGE POINTER FAILS
TO RETURN TO ZERO.
,
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GAGE DOES NOT REGISTER
PROPERLY.
1
GAGE HAS ERRATIC
~
OPERATION.
Faulty mechanism. Replace instrument.
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Worn o r bent movement.
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Replace instrument.
15-16