Cessna 1969-1976 172 Series Service Manual page 229

15-26. TROUBLE SHOOTING--VACUUM SYSTEM
Gyros function normally-relief
valve screen clogged, relief
valve malfunction.
TROUBLE
HIGH SUCTION GAGE READINGS
NORMAL SUCTION GAGE
READING, SLUGGISH OR
ERRATIC GYRO RESPONSE.
LOW SUCTION GAGE
READINGS.
SUCTION GAGE FLUCTUATES.
Check screen, than valve. Com-
p a r e gage readings with new gage.
Clean screen, r e s e t valve. Re-
place gage.
REMEDY
1 ~
PROBABLE CAUSE
Instrument
air
f i l t e r s clogged.
Check operation with f i l t e r s re-
moved. Replace filters.
Leaks o r restriction between
instruments and relief valve,
relief valve out of adjustment,
defective pump.
Central air filter dirty.
Deiective gage o r sticking relief
valve.
TROUBLE
I
I
HORIZON BAR FAILS TO
RESPOND.
A
HORIZON BAR DOES NOT
I
SETTLE.
Check lines f o r leaks, disconnect
and t e s t pump. Repair or replace
lines, adjust or replace relief
valve, r e p a i r o r replace pump.
Check operation with filter r e -
moved, Clean o r replace filter.
Check suction with
test gage.
Replace gage. Clean sticking
valve with Stoddard solvent.
Blow dry and
test.
If valve
sticks after cleaih:~g, replace
I
valve.
15- 27. TROUBLE SHOOTING-
-
GYROS.
PROBABLE CAUSE
Central filter dirty.
Suction relief valve improperly
adjusted.
Faulty suction gage.
~~
Vacuum pump failure.
Vacuum line kinked o r leaking.
Defective mechanism.
Insufficient vacuum,
~~ ~
Excessive vibration.
REMEDY
Check filter. Clean o r replace
filter.
Adjust o r replace relief valve.
~
Substitute known-good suction
gage and check gyro response.
Replace suction gage.
Check pump. Replace pump.
Check lines for damage and leaks.
Repair or replace damaged lines,
tighten connections.
I
Substitute known-good gyro and
check indication. Replace in-
strument.
Adjust or replace relief valve.
I
Check panel shock-mounts.
Replace defective shock-mounts.