Cessna 150M - Trouble Shooting-Vacuum Pump; Vacuum Relief Valve Adjustment; Engine Indicators; Tachometer

15-28. TROUBLE SHOOTING--VACUUM PUMP.
TROUBLE
PROBABLE CAUSE
REMEDY
OIL IN
DISCHARGE.
Damaged engine drive seal.
Replace gasket.
HIGH SUCTION.
Suction relief valve filter
Check filter for obstructions.
clogged.
LOW SUCTION. Relief valve leaking.
Vacuum pump failure.
LOW PRESSURE.
Safety valve leaking.
Vacuum pump failure.
15-29. REMOVAL AND INSTALLATION OF COMPO-
NENTS. The various components of vacuum system
are secured by conventional clamps, mounting screws
and nuts. To remove a component, remove mount-
ing screws and disconnect inlet and discharge lines.
When replacing a vacuum system component, ensure
connections are made cor'rectly. Use no thread-lube
on any connections. Teflon tape may be used on male
threads. Avoid over-tightening connections. Before
re-installing a vacuum pump, place mounting pad gas-
ket over studs. After installing pump, before con-
necting plumbing, start engine and check for evidence
of oil in the discharge which would indicate a leaking
engine drive seal.
15-30. CLEANING. In general, low-pressure, dry
compressed air should be used in cleaning vacuum
system components. Suction relief valve, exposed
to engine oil and dirt, should be washed with Stod-
datd solvent, then dried with a low-pressure air
blast. Check hose for collapsed inner liners as well
as external damage.
Never apply compressed air to lines or com-
ponents installed in aircraft. The excessive
pressures will damage gyros. If an obstruc-
ted line is to be blown out, disconnect at both
ends and blow from instrument panel out.
15-14
Change 2
Clean or replace filter.
Replace relief valve.
Substitute known-good pump
and check pump suction.
Replace vacuum pump.
Replace safety valve.
Substitute known-good pump
and check pump pressure.
Replace vacuum pump.
15-31. VACUUM RELIEF VALVE ADJUSTMENT.
A suction gage reading of 5.3 inches of mercury is
desirable for gyro instruments. However, a range
of 4.6 to 5.4 inches of mercury is acceptable. To
adjust the relief valve, remove control air filter,
run engine to 1900 rpm on the ground and adjust re-
lief valve to 5.3 ±.1 inches of mercury.
/CAUTION\
Do not exceed maximum engine temperature.
Be sure filter element is clean before installing. If
reading drops noticeably, install new filter element.
15-32. ENGINE INDICATORS.
15-33. TACHOMETER.
15-34. DESCRIPTION. The tachometer used on
Cessna Single-engine aircraft is a mechanical indi-
cator driven at 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 sharp bends.
There should be no bend on a radius shorter than six
inches and no bend within three inches of either
terminal. If a tachometer is noisy or pointer oscil-
lates, check cable housing for kinks, sharp bends
and damage. Disconnect cable at tachometer and
pull it out of housing. Check cable for worn spots,
breaks and kinks.