Cessna 150 Series Service Manual page 204

16-48. TROUBLE SHOOTING (Cont).
TROUBLE
PROBABLE CAUSE
REMEDY
INSTRUMENT LIGHTS
Short circuit in wiring. 1. Inspect circuit breaker. if open,
WILL NOT LIGHT OR DIM proceed to step 2. If OK, proceed to
CORRECTLY. step 3.
Defective wiring. 2. Test circuit until short is
located. Repair or replace wiring.
3. Test for open circuit. Repair or
replace wiring. If no short or open
circuit is found, proceed to step 4.
Defective rheostat. 4. Check voltage at instrument
light with master switch on.
Should read battery voltage with
rheostat turned full clockwise and
voltage should decrease as rheostat
is turned counterclockwise. If no
voltage is present or voltage has a
sudden drop before rheostat has
been turned full counterclockwise,
replace rheostat..
Lamp burned out. 5. Test lamp with ohmmeter or
new lamp. Replace lamp.
Defective transistor or open or 6. Check for opens or shorts at
short circuits between transistor transistor sockets or between
and heat sink. transistor and its heat sink. Heat
sink should not ground transistor.
If light will not dim, check
transistor. Defective or partially
shorted transistor can burn out
rheostat. Replacement rheostats
will continue to burn out until
transistor problem is corrected.
Replace transistor or insulator as
required.
16-18 Change I