Cessna 150M - Page 222

16-46. TROUBLE SHOOTING (Cont).
TROUBLE
INSTRUMENT LIGHTS
WILL NOT LIGHT.
CONTROL WHEEL MAP
LIGHT WILL NOT LIGHT
THRU 1970 AIRCRAFT
ONLY.
PROBABLE CAUSE
Short circuit in wiring.
Defective wiring.
Defective rheostat.
Lamp burned out.
Nav light switch turned off.
Short circuit in wiring.
Defective wiring.
Defective map light assembly.
REMEDY
1. Inspect fuse/circuit breaker.
If open, proceed to step 2. If OK,
proceed to step 3.
2.
Test circuit until short is lo-
cated. 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.
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 counterclock-
wise, replace rheostat.
5. Test lamp with ohmmeter or
new lamp. Replace lamp.
1. Nav light switch has to be ON
before map light will light.
2. Check lamp fuse on terminal
board located on back of stationary
panel with ohmmeter. If fuse is
open, proceed to step 3. If fuse is
OK, proceed to step 4.
3. Test circuit until short is lo-
cated. Repair or replace wiring.
4. Test for open circuit. Repair
or replace wiring. If a short or
open circuit is not found, proceed
to step 5.
5. Check voltage at map light
assembly with master and nav
switches on. If battery voltage
is present, replace map light
assembly.
Failure to observe polarity shown on wiring diagrams 11. 7. 0,
will result in immediate failure of the transistor on the map
light circuit board assembly.
16-16
Change 3