Section XVII SERVICE LIGHTING SYSTEM
The service lighting system provides illumination required while operating the vehicles. This system
is comprised of the following subsystems:
Headlamps and Tail Lamps (para 2-44).
Marker and Clearance Lamps (para 2-45),
Turn Signals and Hazard Warning Lamps (para 2-46).
Backup Lamps (para 2-47).
Work Lamps (para 2-48).
Instrument and Cab Dome Lamps (para 2-49).
Stop Lamps (See brake switches and indicators (para 2-63)).
Individual circuit descriptions, except stop lamps, follow in this section. You can find a complete
electrical schematic diagram and wiring harness diagrams in Appendix D.
2-44. HEADLAMPS AND TAIL LAMPS.
When headlamp switch is on, and operation lamp switch is in NORMAL, battery power is supplied
to illuminate the headlamps and tail lamps as well as the trailer tail lamps on the M915, M916, M917,
and M920. The traiIer tail lamps are powered through tractor chassis receptacles.
HEADLAMPS. Each assembly has two filaments in a sealed unit, a high beam and a low
beam. Selection of high or low beam is controlled by the dimmer switch.
RELAY (K6). Normally open contacts; energized by 12-volt power from operation lamp
switch when in NORMAL and headlamp switch is in on position. When relay is closed,
24-volt power is applied to trailer tail lamps, through circuit breaker and tractor receptacles.
ClRCUIT BREAKER (CB-8). Protects electrical components of 24-volt relay circuit by
opening when load exceeds 20 amps. Automatically recycles until overload is removed.
OPERATION LAMP SWITCH. Two-position safety switch for NORMAL and BLACKOUT
modes of operation. To select mode, switch is pulled out and placed to either position.
Set to NORMAL, 12-volt power from the lamp switch passes through operation lamp switch
contacts to energize; tail lamps, headlamps (through dimmer switch), and coil of relay.
Set to BLACKOUT, 12-volt power is removed from regular service lamps, electric horn,
and backup alarm, while 12-volt power from the lamp switch is supplied through the switch
to blackout tail lamps, marker lamps, and headlamp.
TAIL LAMPS. Each assembly contains two bulbs: A double-element bulb provides tail
lamp, turn signal lamp, and stop lamp. A separate bulb is used for the backup lamp. The
tail lamp circuit is energized through the operation lamp switch when headlamp switch is
in either ON position. (The circuits for turn signals, stop lamps, and backup lamps are
described in paragraph 2-46 and 2-47).