Section IV TRANSMISSION AND CONTROLS
The Caterpillar D7155 transmission is used on all M915 thru M920 trucks. It is a mechanical drive
unit with 16 forward speeds and 2 reverse speeds. The transmission is bolted directly to engine fly-
2-12. TRANSMISSION AND CONTROLS.
ENGINE RUN SWITCH. Energizes the 12-volt electrical system, including the transmission
oil temperature gage circuit.
TRANSMISSION OIL TEMPERATURE GAGE. Signal from sending unit actuates pointer
gage showing transmission oil temperature.
ClRCUIT BREAKER (CB-6). Protects electrical components of transmission oil temperature
circuit when load exceeds 20 amps. Automatically recycles until the overload is removed.
RATlO SELECTOR UNIT. Floor-mounted, houses transmission controls.
GEAR SELECTOR. Allows operator to select desired gear.
TRANSMISSION AlR CHARGING VALVE. Used to charge control line when starting up.
CONTROL LINE. Carries pneumatic pressure which controls pressure from supply line.
Normal air pressure is 90-120 psi (transmission is disengaged if air pressure drops below 60 psi).
BREATHERS. Prevent air pressure buildup in case.
SUPPLY LINE. Pneumatic pressure, controlled by line from ratio selector and sets transmis-
sion to selected gear. Normal air pressure is 90-120 psi (transmission is disengaged if air
pressure drops below 60 psi).
SPEEDOMETER CABLE. Provides engine speed input to tachograph.
TRANSMISSION OIL COOLER. Two hydraulic lines carry warm oil from the transmission
to an oil cooler mounted in front of radiator. Once cooled, the oil is returned to the
DIPSTICK AND OIL FILL PORT. Used to inspect and replenish oil supply. Normal
capacity: at oil change 5.5 gallons (21 liters) of engine oil.
DRAIN PLUG. Provides access for draining oil. Has magnetic base to attract foreign parti-
cles in the oil and warn of unusual wear of the internal parts.
SUPPLY RESERVOIR. Provides air supply line. Normal air pressure is 90-120 psi (transmis-
sion is disengaged if air pressure drops below 60 psi).
ACCESS PLATE. Provides opening for access to oil pump suction line screen, There is an
identical plate on the opposite side of the transmission.
TRANSMISSION OIL TEMPERATURE SENDING UNIT. Provides electrical signal to tem-
perature gage for indicating transmission oil temperature.