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|  MIL-T-62735
3.5.24.3 Heaters. Heaters shall be as follows:
(a) The engine block or the lower coolant inlet hose shall have a coolant heater,
1,000-watt (W) minimum rating on trucks up to class 22 and 1,500-watt minimum
rating for larger trucks. An engine thermostat with an operating range of 77C to
90C (170F to 195F) shall be installed.
(b) The oil pan shall have a heater of the permanent external surface mount, permanent
in-pan mount, or immersion type that meets the following requirements:
(1) Not less than 16 W/L (16 watts per quart) heating capacity
(2) Not more than 2.8 watts per square centimeter (W/cm2) (18 watts per
square inch) heating capacity
(3) Thermal balance design or thermostat control providing for uninterrupted
operation
(4) Provision for mounting below minimum service oil level.
(c) Diesel engines shall have a fuel warmer to prevent clogging of fuel filters due to wax
crystallization in the fuel. The fuel warmer shall use engine coolant to transfer
sufficient heat to the diesel fuel to heat it from an inlet temperature of -40 (-40F) to
an outlet temperature of -13C (+9F), with a fuel flow rate not less than the
maximum fuel demand of the engine fuel system. The system shall include a
coolant shutoff valve for the coolant inlet side of the fuel warmer unit. The system
shall not cause heating of the fuel above 27C (80F) under any possible condition.
(d) Diesel engines shall have an in-tank fuel warmer. The warmer shall use engine
coolant to transfer heat to the fuel in one fuel tank. The warmer shall not cause
heating of any fuel above 27C (80F) under any possible condition, shall not
disable or cause elimination of the fuel gage sending unit and shall not violate 3.1.1
or any Federal Motor Carrier Safety Regulation. A coolant shut off valve shall be
included. The units required by 3.5.24.2(c) and (d) may be combined.
3.5.25 Fuel fired engine preheater. When specified (see 6.2), for diesel engine driven
vehicles, the vehicles shall have a diesel fuel fired engine water heater to preheat the engine and
the fuel. The heater shall include a timer, a thermostat, a circulating pump and connection into
the engine coolant system. The heater shall be capable of starting and operating at -51C
(-60F). The heater shall heat the engine from -51C (-50F) to 4C (+40F) in not more than
1 hour.
3.5.25.1 Preheater controls and indicator light. The system shall have a starter on/off
switch, accessible to the seated driver. The system shall include fuel warmers in accordance
with 3.5.24.2(c) and (d). The system shall be have an indicator light, visible to the seated driver,
to indicate that the preheater is operating. The purpose of the switch and light shall be indicated
by marking in accordance with 3.10.5.
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