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|  MIL-T-62021E(AT)
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3.4.1.7 Fan clutch. A fan clutch shall be provided. The fan clutch shall reduce the fan
speed automatically when the fan is not required for engine cooling. The fan clutch shall be
asbestos free.
3.4.1.8 Silicone rubber hoses. When specified (see 6.2), silicone rubber radiator and
heater hoses shall be furnished.
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3.4.1.9 Power plant heaters and fuel warmer. When specified (see 6.2), a coolant heater,
an engine oil heater, and fuel warmers (diesel engine driven vehicles only) shall be provided.
Heaters shall operate on 110 volts alternating current (ac), and shall be wired through a junction
block, including a fuse or circuit breaker, to a single three-pronged (male), weatherproof, slave
receptacle for receiving external power and grounding the vehicle. The receptacle shall be located
on the streetside of the vehicle, as near the cab door as practicable. A three-wire connecting
cable, not less than 7600 mm (25 feet) long and of adequate line capacity to supply power for all
heater units simultaneously, shall be furnished. Connecting cable shall include a matching female
connector at the vehicle end and a standard, three-pronged (two power plus one ground) male
connector at the other end. Electrical apparatus shall conform to Federal Motor Carrier Safety
Regulation 393.77(c)(7). The electrical insulation of the connecting cable shall withstand normal
operating stresses in low ambient air temperatures (down to -51C (-60F)) without cracking or
loss of dielectric capacity. All heater lead wires shall be installed without interfering with vehicle
component operation and without loose excess wire. Provisions for stowage of the cable shall be
provided in the vehicle cab. Heaters and fuel warmers shall be furnished as follows:
(a)
A coolant heater, 1,000-watt (W) minimum rating on trucks up to class 22 and
1,500-watt minimum rating for larger trucks shall be installed in the engine block or
in the lower coolant inlet hose. An engine thermostat with an operating range of
77C (170F) to 90C (195F) shall be installed.
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(b)
An oil pan heater of the permanent external surface mount, permanent in-pan mount,
or immersion type that meets the following requirements, shall be installed:
(1) Not less than 16 W/L (15 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.
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(c)
An in-line fuel warmer or preheater unit shall be provided on diesel engine driven
vehicles to prevent clogging of fuel filters due to wax crystallization in the fuel. The
unit shall use engine coolant to transfer sufficient heat to the diesel fuel to heat it
from an inlet temperature of -40C (-40F) to an outlet temperature of -13C (+9F),
with a fuel flow rate not less than the maximum fuel demand of the engine fuel
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