TM 9-2350-256-20-1
UNIT MAINTENANCE
RECOVERY VEHICLE, FULL TRACKED: MEDIUM, M88A1
NSN 2350-00-122-6826, EIC AQA
AUXILIARY POWER UNIT (APU) THEORY OF OPERATION
APU
General
The APU consists of an overhead-valve, two-cylinder, four-cycle, air-cooled, diesel engine connected directly to a 300-A,
28-V, shunt-wound, dc generator (which is also used as a starter), and an auxiliary hydraulic system pump. The APU provides
electrical power to charge the vehicle batteries, and hydraulic power to raise and lower the spade, stow and raise the boom, and
retrieve the main and hoist winch cables.
Figure 1.
APU.
Starting and Generating Systems
The 300-A, 28-V generator mentioned above is also used as a starter motor. This is accomplished by first feeding 24 V dc to
the field coil of the generator from the preheat relay when the preheat switch is activated, and then by applying 24 V dc to the
generator armature through a starting relay when the start switch is activated. This motorizes the generator, which is directly
coupled to the engine by a drive chain. When the engine starts, the current in the generator armature reverses direction, and
the system automatically changes from a motorizing mode to a generating mode. After the engine starts, the start and preheat
switches must be released to prevent loading down the engine since the voltage regulator is bypassed during this start cycle, and
full voltage is being applied to the field. After the engine has started, and the preheat and start switches are released, the generator
output is controlled by a solid-state voltage regulator and a current-limiting device.
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