Abstract: An auxiliary power assembly (5) for use with a heavy-duty diesel engine (8) of the type used in the trucking industry and including a small auxiliary diesel engine (9), an air compressor (10), a compressed air accumulator (17) fluid coupled to the air compressor (10), and a pneumatic starter (21) fluid coupled to the accumulator (17) and mechanically coupled to start the heavy-duty diesel engine (8). The auxiliary power assembly (5) further drives: a coolant pump (51) connected to circulate coolant between the auxiliary engine (9) and main diesel engine (8), a refrigerant compressor (41) formed to pump air conditioning fluid from the main diesel engine air conditioning system to the vehicle cab (70), and a pre-oiler pump (83) used to pressurize lubricating oil in the main engine (8). A method of operating the overall assembly and for retrofitting an existing heavy-duty diesel engine (8) with the auxiliary power assembly (5) also are disclosed.