OperationAir Compressor CyclingThe engines can be equipped with one of two different types ofair compressor and may have one or two cylinders.• Head Unloaded Air Compressor: This single cylinder,constantly engaged air compressor works in conjunctionwith the air governor and air dryer to pump compressedair to the air dryer and air tanks. When additionalcompressed air is not needed, air is shut off from theair compressor discharge line and confined to the aircompressor cylinder and its head.• Clutched Air Compressor: This high-capacity, twocylinder, air compressor is periodically engaged byusing an On-Off clutch. It works in conjunction withair governor and air dryer to pump air to the air dryerand air tanks. When additional compressed air is notneeded, the clutch is disengaged, compressor speedgoes to zero, and all air pumping ceases. This systemprovides relatively low cylinder temperatures and soundversus other compressors during the unloaded cycle. It isused to promote maximum fuel economy by eliminatingpumping energy loss during the unloaded cycle.As the compressor reaches approximately 130 psi (896 kPa),the air governor will, through various methods, stop the aircompressor from pumping pressurized air to the air system.When the air pressure reaches approximately 110 psi (758kPa), the governor signals the air compressor to resumepumping pressurized air to the air system. During normalengine operation, this cycle will be evident by the fluctuation ofthe primary air tank pressure gauge.Cooling SystemWARNINGTo prevent property damage, personal injury,and / or death, use only the following procedureto remove the pressure cap from the radiatoror expansion tank. Allow the engine to coolfirst. Wrap a thick, heavy cloth around the cap.Unscrew the cap slowly to allow pressure torelease from under the cap. After the pressurehas been released, the pressure cap may beremoved.WARNINGTo prevent property damage, personal injury,and / or death, exercise great care when workingon vehicles with running engines that areequipped with an automatic fan clutch. Thefan engages when engine coolant reaches apredetermined temperature or the refrigerantpressure (if equipped with air conditioning)reaches a predetermined setting. The fan willstart with no advance warning.154
