Try to turn the compressor by hand. It should be possible to turn it around. It should turn around smoothly, without any 'hickups' (be sure to completely shut down the air compressor!). Check the isolation of the motor windings.
[pdf] An EU/EPA approved diesel air compressor is a type of air compressor that is powered by a portable air compressor diesel engine and has been certified to meet the stringent emission standards set by the European Union (EU) and the United States Environmental Protection Agency (EPA).
[pdf] An internal diesel motor powers these portable compressors and they work in a similar way to electric compressors. The air inside the chamber of the tank is pressurized either by rotating impellers or by collapsing the air containment chamber, which will force the air to compress in a smaller space.
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