SWITCH AS-PRESSURE 1149281 - Caterpillar

Do not disconnect the air line from the air compressor governor without purging the air brake and the auxiliary air systems. Failure to purge the air brake and the auxiliary air systems before removing the air compressor and/or the air lines could cause personal injury.
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Typical installation of an air compressor
If the air compressor pressure relief valve that is mounted in the air compressor cylinder head is bypassing compressed air, there is a malfunction in the air system, possibly ice blockage. Under these conditions, your engine may have insufficient air for normal brake operation.Do not operate the engine until the reason for the air bypass is identified and corrected. Failure to heed this warning could lead to property damage, personal injury, or death to the operator or bystanders.
The function of the air compressor pressure relief valve is to bypass air when there is a malfunction in the air compressor system. The air compressor pressure relief valve releases air at 1723 kPa (250 psi). All personnel must stand clear of the air compressor pressure relief valve when compressed air is released. All personnel should also stay clear of the air compressor when the engine is operating and the air compressor is exposed.Refer to the Disassembly and Assembly Manual, "Air Compressor - Remove and Install" for more information or consult with your Cat dealer.