ISM, ISMe, and QSM11 – Air Compressor Cycles Frequently

Air system leaks

Block the vehicle wheels and check the air system for leaks with spring brakes applied and released. Check for leaks from the air compressor gaskets and the air system hoses, fittings, tanks, and valves. Refer to Procedure 012-019 and the OEM service manual.

Air governor is malfunctioning or not set correctly

Check the air governor for correct operation. Refer to Procedure 012-017.

Air system component is malfunctioning

Check the operation of check valves, alcohol evaporators, air dryers, and other OEM-installed air system components. Refer to manufacturer’s instructions.

E-type system is not plumbed correctly (Holset® only)

Install an Econ valve, a check valve, and system hoses. Refer to the Master Repair Manual, Holset® Air Compressors, Bulletin 3666121.

Carbon buildup is excessive in the air discharge line, check valve, or cylinder head

Check for carbon buildup. Replace the air compressor discharge line, if necessary. Check the turbocharger for oil leaks. Check the intake tube for oil. Refer to Procedures 012-003, 010-033.

Air dryer desiccant cartridge needs replaced

Replace the desiccant cartridge on the air dryer (if equipped). Refer to the OEM service manual.

Air dryer outlet check valve is sticking

Lubricate or replace the air dryer outlet check valve assembly. Refer to the manufacturer’s instructions.

Air installation issues

Refer to the Automotive and Bus Installation Recommendations – Air Compressor Systems, Bulletin 3884946.

Air compressor not designed for application

Check the air compressor duty cycle. Install a larger air compressor, if necessary. Refer to Section G in the Master Repair Manual, Holset® Air Compressors, Bulletin 3666121.