analysis ac clutch related failures

Unburnished Clutch

Burnishing is the cycling of the clutch to allow a wearing in of the engagement surface area. The reason for burnishing a clutch is to increase the initial starting torque. Most technicians fail to follow this important procedure when replacing a compressor or clutch. An unburnished ac clutch can produce a low torque condition, causing the clutch to slip and thereby fail. When replacing a clutch or compressor & clutch assembly, follow this important burnishing procedure. Run the engine at 1,500 to 2,500 RPM. Using the controls on the dash, cycle the clutch ON and OFF at a rate of 10 to 15 times per minute for a total of 50 cycles minimum. This should bring the clutch up to operating torque capacity.

Improper Rotor to Hub Air Gap

An incorrect air gap can cause a clutch to engage or disengage improperly. This is particularly true on Sanden, Seltec, and Frigidaire/GMC style ac compressor clutches. Before operating a compressor of this type, check the hub/armature to rotor/pulley air gap. Check the required specifications for your particular type of compressor clutch in the illustrated section of the A/C parts catalog. These clutches may involve adding or removing shims to properly gap the hub. New manufactured compressor & clutch assemblies are properly gapped at the factory. Most clutch failures or problems relating to improper air gap can be traced to improper clutch replacement in the field. Check to see if this applies before returning possible warranties.

Misaligned Belt or Use of Wrong Clutch

In some cases ac clutch failure can be contributed to a slipping, under or over tensioned, or misaligned pulley belt. Further checking may discover that the wrong ac clutch (with improper mounting distance specifications) was installed onto the ac compressor. A clutch with as little as 1/8" offset in alignment can cause problems. Sometimes a misalignment condition can be traced to a cracked or loose compressor mount bracket on the engine. Look for excessive wear and/or indications of rubbing on the inside of the pulley groove before returning possible warranties.

Open Circuit inside Field Coil

This type of condition is rare, and can only be verified by removing the clutch pulley and hub from the compressor. The field coil core is held in place by an outer epoxy resin. If the bond between the epoxy and the winding fails, the coil may be subject to move, which could lead to a break in the coil wire, producing an inoperative clutch. With the exception of a fine crack around the outer diameter of the epoxy, there is no outward indication of a problem. However, the open circuit can be found with a resistance (ohm) meter. If an open circuit is found, with no evidence of melted resin or installation damage, the coil itself may be warranted.