Car A/C Clutch Guide

At the heart of the air conditioning unit of a car is the compressor that pressurizes the refrigerant and pumps it throughout the A/C system. It gets its power directly from the engine flywheel and, in a similar way to the drive chain of the car, a clutch controls its operation. These two parts are the most working components in a vehicle’s A/C system, and maintaining them in good working order is essential to its efficiency.

The engine drives the pump rotor in the compressor directly with a belt and so the compressor is required to be able to be turned on and off on demand. To get this, automotive air conditioners use an electromagnetic clutch mechanism to engage the compressor to activate the A/C unit in the cabin. The air conditioner clutch has a friction plate that is adjacent towards an electromagnetic coil. When a current runs throughout this coil it creates a powerful magnetic force towards the friction plate in the clutch. The compressor’s rotor has another friction plate that is attached to it which is next to the first one and the electromagnetic force draws it towards the friction plate in the clutch which drives the pump rotor in the compressor when it is engaged. The friction plate uses springs for returning back to their disengaged positions as soon as the current stops passing across the magnetic coil to switch off the air conditioner.

A/C Clutch Component Function
Clutch rotor Used for constantly spinning a friction plate using power from the motor
Electromagnetic
coil
Helps to produce a magnetic field around the rotor to attract the armature
Armature The armature attaches a friction plate magnetically with the rotor plate
Output hub Directly transfers power towards the compressor pump rotor
Armature springs Helps to disengage the armature plate from the clutch rotor to stop the compressor

A/C clutch mechanisms helps to control the transfer of power from the engine to the vehicle’s air conditioner using only five simple components. Turning on the car air conditioning in the cabin activates the A/C clutch, which drives the compressor to pump refrigerant through the heat transfer cycle. Usually, the A/C clutch rotor is constantly under load and the output hub permanently attaches the armature to the compressor pump rotor. This helps in the minimal amount of slippage while engaging the friction plates and provides excellent torque transfer efficiency.

Examining Air Conditioning Clutch Failures in the Field



This guide can be used as a reference to cure clutch failures, and is specifically made to help you differentiate between unwarrantable clutch failures and those that would normally fall under possible warranty consideration. 

Next to hose leaks and compressors, clutches show the next highest rate of failure of any a/c component. Using this guide will enable mechanics to properly diagnose primary causes of clutch failures. Observance and examination of both the compressor as well as the clutch during diagnosis will improve your analysis a lot in determining exactly what happened. 

Since subsequent problems or failures often happen because of primary failure causes, this guide is divided into the following sections: 

  • Clutch Related Failures
  • Compressor Related Failures & Causes
  • Installation Related Failures

A Typical Clutch Assembly

CLUTCH RELATED FAILURES

The following are the most common symptoms and causes of clutch related problems.

Burnt Hub, Pulley, and/or Coil

Inadequate voltage to the coil will lead to overheating of the internal winding and cause the clutch hub to slip against the face of the pulley. A compressor in the stages of failing because of slugging or loss of lubrication, or operating under a high pressure condition will cause the clutch to slip. The slipping take place continuously during compressor operation and can subject both the clutch and coil to extreme temperatures up to 1,200 degrees F. This will quickly destroy the clutch and all its internal components. Failure of this type is not warranted.

Bearing Failure

Bearing failures are mostly caused by system problems. Very seldom is a bearing itself which is the culprit of a failed or locked up compressor. For instance, if the clutch slips severely due to a high pressure or low voltage condition, or due to compressor slugging, the excessive heat generated by the slipping will quickly melt and destroy the bearing seal. The bearing then loses its grease and locks up or falls apart, mostly destroying the clutch in the process. This type of bearing failure is not covered under warranty as the bearing was not the root cause of the problem. Warranty may be considered only if the bearing is intact, and there is no proof of excessive heat, a compressor or a/c system problem, or signs of installation related problems. Manufacturing defects in cutch bearings are usually rare, and warranty allowances are mostly associated with excessive noise symptoms only.

Noisy Bearing

If the complaint is noise only related, a defective clutch pulley bearing could be the source of the problem. Check for a rough or poorly functioning bearing by holding the clutch armature hub stationary, and rotating just the pulley. If excessive noise or difficulty in ease of rotation is noticed, the bearing may be suspect. However, as with the failed bearing situation described above, several factors can lead to the bearing becoming noisy or rough in operation. A rough bearing can also be one that is in the early stages of failing because of system problems involving extreme heat. This is mostly the case if the compressor clutch has been operating sufficiently for some time and then becomes noisy. Most factory bearing defects become noticeable in the first hours of compressor operation. Warranty consideration for failed compressors and clutches described as noisy will be given only if there are no obvious signs that the failure was caused by other factors.

Unburnished Clutch

Burnishing is the cycling of the clutch to help a bearing in of the engagement surface area. The reason for burnishing a clutch is for increasing the initial starting torque. Most technicians fail to follow this crucial procedure when replacing a compressor or clutch. An unburnished clutch can give a low torque condition, causing the clutch to slip and thereby fail. While replacing a clutch or compressor & clutch assembly, follow this important burnishing procedure. First, run the engine at 1,500 to 2,500 RPM. Then, 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 towards the operating torque capacity.

Improper Rotor to Hub Air Gap

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

Misaligned Belt or Use of Wrong Clutch

In some cases clutch failure can be contributed towards a slipping, under or over tensioned, or misaligned pulley belt. Further checking might discover that the wrong clutch (with improper mounting distance specifications) was installed onto the 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 to a loose compressor mount bracket on the engine. Check 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 quite rare, and can only be verified by removing the clutch pulley and hub from the compressor. The field coil core is held in place through an outer epoxy resin. If the bond between the epoxy and the winding fails, the coil may be subject to move, which could result in 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 any problem. However, the open circuit can be noticed with a resistance (ohm) meter. If an open circuit is found, with no proof of melted resin or installation damage, the coil itself may be warranted.

Failed Field Coil Mounting Flange Welds

This condition is limited only to CCI/York/Tecumseh style clutches. The field coil assembly has a magnetic field core that encloses the coil, and a flange which mounts onto the compressor. The field core and flange are assembled at the factory and joined together by spot welds. Although it seldom occurs, a faulty spot weld can cause rubbing between the outside of the field core and inside of the pulley, eventually causing an open circuit and inoperative clutch. Be careful that in most cases involving failed welds and broken coil flanges, a severe vibration problem usually exists. This is quite true for Detroit Series 60 engines. In some cases the vibration is so extreme that the screws holding the coil onto the compressor will just vibrate loose, causing the clutch assembly to become unattached from the compressor shaft. Without having a secure mount, the clutch and coil will just be torn apart by the force of the drive train belt attempting to turn the clutch pulley. The compressor shaft is generally also destroyed in the process. An occurrence of this nature voids all warranty on both the compressor as well as the clutch.

Faulty Lead Wire

If the lead wire connected to the field coil is faulty, the clutch can’t operate. This will be noticed at the time of installation, and seldom occurs afterwards. However, evidence of obviously damaged or cut lead wires is not a reason for warranty consideration.

COMPRESSOR RELATED FAILURES & CAUSES

The following are the most common causes of clutch failures. These all can be traced to a problem in the compressor and/or a/c system. Please see that in nearly every case involving compressor related clutch failure, warranty is not applicable.

Seized or Slugging Compressor :

The clutch’s aim is to engage and disengage the compressor from the vehicle’s accessory drive system. If there is a system problem that doesn’t let the proper rotation of the compressor input shaft while the clutch is energized, the clutch will slip. A slipping clutch can generate tremendous heat which can be up to 1,200 degrees F. In a matter of seconds, parts can start to fail. There is a progression of effects which is generated from slipping, all leading to overall clutch and/or compressor failure.

Symptoms and causes are listed below:

  1. Loss of Lubrication: 
   – Loss of refrigerant will also stop oil from returning to the compressor causing binding or lockup. 
   – A blockage or restriction somewhere inside the system will also inhibit or prevent oil return to the compressor, thereby causing slipping, binding, or lockup.

2. Excessive A/C System Pressure 
The following problems lead to increase in system pressure. Just as too high blood pressure can lead to the heart to overwork, and prematurely fail, excessive A/C system pressure can cause the compressor to overwork, begin to seize, and increase the torque requirements of the clutch. The clutch will then begin to slip, which produces tremendous heat build-up. Resultant clutch and bearing failure follows.

3. Incorrect Amount of Oil in System 
Too much oil can lead to compressor slugging and a slipping clutch. Excessive oil can accumulate in the a/c condenser which will increase the system discharge pressures. Not enough oil will result in compressor binding and clutch slipping.

4. Inadequate Air Flow across the Condenser 
If the condenser is restricted internally, or has inadequate air flow across it, it will cause a higher discharge pressure. This can cause the clutch to slip. Check the inlet and outlet temperatures of the condenser. A difference of over 35 degrees can indicate a problem. An inoperative or faulty radiator or condenser fan could also be suspect, or shutters not opening properly.

5. A Blockage in the High Pressure Side of the A/C System 
A blockage will make the discharge pressure to increase and can cause clutch slipping. A blockage in the system can generally be pinpointed by an immediate temperature (pressure) drop just following the blockage. Note that a rise in pressure means a rise in temperature, and vice versa.

The causes are as follows –

System Overcharge 
An overcharged a/c system will make pressures to increase and cause slipping. When checking for an overcharged system, ensure check for the presence of non-condensables (air) too. This condition will also help to increase system pressures.

Leaking Compressor Shaft Seal:

Another clutch failure that can be attached to a system problem is a failed bearing due to impingement by compressor oil. This might occur if the seal on the compressor shaft leaks compressor oil, and finds its way into the clutch bearing. This oil can contaminate the bearing grease, resulting it to be less effective, and also lead to grease purging through the bearing seals. The result is a gritty or a failed bearing. Remember that an excessive heat condition in the system can also lead to a shaft seal failure.

Mis-Machined Compressor Mounting Boss:

If there is a dimensional glitch with the mounting bosses used for attaching the field coil assembly to the compressor, an interface condition might be created between the field coil and rotor assembly. This problem is mostly associated with poor quality remanufactured compressors.

Re-Machined Compressor Shaft:

During the course of remanufacturing a compressor, the taper of the input shaft might be re-machined. As a result, the shaft will be shortened and less clearance will remain between the clutch as well as the compressor. This can create an interference and also hinder the rotation of the rotor assembly. Again, this problem is mostly linked to poor quality remanufacturers.

Symptoms of the preceding problems are-

  • Discoloration of Pulley and/or Armature Disc: 
    The extreme heat generated from slipping will quickly cause the part/parts to take on a discoloured appearance. They will appear “frosted”, or “blued”, or be charred.
  • Melting of Rubber Spacers in Armature: 
    If the slipping is still there, the temperature can get to a point where the rubber spacers between the armature plate and disc begin to melt. Not all clutches can utilize rubber spacers.
  • Melting of Epoxy in Field Coil: 
    The extreme heat can also affect the field coil, making the epoxy to show visual signs of melting or “charring”. Under extreme situations, the coil winding will be exposed and/or unseated.
  • Melting of Bearing Seal: 
    The heat will eventually lead to the bearing seal to melt, helping the grease to escape. Without grease to lubricate the bearing, it will be quickly destroyed.

INSTALLATION RELATED FAILURES

The following are the most common and frequent causes of installation related clutch failures. Please see that clutch failures resulting from improper installation are not warrantable.

– Loosen Field Coil Mounting Bolts 

If the bolts used to mount the field coil are not tightened adequately, vibration in the vehicle will cause the coil to become loose. Eventually, the field coil will start to contact the rotor assembly, as evidenced by rubbing on the field core outer diameter. This rubbing might generate heat that melts the field coil epoxy, leading to an open circuit and inoperative clutch. This condition is limited to CCI/York/Tecumseh style compressors. The replacement clutches for these type of compressors are now usually supplied with special Loctite treated fasteners. Use of these fasteners normally solve such a problem.

– Short or Open Circuit in Field Coil Wire 

If the clutch leading wire is routed in such a way as to subject it to possible rubbing or cutting by another object, the wire’s insulation can be rubbed through and the bare wire exposed. A short circuit or open circuit will mostly develop, causing the clutch to become inoperative. Such a failure is evidenced by a lead wire worn down to bare metal in one or more locations along its length, or a lead wire that gets cut in half.

– Shaft Key not Seated Properly in Armature Hub 

If the key is not properly inserted into the keyway on the compressor input shaft when a clutch installation takes place, a severe misalignment of the pulley relative to the shaft will take place. This misalignment will create interference between the rotor as well as field coil assemblies. Evidence of severe rubbing of the pulley on the field coil will be noticed.

– Improperly Installed or Wrong Snap Ring 

Use of a wrong snap ring, or an improperly installed snap ring on the field coil or pulley can result in the clutch being noisy, fail to disengage, or in some cases come off. Resultant clutch damage will follow. This is especially true for some Ford FS6 compressor clutches, where different types and thicknesses of snap rings were originally used. Consult your manual for further instructions.

– Mismatched Clutch Components 

Especially in the case of Ford FS6 and FX15/FS10 type clutches, complete replacement of the clutch and coil assembly is needed. Original or replacement Ford O.E. clutch components cannot be mixed with aftermarket clutch components type (such as a Ford O.E. coil and an aftermarket pulley and hub). Mixing these different types of components together can result in clutch failure and/or faulty clutch operation.

– Improperly Installed Coil Ground Wire 

On CCI/York/Tecumseh style field coils, the ground wire terminal needs to be installed on top of the coil mounting flange. If the terminal is not properly installed underneath the flange, the alignment of the outer coil and inner pulley interface will be adversely affected and create the potential for rubbing between the rotor and coil assemblies.

– Use of Excessive Torque on Compressor Shaft Mounting Bolt Broken or twisted bolts

These are the result of excessive torque applied during the process of installation, such as from use of an impact wrench. Check manufacturer’s torque specifications while installing clutch hardware.


Comparison of New Clutch and
Clutch With Failed Rotor
Assembly Bearing

Field Coil with Failed Epoxy Bond
(Note Gap Around O.D. of Epoxy Ring)

Three Field Coils in Different Stages of Failure Due to Extreme Heat

Wear on O.D. of field Coil Assembly on Clutch with Elongated Field Coil Mounting Flange Holes

AC Clutch Replacement in Five Steps

Step 1: Buy a New Compressor

To replace the electric clutch in any air conditioning system, the whole air compressor has to be replaced. Find out what kind of air compressors are suitable for your vehicle and buy a replacement.

Step 2: Get Access to the Compressor

Open the hood and allow the engine to cool completely before starting to work on the car. Disconnect the battery by first removing the black negative cable and then the red positive cable. Just identify the air compressor; the clutch is found inside of it.

Step 3: Remove R134-a From the System

R134-a is a chemical which is similar to Freon that must be removed from the air conditioner system before the hoses can be unhooked. It is dangerous if touched or if it is inhaled, so you will have to use a refrigerant recovery machine to get it out. If you don’t have access to such a machine then take your car to an air conditioning mechanic to have the R134-a removed.

Step 4: Remove Air Compressor

Unhook any hoses attached to the air conditioning or with the heating system, as well as the wire that leads to the electromagnet in the AC clutch. If there are any other types of restraints that keep the air compressor in place, remove them. Take out the compressor to inspect it for any damage.

Step 5: Install the New Clutch

Place the new compressor in the same place as the old one, and make sure that the friction plates are spaced correctly. Reconnect the hoses and wires in the same way that they were attached to the old compressor. Reconnect the car battery and start the car to ensure the new clutch is working properly. When you turn the A/C on, the fan behind the grate should be blowing and the friction plates needs to be pressed together and turning.

AC clutch replacement is quite an easy process, and it costs a lot to have it done by a professional, so it is a good idea to save money by installing your new air conditioning clutch by yourself. A worn out AC clutch can have a noticeable negative impact on the life as well as on the efficiency of your vehicle’s entire air conditioning system.

Video of how to replace AC compressor clutch

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