Auto A/C Troubleshooting 2An underhood adventure around the refrigerant cycling system
When your car's air conditioning blows hot instead of cold, you should start by checking out the controls of the Air Management System on the dash, as covered in Part 1 of this article. If that doesn't do the trick, the next step is to look under the hood at the Refrigerant Cycling System. Before you do anything, though, take a look at a service manual for your car, not only for information on how the system works, but also specifics involving its pressure and temperature control features. The manual can tell you both procedures to follow and how far you can go with the basic testing that we're going to discuss. It will also inform you of the hazards and safety precautions when working on the system.
Since you can "see" a lot just by looking, start with a visual inspection of the refrigerant cycling components under the hood with the engine off. First, look at the compressor and its drive belt. Is the belt connected, sufficiently tight and in serviceable shape? Replacing a missing belt may be all that's required for a quick fix (and check the other belts while you're at it, too).
Inspect the refrigerant hoses for damage (don't be fooled by surface damage to the insulation that may be covering the hoses). Since a small amount of lubricating oil is contained within the system, check if any areas or connections are oily, with no other possible outside source. Oil found on any A/C component, or at any of the hose or line connections without an outside source, usually indicates refrigerant loss at those points, which may require replacing those components and a recharge and retest of the system.
How about the wiring to the compressor, any temperature or pressure switches, and condenser fan (if so equipped). It should be securely connected and free from damage. Also, makes sure it's not routed near to moving or hot components. Any damaged wiring will have to be repaired, and the cause corrected before re-testing the system.
Is there any indication of a catastrophic failure of the compressor, such as an off-center drive hub, or paint damage or rust indicating severe overheating? Is there oil on it that doesn't seem to have come from anywhere else? All of these are signs that you may need to install a new one. If the compressor appears to be okay, grab hold of its clutch hub (the round part that protrudes just forward of the belt pulley) and make sure it turns with only moderate resistance. On the other hand, if there is evidence of a complete failure of the compressor, well, that's about as bad as it gets. If the vehicle is much more than five or six years old, the proper way to address this problem is usually by replacing the entire refrigerant cycling system, which unfortunately is big-dollar job.
Run & Check
After you've done an initial inspection under the hood, and you still don't see an obvious cause for the problem, next proceed to do some engine-running inspections. With the engine running and the A/C controls on maximum, observe the operation of the compressor, and answer the following questions:
> Does the clutch hub that you previously turned by hand, engage and spin at belt speed?
> Does the belt, although sufficiently tight, appear to be slipping or squealing?
> Does it stay steadily engaged, or does it cycle on and off in approximately two- to three-second intervals? If it's steady, test the temperature of the hoses/lines near the connection to the compressor.
> Does the larger diameter one feel cold to the touch, and the one of smaller diameter feel hot, but not scalding (think safety here)?
> Does the condenser fan, or engine-driven fan operate with sufficient speed and volume?
After you've noted the answers to all these items, here's what they mean:
If the compressor is steadily engaged, and the sufficiently tight belt is squealing or slipping (with no presence of liquid on the belt), then likely there is a problem internally with the compressor (as long as no other components that may share the same belt are not causing this condition). Any fluid leaks onto the belt, or malfunctioning components on the same belt would obviously need to be repaired before making this determination, though.
If the compressor clutch does the engage-disengage thing, it usually indicates low system pressure, more often than not a result of an insufficient charge of refrigerant. This problem can occur over the course of several years and may only necessitate a visit to a mechanic for a recharge and inspection. If, however, you notice the aforementioned oil accumulations, which are a sign of a more rapid refrigerant loss, you'll have to discharge the system, repair the leaks, recharge and retest. Fairly routine repairs like these are less costly than an entire system replacement.
If the compressor clutch does not engage at all, here's a little tip that can help, although it can't be used on all vehicles (consult your manual to determine whether damage may occur to the computer if you perform the following test procedure).
Find the low-pressure sensing switch and bypass it with a jumper wire or modified paper clip of appropriate thickness (sometimes disconnecting the switch is the bypass procedure), and observe the compressor clutch for engagement. No engagement usually indicates a blown fuse, a circuit problem, or a compressor clutch problem. You probably know how to deal with the fuse, but the others will likely require a pro to repair.
On the other hand, after bypassing the switch, and you have clutch engagement, perform the informal hose/line temperature tests. If the larger hose is not cold at the compressor, or at any point traced back to the firewall, then there is either an internal problem with the compressor, or the system is completely discharged-and likely has a serious leak.
Now, if the hose is instead cold, and a quick check of the dash vent outlet temperature indicates that sufficiently cool air is now present, the odds are that the low-pressure switch has failed. Consult your manual to see if you can replace it without violating any environmental laws, and doing so doesn't require the charge/evacuate/recharge procedure to be performed by a pro. Lastly, if the hose is only somewhat cold, as is the air at the dash vent outlets, a recharge/retest is in order.
If all is well initially, but then the large hose gradually warms up and the small hose gets very hot, inspect the airflow through the condenser and coolant radiator. If the airflow is not vigorous, check for obstructions at the front of the condenser. Simple things, like an accumulation of leaves or other road debris (plastic grocery store bags are common culprits) can really wreak havoc on the airflow. An accompanying rise in the engine coolant temperature is typical with this kind of problem. Remove the debris with the engine off, being careful not to damage the cooling fins on the condenser.
When there are no obstructions at the condenser, suspect a worn or faulty condenser fan (slow or no rotation), or a malfunctioning control system or radiator fan clutch. As confirmation, you'll notice high engine coolant temperatures when the vehicle is stationary, but normal temperatures when the vehicle is moving steadily at 30 mph or more on a level road.
After you trace down the cause of your air conditioning's problem, you may not be able to fix it yourself, but at the very least, you now have a greater understanding of what to expect in the way of repairs. And good service professionals appreciate a customer's informed input, as that helps them get the job done more easily. Of course, if you're able to repair the problem yourself, you'll have the satisfaction of a job well done, not to mention saving some dough.