Auto A/C Troubleshooting 1

Auto A/C Troubleshooting 1

How to diagnose your car's air conditioning problems

Article updated October 2011


When your car air conditioning goes on the fritz, the driver’s discomfort is twofold. You’re not only sweating bullets, but also sweating hefty repair bills. If you’ve got some basic understanding of your air conditioning system, though, you can stay cool.

Normally, even though it's stifling-hot inside your vehicle, you know that relief is just a twist of a key, a push of a switch, and a few blocks of driving away. Except that this time, something is amiss: Only hot air is coming out of the dash vents—or no air at all. So now, besides the discomfort you're feeling, you're also anxious over the thought of a visit to the repair shop.

Here are some tips for diagnosing the problem yourself.

Even when performed by conscientious professionals, air conditioning work can be quite expensive, and much more costly if those professionals are less than scrupulous. Before your heat-stroked brain overwhelms you with worry, here are some simple tips for diagnosing the problem yourself-so you know whether the problem is with a simple switch or something major.

Diagnostic Tips

First, we should cover a few fundamentals. An automobile's air conditioning system can be divided into two basic sections: The Air Management System and the Refrigerant Cycling System. We'll cover the former in this article, and the latter in a separate story. Starting with the Air Management System, the first things to inspect are the system controls on your dashboard. Has anything changed about the way they're operating? There are a lot of different designs for both manual and automatic climate controls, but here are a few basic questions that can apply to most of them:

> Did you mistakenly hit the air-recirculation button instead of the A/C switch?

> Are all of the switches working correctly, offering the proper amount of resistance and pausing appropriately at the detent points (where the vent function or blower fan speed would change)?

> Are the appropriate pilot lights lit and graphics displays legible?

> Is the blower fan working? (There should be an accompanying sound coming from the center or passenger side under the dashboard.)

> Is the air being directed to the dash vents, or is it going somewhere else?

> Is the air temperature approximately what the ambient air temperature is, or is it considerably hotter?


Now, let's make some sense out of the answers to these questions. If the controls and switches do not feel as they did before the problem began (such as moving with no resistance, no detent points, excessive resistance, limited movement or no movement at all), or the graphics display is blank or blacked out, the problem is likely at the control panel. These can be fairly difficult to repair or replace, so it would be time to see a service professional.

If the controls and switches feel okay, but they are having no effect on blower fan operation whatsoever, check the appropriate fuses. They are usually under the dash and labeled, although with later-model cars, it may be necessary to consult at least the owner's manual or even a service guide (not a bad idea to have on hand) to determine the location of the fuse box and identify the individual fuses.

Physically remove the related fuses with the tool that is usually attached to the fuse box cover or inserted into a slot in the fuse box itself. Blown fuses are normally pretty obvious; but, if in doubt, replace them with a fuse of the same rating. If the fuse continues to blow, you probably have a circuit problem, so resist the urge to install a fuse of a higher amperage rating. If it's the blower fan fuse that continues to blow, there's a good chance that the blower motor has failed. Consult your service manual for repair options, as some blower motors can be fairly easy to replace, especially on older cars. If the blower fan only operates on the highest speed setting, or is missing one or more speed settings below that, most likely the blower resistor has failed. Again, consult your manual for repair options.

Regarding the directional and temperature controls: Most manufacturers are using a combination of sub-systems to accomplish proper function of the entire system. These are 1) an electrical system, 2) a cable system, 3) a vacuum system, and 4) an electronics/computer-controlled system. Your service manual will indicate which ones your vehicle is using.

Basic Tips

Here are some basic tips that do apply to most vehicles: If the fan is working, but the air is going only to the defroster vents (top of dash at windshield base), possibly even being cold at the outlet, there is likely a problem with the vacuum-system supply line. It conducts engine vacuum (usually via a vacuum reservoir) to the ventilation, and often, the temperature controls. Look for a small-diameter black plastic tube routed through the engine compartment firewall near where the refrigerant lines are also routed through. It will eventually lead to the vacuum source-either a reservoir (plastic bottle or metal can) or a manifold. Inspect the tubing and the rubber end connectors for damage, including the one supplying the reservoir.

If the directional controls are working properly, but the output air is lukewarm or hot, check the heater valve, if your vehicle is so equipped. This valve is usually located in-line in one of the two heater hoses, which also pass through the firewall, often in the general vicinity of the refrigerant lines.

With the engine warmed-up and running, and the A/C controls set to max, locate the two heater hoses where they pass through the firewall, and determine if they are both hot to the touch. One should be noticeably cooler than the other, indicating that the valve is closed to coolant flow in order for the cabin air to be at its coolest. If they are both hot, you probably have a problem with the valve control or the valve itself. When the output air at the dash vents is still not cold, either a fault lies within the air blend system under the dash (repair pro needed), or in the refrigerant cycling system, which should be inspected under the hood. We'll cover tips on diagnosing that system in Part 2.

Troubleshooting your car air conditioning is the easy part. Hopefully, identifying the problem with you’re A/C system will make the repair easier as well.

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