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posted: 8/5/2012 5:20 AM

Ice buildup can block air flow from A/C

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Q. I own a 2009 Mazda 3 with a 2.0 liter engine and mileage of 26,000. The air conditioning works fine under normal driving conditions. I recently took two long 600-mile trips in 90-degree weather. After about four hours of driving at highway speeds the cabin of the car started to get warm. The temperature gauge read in the usual range but I noticed the air was not blowing at full force from the vents.

The air was cold and the fan sounded as if it were at full speed but only a minimal amount of cold air was coming through. I tried adjusting the fan speed and I could hear the fan motor change speed but not the air flow. After stopping for lunch the A/C worked fine for another hour before the same thing happened. I repeatedly needed to turn off the air for a period of time to get the full fan speed blowing.

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After the first trip all fluids checked out fine. I let the car run for extended periods of time and the A/C operated normally until the same thing happened on the second trip. Any ideas on what would cause this? I am not sure I can duplicate the same condition at a repair shop.

A. It sounds like perhaps you were getting some ice buildup on the evaporator, reducing the air flow. When you stopped for lunch it melted and you were good to go for a while. You may want to have your shop perform an evacuate and recharge on the system to be sure it has the exact amount of refrigerant in it.

The combination of humidity and improper pressures could have caused the problem. Give that a try and let me know how it works out.

Q. My 1996 Sonoma four-cylinder does not start sometimes. It can start flawlessly for weeks, then not start. It can happen when the truck has been sitting, or if it was just running. There's no rhyme or reason for the pattern. When it doesn't start it may take only one more turn of the key, or five minutes worth of attempts.

I replaced the starter, since it had a few years on it. The battery is relatively new. The connections are all good and clean. One quirk I discovered was, when I tried starting it without engaging the clutch, it should not have turned over, but it did. So I replaced the neutral safety switch. No luck. My next move may be the ignition switch. Is there anything else to consider here?

A. This is a little tough to answer without doing some of my own testing but it seems as though you are on the right track. If there is any adjustment on the clutch lockout switch make sure it is correct. There should be a starter relay in or near the power center; make sure the connections are good and if they are, you may want to try a new relay. It is inexpensive and easy to change!

We have been tricked by bad new parts before, so next time the truck fails you might want to see if the starter is getting power. Since it is so intermittent, maybe you could run some kind of a test wire inside the auto with a light on it so you will know if the starter is getting a signal or not.

• Douglas Automotive is at 312 S. Hager Ave., Barrington, (847) 381-0454, and 123 Virginia Road, Crystal Lake, (815) 356-0440. For information, visit douglasautomotive.com. Send questions to underthehood@dailyherald.com.

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