Can running the air conditioner actually reduce your braking power?

Dear Car Talk

Dear Car Talk | Oct 01, 2006

Dear Tom and Ray:

I have a lady friend I e-mail with on a regular basis. She lives in the Midwest and swears that running the air conditioner in her car "softens" her brakes. During this past summer's dreadful heat wave, she had to take a long trip to see her ailing mother, and didn't use the AC for fear she would lose her brakes on the interstate. She swears her dealer agrees that this is a problem with the vehicle. It's a Honda, relatively new. I find it difficult to believe that this is possible, and even more difficult to believe that the dealer would admit it and then let her drive off. Please advise. -- Caroll

TOM: Geez, what a terrible predicament for this lady's mother. On the one hand, she's ailing and would love to have a visit from her daughter. On the other hand, without AC in a heat wave, the daughter's going to arrive smelling like my brother's gym bag. Tough call.

RAY: Actually, this is a stumper, Caroll. I can't think of any way the air conditioner would hinder the operation of the brakes. So I'm going to have to assume that your friend and the dealer had a misunderstanding.

TOM: There are two possible ways the AC could affect the brakes in small ways. One is via the idle air control (IAC).

RAY: When a major accessory, like the air conditioner, is turned on, the car's computer signals the idle air control to boost up the idle speed. That's because the air conditioner takes a lot of power from the engine, and the boost in idle speed keeps the engine from stalling and running rough when you're stopped at idle.

TOM: Normally, the IAC just boosts the idle speed back up to where it was before you turned the air conditioner on, so you don't even notice it. But I suppose if it's malfunctioning, it could be boosting the idle speed too much. And that could make your lady friend FEEL like the car is harder to bring to a complete stop -- because the engine is running faster and trying to keep it moving.

RAY: But that would have absolutely no effect on the brakes themselves, and certainly no effect at highway speed.

TOM: The other possible explanation has to do with the fact that the air conditioner's compressor cycles on and off continuously. If you're stopped at a light with your foot planted on the brake pedal, you might feel the pedal drop a tiny bit when the compressor cycles off and increases the engine vacuum slightly.

RAY: But again, this is completely benign, and would have virtually no effect at all on the brakes' ability to stop the car.

TOM: So here's what you should do, Caroll. First, get her a case of Handi-Wipes and a 55-gallon drum of Secret antiperspirant. That should help her socially until the mechanical problem is addressed.

RAY: Then ask her to get some more information from the dealer. Or you might even want to talk to the mechanic yourself. Find out what he thinks she's experiencing. I'm guessing it's something benign, and that it wasn't explained well to her. I can't imagine a dealership diagnosing a dangerous brake problem and then letting a customer drive away in the car.

TOM: Once she understands that there's no danger, she'll relax and go back to using her air conditioning. But I'm guessing this was more a communication failure than a brake failure. Write back and let us know.

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