Why does my car A/C only work around town but not on the highway?
June 13, 2008 5:58 AM   Subscribe

The A/C on my car ('92 Honda Prelude) works great from 0-40 mph. Above 40 mph, the ratio of cool to hot air moves decidedly in favor of hot. Any ideas (besides the obvious: take it to the shop).
posted by wheat to Travel & Transportation (6 answers total) 2 users marked this as a favorite
Sounds like the compressor isn't working at that speed. If it were a slipping belt, you'd hear that, so I'm guessing it's a bad clutch in the compressor.
posted by bricoleur at 6:45 AM on June 13, 2008 [1 favorite]

When's the last time you had the compressor charged?
posted by Guy_Inamonkeysuit at 7:10 AM on June 13, 2008

Beyond a certain throttle point, the a/c kicks *off* to give you more power at the wheels. I'd guess the sensor on the throttle is effed.
posted by notsnot at 7:14 AM on June 13, 2008

Best answer: Beyond a certain throttle point, the a/c kicks *off* to give you more power at the wheels. I'd guess the sensor on the throttle is effed.

This is my thought too, but I was wondering is it the compressor not coming back on once the engine rpm's level back off at cruising speed, or is there something in the engine computer that's sending a signal that the engine is still under stress even though rpm's have leveled off.
posted by Pollomacho at 7:18 AM on June 13, 2008

Honda A/C compressors do have a tendency to go bad over time. If it is the clutch then it's not something you can easily replace without swapping the entire compressor, and to do that you'll need a shop at very least to flush and replace the refrigerant. Perhaps get the shop to check your refrigerant levels first?
posted by holgate at 7:44 AM on June 13, 2008

Best answer: A '92 Prelude A/C system was originally an R12 refrigerant based system. It's quite possible, in a vehicle that old, that the system has since been changed over to R134a type refrigerant. If so, you may find a sticker on the compressor or evaporator that indicates that has been done.

What often happens in older A/C systems, particularly those that have been opened or changed over to newer refrigerant, is that the A/C filter, or the accumulator/drier has become plugged. The system can pass a small volume of refrigerant at low compressor speed, but at higher volumes, flow is impeded to the point that the compressor shuts off on low inlet pressure, or excessive system pressure indications. This happens as a result of sensors in your A/C system being tripped by low or excessive refrigerant pressure indications, caused by flow restrictions or coolant loss, as a means of saving your expensive compressor, evaporator, and hoses. Thus, it is worth having your filter or accumulator/drier changed, if you are having the system serviced.
posted by paulsc at 12:17 PM on June 13, 2008 [1 favorite]

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