Air Conditioning problem
1999 Isuzu Rodeo Question 6 cyl Four Wheel Drive Manual
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Every time I turn my defrost on, it turns on my a/c. My gas mileage is terrible when it's on. Why does it do this?

This is typical of many cars. The A/C unit is meant to dry the air before it gets blown onto the windshield. Nothing worse than blowing damp air onto a frozen window. My advice is to only use it when needed, otherwise use the "heat". The alternative is to disconnect the power wires to the clutch so it cannot engage. And just plug it back in come summer. Ba dthing about that is even during the winter you are supposed to cycle your A/C at least once a month to keep it oiled.