Leaving AC on

My wife always shuts the car off with the air conditioning still switched on. So, naturally, it comes on immediately when she restarts the engine. Is there any harm in this habit? Isnt’ it better to turn the unit off when you turn off the car? Does it matter?

No harm. It is totally a non-issue

Doesn’t matter at all.

The AC compressor is disengaged when the key is in the “start” position, so there’s no additional drag on the starter. Once the engine is running the compressor goes back to its normal on-and-off cycle.

Mine is pretty much never turned off, it’s controlled by the climate control system and runs (even with the heat) for dehumidification unless the ambient temperature is very low.

It’s not a problem.

On my car I have fully automatic climate control, but have been experiencing a problem with a faulty pressure sensor preventing the body computer from energizing the A/C relay. So I bypassed the relay and A/C control is done by a little red button I worked into my center console. I plugged the wires directly into where the power terminals of the relay would plug in the relay center. So I’m wondering, since my switch is just an incredibly long jumper wire with a switch, is it posible my A/C compressor still turns with the engine at starting? I’ve gotten into the habit of ALWAYS having the switch OFF before starting.


I would be more concerned about running the compressor without a low pressure switch. Replacing the compressor will be more expensive than replacing the switch.

It actually can cause mustiness. What happens is that the very cool vents shut off when you turn the car off, then the warm humid air condenses on them - -that’s a perfect growth medium for bacteria and mold. If you switch to vents about a minute before you park, the warm air will hit it, but the condensation will be blown away because the vents are still on.

Of course, the other solution is not to worry about it and if it starts smelling musty, turn the vents on full, bi-level and empty a can of Lysol into the intake vent (just in front of the windshield)

A low pressure control is only about $20 or so so the hack is not worth it! Call a local HVAC supply store and ask for the price of some pressure controls and the price for a new compressor. You’ll understand why pressure controls are highly recommended.

You can use adapters to connect the controls to the service nipples to allow installation without recovering the charge.

Forgot to mention that when you add in the pressure controls, also add a delay-on-break timer. Otherwise, the controls might short-cycle your compressor.

In the Gulf South you would have to run the blower on high for at least a half hour without the AC to dry it out.