When you turn the selector to the defrost, the A/C light and compressor will turn on to help with the defrost/defog the inside of the windshield. Having the A/C on while trying to defrost the outside of the windshield does not help.
For the 01-03 Elantra XD's it's a simple modification by disconnecting(cutting) one wire. This will not affect the normal usage of the Air Conditioner or the defroster. If you do require the A/C to be on, you can simply push the button to activate it.
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it's very easy: what you have to do is remove the dash fascia (four screws and some clips...there is an "easy dash removal" DIY for this on the DIY page). then remove the four screws holding the HVAC control cluster in place. there are several wiring harness connectors plugged into the back. you need to remove connector M20 and M22, indicated in the diagram. M20 is the A/C control module connector and M22 is the location (vent, floor, defrost, etc) switch connector. there is an orange-striped black wire on M20 pin 14 and M22 pin 8. this wire is just a loop, which generates a ground signal when the location switch is set to defrost or floor/defrost. the ground signal is sent to the A/C control switch and triggers the compressor to switch on. this is called the "defogger mode logic" circuit. the A/C button toggles the same ground signal, and overrides the "defogger mode logic" circuit.
all you have to do is cut the defogger mode logic wire (circled in red in my diagram)! the A/C button will still work as normal.
all you have to do is cut the defogger mode logic wire (circled in red in my diagram)! the A/C button will still work as normal.