Hyundai Repair: 99 Hyundai Elantra air condition quits working, coolant temperature sensor, hyundai elantra

Question
My elantra a/c comes on and works great at first. After about 5 minutes run time the compressor quits running and the pressures are OK. If I remove the relay and reinstall it the clutch comes on and shuts off. I switched relays and the same thing. something is not allowing it to come on, where do I check next?

Answer
Your air conditioning compressor is controlled by the engine control module (ECM).  There are four basic reasons that the ECM will turn off the compressor:
1.  Very high engine load for the purpose of additional power.  (I presume this isn't it.)
2.  Engine coolant temperature too high (or perhaps defective coolant temperature sensor).
3.  Thermostatic switch in evaporator opens (indicating freezing evaporator).
4.  Pressure switch opens (indicating high side pressure less than 29PSI or more than 455PSI).

Probably the easiest thing to check is whether the engine is overheating.  If you have access to a scan tool, you can also check whether you have a diagnostic trouble code for the coolant temperature sensor and monitor the sensor's readings.

To check the thermostatic switch, unplug the pressure switch under the hood.  Under normal circumstances, power leaves the thermostatic switch and arrives at the pressure switch on the black/blue wire.  If you have no power on this wire, you should immediately suspect that the thermostatic switch has opened.  Note that if you have an improper amount of refrigerant in the system, this can cause the evaporator to begin to freeze, so the fact that the switch is open doesn't necessarily indicate the switch itself is defective.

To check the pressure switch, simply use an ohmmeter (or other continuity testing device) to verify it's closed.  If open, the pressure in the system is outside the specifications listed above or the switch is defective.  An alternate method of checking the pressure switch is to, when the compressor is not running, unplug the pressure switch and jumper the terminals on the connector together.  If this makes the compressor run, then you know the switch was open.