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1990 964 C2 A/C Freeze-up

Heating and Ventilation

Vehicle Information: Year: 1990; Body Type: Coupe;

Hers is the repair list so far on my A/C: I've Replaced hoses in engine bay.Replaced receiver dryer.Replaced expansion device.Used new o-rings purchased from dealer (bandits).Evacuated and leak checked system, no problem found.Refilled with R12 juast til sight glass cleared, using refrigerant froma new 30# cylinder I tucked away 10 years ago. Head pressure and suction pressures are reasonably within limits defined in Porsche manual.A/C vent discharge within tolorence defined in Porsche manual.Cools wonderfully for about 1 hour. After that, the dischargetemperature begins to rise and there really is no more cooling. I suspect the evaporator is freezing. This model has electronic temperature control and uses a RTD (resistance temperture device) for sensing evaporator temperature. I checked the sensor at the connector where it plugs into the climate control system on the dashboard - resistance at ambiant temperature(about 80 Degrees F on the day I checked it) is well within the boundries established in the Porsche manual, which reads:0 deg C (32 F) 28.8 - 36-4 kOhms25 deg C (77 F) 2.6 - 2.9 kOhmsI also checked both leads for grounding as specified in the manual, no grounding was found. I have not checked the ice point resistance for the sensor, I will do so by removing and placing the sensor in a glass filled with ice chips covered with water, a true 32F at sea level. I'm wondering if perhaps the sensor is placed too deep in the evaporator or perhaps the resistance is out of range at the ice point. Based on my understanding of the climate control system, I believe the compressor clutch should de-energize when the evaporator temperature reaches a point near freezing, allowing it to thaw before re-energizing the clutch.

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