| I have a 1994TT with factory R134a refrigerant and auto a/c. Having ruled out, low refrigerant, a weak compressor clutch electromagnet, a malfunctioning Dual Pressure Switch, a bad Relay, a bad Fuse, a bad Ambient Temp Sensor and a bad a/c Computer, I have determined clutch cycling is being caused by the ECU disconnecting the ground to the relay coil. One more thing that shuts the compressor off is the application of full throttle but this is not in play here. The evaporator temperature is controlled by an expansion valve sporting an oil filled tube wrapped around the evaporator's outlet line. When it gets too cold (in order to prevent atmospheric water from freezing on the evaporator and blocking air flow and cabin cooling) contraction of this oil forces the refrigerant to be diverted towards the compressor elevating the temperature in the evaporator. Normal refrigerant flow through the evaporator resumes when temperatures rise and the oil expands. In addition to this system the car has another temperature sensing device to further protect the evaporator from freezing. Nissan calls it the Thermo Control Amp. From the 1994 FSM, page HA-14: "FREEZE PROTECTION
If evaporator coil temperature drops below a specified point, the TCA interrupts compressor operation. When evaporator coil temperature rises above the specification compressor operation will resume. This condition (compressor cycling) indicates a malfucntion in the system." Pin #46 (ECU connector), the wire from the Thermo Control Amplifier to the ECU, goes hot every time the compressor disconnects. The manual says on/off values are approximately 12 and 0 volts. I measured 8.4 and .22, not exactly "approximately" but non-the-less co-incidental. There is no doubt the cycling is being ordered by the TCA. This is supposed to happen at center grille discharge air temperatures 37 to 39F but it is doing it much warmer. Restart is at 39 to 43F. To fix my a/c I now have to either disable or replace the TCA if I can get to it. I have no idea where it is or how to get to it.
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