Toyota Repair: 1992 camry airconditioner not cooling, rain storm, expansion valve

Question
QUESTION: 1992 camry 4 cyl with replaced compressor and coversion to new freon about 5yr ago when no cooling and blinking ac button.

Presently the ac would not cool at idle only moving at higher speeds.
I added small amount of freon to system to upper limits of the range of gauge that came with the freon can for the 98 degree temp in miami. this did not help in fact now only warm air blows.
No abnormal compressor noise. no blinking lite.
At this same time my tachometer works intermittently infact when there is a  rain storm the tach begins working and the speedomoter indicates 50 or 60 mph in park. after a few min. when the engine warms up the tach goes to 0 and the speedometer reads about 10 mph less than it should. Are these related?

ANSWER: I don't think these problems are related unless there is some electrical wire damage or short circuit somewhere,if the compressor turns on there may be a blockage in the system, this could be due to a stuck expansion valve ,clogged receiver drier or blocked condensor. You would need to connect a/c pressure gauges for further diagnosis.

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QUESTION: what would the gauge readings be to differentiate bad compressor from stuck expansion valve or clogged dryer or blocked condensor and can these be unblocked by adding some additive to the system?

thank you

Answer
Normal A/C pressures with the engine running and the compressor engaged are approximately 28-40psi on the low side and 125-225psi on the high side. If the compressor is not pumping freon the pressures would be about equal, if there is a restrction at the expansion valve it would be higher than normal on the low side(more than 180psi) with the high side the same or lower than normal. A restriction past the expansion valve would indicate a higher than normal high side reading (more than 250psi)this could be a receiver drier blockage. I know of no additive than can be added to cure this.