Roger's correct. The system is low on refrigerant. But it's the cycle switch up front on the receiver-dryer that detects the pressure and turns the compressor on and off. The ECM comes into play by kicking the RPM up when the compressor is energized. The system should hold about 32 oz. of R-12 refrigerant. Systems converted to R-134a don't require as much because of the higher expansion rate of R-134a. Only about 80% or 25 oz. R-134a is required.
My technique has been to slightly loosen or depress the schrader valve in the receiver-dryer and drain all the refrigerant, being careful not to drain so fast that the oil blows out. Then I replace the R-134a with 2 12 oz. cans. One can being the type with the stop leak included. I have a gauge on my fill line that lets me know if the system is over-pressurizing or failing to pressurize, indicating blockage or leaks. Just picked up 2 cans at Dollar General and charged the 86SE, yesterday.
