1992 Cadillac Deville AC malfunction

The system will default to econ in response to a fault, usually a low refrigerant type of fault. The compressor and dryer are not the only possibilities for leaks, in fact late 80's and early 90's Cadillacs were common for leaking evaporator cores. The evap core can be viewed, or tested through the blower power module access port, that is in the engine compartment in the center of the HVAC case, to the passenger side of the blower motor. I would recommend checking system running pressures, and if the low side is dipping to low too rapidly(below 10 psi), I would have the system recovered to check the weight of the charge. If it is significantly less than capacity(approx 1.8 lbs), I would suspect undercharge due to a leak. If it is close to, or at capacity, look for a sensor fault, or restriction. A restriction would be indicated by low side pressure lower than normal, hi side pressure higher than normal, with an adequate charge. A low side pressure sensor, or circuit fault, could falsely register a low charge condition as well.