Lsmalley, most of us understand your exasperation with the A/C systems from 20 years ago, particularly if they've been converted to R134A. The photo below shows the drier/receiver with its high pressure switch. The pigtails come off the high pressure switch to activate the aux fans on low speed. If your fans do not engage at low speed, the most likely causes are: (1) low refrigerant pressure as I mentioned in my earlier post above; (2) bad pressure switch; (3) bad dropping resistor; or (4) bad fan relay. You know that the fan motors and wiring are good, you proved that when you disconnected the blue base temp sensor and got high fan speed. It's the same circuit for both fan speeds, except for the resistor and the relay. The pigtail jumpering test helps confirm the rest of the low fan circuit, except for the switch itself. The pigtails are normally covered by plastic sleeves (as shown in the photo), but yours may not have the sleeves through wear and aging. The test is to slide the sleeves back and touch the wires to each other (KOEO). That should trigger low fan. If no, there's an upstream problem, probably the relay or the resistor. If yes, it's likely the switch is toast.
