My 36 year old central air conditioning system has lately developed a problem.

When set to cool, everything starts normally,
but after a random number of minutes,
the Indoor air circulating blower motor unexpectantly stops,
but the outdoor compressor unit continues to run pumping R-22 to the now “no airflow” evaporator coil,
the LED also stays on for cooling at the ten year old Venstar T1010 digital single day programmable thermostat.

If I turn everything off,
restart the circulating blower motor using the Thermostat “Fan Always On” setting,
run In “fan only” long enough to de-ice the evaporator coil,
then energize “Cooling” again,
but with the “Fan Always On” setting still active,
the AC will run successfully this way for more than 2 days with no further Stoppages of the circulating blower motor.

But if I turn the “Fan Always On” setting off,
and try to run the AC with its normal Cooling setting that powers the fan up when cooling is called for by the thermostat temperature setting, random motor stoppages occur again.

The 24 volt AC contactor control circuit coil that powers the single phase 120 volt capacitor start multi-speed motor of the air circulating blower is part of the Amana natural gas furnace.
This contactor has a high speed motor lead that engages during Heating.
When Cooling is called for by the Thermostat another section of the contactor engages a Medium speed motor winding.

I know when “Fan Always On” setting is activated the Green thermostat wire should get 24 volts AC on it.

I know when Cooling setting is activated the Yellow wire at the thermostat should get 24 volts AC.

I know that in our unit the Yellow and White wires from the thermostat are connected inside the Gas Furnace to a Red/White pair that runs out to the outdoor AC unit. The low voltage wiring diagram for the 3 ton outdoor AC unit shows a “High Pressure Cut Out Switch” between the Y terminal and the 24 volt AC coil of the dual contactor that energizes both the 240 volt single phase compressor and condensor fan.


About ten years ago that Red/White pair running out to Outdoor AC unit had to be spliced,
but since the outdoor unit continues to run, I assume that pair is at least low enough resistance to engage the compressor and fan contactors out there.

Amps on the circulating blower motor medium speed lead is
5.1 amps with one clamp on amp meter and
5.7 amps with a second meter.
Full Load is 6.5 amps.
Ten year old 10 uF starting capacitor.
Ten year old half horsepower motor is marked as internally thermally protected.

What are ideas as to what is going wrong,
and what additional things should be checked?