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?

Couple thoughts.
One - A bad connection in thermostat. This may be internal and not on the terminal strip. Although I would make sure the terminals/wires are making good connection.

Two - The spliced wire to the outdoor unit. If you have a volt-ohm meter. Check the resistance of the wires to the outdoor unit. Secure power (Inside and outside) and connect the wires together on the outdoor unit. Then disconnect at the other end and measure the ohm reading. It should be low 0-1 to maybe 2 ohms resistance. If much higher, you will need a new pair of wires or find and fix the bad connections.

Three - There may be a bad connection to the medium speed winding. If you have a push together connector for the indoor fan motor wires, separate and check the male/female connections for clean and good connections. The medium winding on the motor may be failing,

Try the simple easy things first.
Ron

Thank you that good advice.

Because it was easy
I switched from the medium speed to high speed motor winding
but the motor quit again after a few hours run.
After stopping, it would immediately restart.

Shorted ends of various thermostat wires and measured loop resistance.
All seem reasonable ohms/foot compared to some new thermostat wires I have.

Measured the motor windings resistances.
High Speed winding 2.6 Ohms
Medium speed winding 3.6 Ohms.

Used a Biddle Megger set at 500 volts to check insulation strength to a ground at motor housing.
Both Windings read more than 1000 Mega-Ohms to ground.

I suspect either the “Fan Relay Contactor”
or the motor internal thermal protection switch.

A local supply house had a Packard PR380 Fan Relay for $12 (Made in China)
to replace the thirty seven year old Essex B12173-1 rated 12 full load amps.

I just installed the new Packard fan relay and the fan motor has run trouble free for one hour.

The 1/2 hp, 115 volt, Four speed, frame 48, 1074 rpm motor is marked “no longer available”
but I bought from Grainger a Century brand
3/4 hp four speed motor that shows the same shaft and exterior dimensions.

Glad you have troubleshot it to the relay or possibly the indoor fan motor. Although the winding to ground readings are very good, it could be the internal overload/over temp device as you stated. Hope it was just a relay. Save the motor for another time.
Ron

Looks like the new Fan Relay fixed the motor drop outs.
To my ear it sounds like the motor runs smoother now too.
Those 37 year old contacts must have been in rough shape.
I am tempted to cut the case open and look.

Do it. That’s how you learn. Post a pic or two hear for others.
Thanks,
Ron

The “made in China” Packard Stock No. PR380 Fan Relay
now has number 6 contact spade lug overheating.

The bright shiny metal of the #6 lug has turned blue from heat.
The spade #6 wiggles
but spades 1-5 are tight.

I originally thought the 36 year old female spade had a loose wire crimp that was causing heating,
but when I replaced the wire and female spade connector and started the unit back up,
my IR gun showed a hot connection developed again.
As I watched I saw small sparks shooting from behind the rivet
that secures the mount of of the male spade lug of number 6 contact to the relay body.

On the Packard relay box it says “Cross Reference 90-380”

Amazon.com told me that refers to a
“Made in USA” Emerson 90-380 Fan Relay with an Amazon cost of $8.47

The original relay from 36 years ago was a ESSEX B12173-1

Going to have to rely on electric heat overnight until Monday.
Luckily the lowest temperature predicted for tonight is 36 F.

The two Emerson 90 380 “Made in USA” Fan Relays I ordered came in from Amazon after a 3 day wait.

What was in the Amazon package were “Made in China” White-Rogers Fan Relays.

These White-Rogers Fan Relays did have copper rivets instead of steel at the Number 6 terminal that overheated on the Packard brand Made in China relay

I installed one this morning and at least for now the gas furnace is working again.