AC not cycling on / off in auto mode

[Prodjbm]

Tempstar FEM4P3000AT air handler with heat strips.
AC model N4A330AKC200
Problem: On auto mode, once the unit cycles off completely, then AC condenser kicks back on, the air handler does not, leaving the AC compressor running without air handler running.
I can put Stat in "fan" mode and AC condenser will cycle on and off. This is how I've gotten through the past few days.
I have tried a different stat, same outcome.
What I have noticed through all this is, there have been times that I couldn't even get air blowing when in "Fan" mode and system "off" then click over to AC and compressor kicks on right away, Stat doesn't blink "cool" first before staying on solid. I just fund that strange because when it seems to operate correctly, cool would blink first, they stay on.
One time after shutting breakers off to AC and handler, then turning them back on trying to get "fan" to run, it took 12 minutes then fan started running.
Sorry for the long post, just trying to get all my observations out at one time.
Thank you

 

[WorthFlorida]

First do check the voltage at the red wire to the thermostat. It should read ~24volts ac. No load maybe as hight as 29v, under load it maybe as low as 18v.

Possibly you're low on refrigerant and the low pressure switch shuts things down by opening the 24v to the compressor relay. Explanes why the compressor shut off and the air handler fan still runs. The low pressure side (the suction side)) gets too low and shuts the compressor off. The high and low pressure sides balance out where the low pressure side get high enough to close the circuit and turn on the compressor, thus cycling.

Homes in the NE seem to run the AC for twenty years and never had the unit serviced. Gauges are needed to check the pressure. Now that it is cooler, the pressures drop. There is a chart on the compressor cover for the electrical connection. It will give the pressures vs ambient temperature.

The control board inside the air handler is possibly bad. Depending on the age of the air handler, old units worked almost directly off the thermostat. Newer units a control board is used. Low Seer units use usually a common off the shelf universal board.

When the fan did not turn on in "on" mode, all thermostats have a cyclic protection for the compressor. It might also keep the fan off during this time. It to allow the high pressure side to drop so the compressor can start up.

 

[Sponsor]

 

[WorthFlorida]

LPS and HPS

 

[Prodjbm]

Thanks guys.
The AC compressor runs without the handler blowing. So I guess that eliminates low pressure on AC.
My handler does not have a printed circuit board (PCB).
I had 28v at stat.
The 12 minute wait does make sense since things were on and off.
The search continues Lol

 

[Fitter30]

At the air handler measure voltage. If equipped door safety switch has to be depressed. Take a pic of wiring diagram.
R to C 24vac
R to G if thermostat is calling for blower 0 vac
C to G if thermostat is calling 24 vac

 

[WorthFlorida]

The fan motor can be iffy. Check the start capacitor, it should not be swollen. Usually when it’s bad the motor may run slow or not at all. While the compressor is running and not the fan, if you can, spin the blower drum and if it runs it’s a bad cap. As fitter suggest, by pass the safety door switch if there is one.

With the fan not running and the compressor is , you should see icing on the coils.

 

[Prodjbm]

I do not have a door safety switch and I do not see a fan capacitor on the air handler.
There are only 3 electrical pieces inside the cabinet. A transformer, rectifier control and 24v relay. I ordered the last 2. Will see what happens.

 

[Fitter30]

Sometimes capacitor is screwed to the blower housing with a strap.

 

[WorthFlorida]

I cannot find parts list for your air handler. It seems to be about 20 years old. Older units used a variable speed motor and it will have a cap. In about the last 15 years, most units use an ECM motor and these do not have an outboard start cap. ECM motors are 3-4 times more expensive than the traditional variable speed motors.

 

[Prodjbm]

UPDATE: After throwing some parts at it, 2 of the 3 that are in the air handler lol (relay and rectifier) transformer tested good. I decided to change the speed tap to high from medium. It has been working for 2 days. I thought maybe the A coil was freezing up from not enough air flow through it. So, it takes a while for my house to cool down in Central fla. and it seems like it can't get house below 74 that I can tell. Would the lack of getting temp down mean the compressor is low on freon?

 

[WorthFlorida]

Yes, probably low on refrigerant. Depending how long it's been since the pressures were checked, there could be leak on the low pressure side. With a sniffer tool, need to check the coil in the air handler. If the tech only adds refrigerant and does not check for leaks with the sniffer, it may leak out. The low pressure side it can be days to leak enough to not cool. The high pressure side it be gone in minutes.

 

[Fitter30]

Air temp check across the air handler would tell if its short of refrigerant with a pocket thermometer not a laser thermometer. Less than 18° across its short.

 

[WorthFlorida]

Low on refrigerant, the coils will usual ice over.