Compressor kicks on and off rapidly. I see from your user forum that this a common problem. Advice?

[CharlesTX]

I installed a Google Nest thermostat Oct 25 2024, it worked fine for a few days. Then it started turning the outdoor compressor unit on-off-on-off rapidly. This is UNACCEPTABLE. The tech specs page indicates: "Some systems, including heating-only, cooling-only, zone-controlled, and heat pump systems, require a C wire or the Nest Power Connector. Go to g.co/nest/cwire for details."  None of these apply in my case. Also reading in the community forum I see that even this drastic solution has only spotty success. Can you offer any workable options? Thanks. 

[CoolingWizard]

@CharlesTX sometimes this can be caused when the HVAC systems power transformer is no longer capable of providing a full current load for the systems being used. Now the nest thermostat only draws about 200 mA of current from that system sends and it is a class 2, 40VA transformer.

You need to get a clamp type amp meter and precisely measure the current draw on the R side of the transformer. 

AC Cooling Wizard

[CharlesTX]

Thanks! I greatly appreciate your advice. I'll see what I can turn up. 

[Charleshall76]

 Sounds to me like you got your reversing valve settings backwards or most likely your contactor is engaging rapidly due to a thermistor on your condenser if you have it set on auto mode and thinking it needs to run but then realize it it doesn't it's a balance point it's very fine we're talking two degrees at point offset. I would definitely have a take look at your refrigerant level and make sure your condenser coils are clean. Most likely refrigerant issue. If it's just a tad low that's all it's going to take for it to bounce back and forth. hvac Master mechanic and I deal with this all the time. The new thermostat has sensors that are more sensitive and our communicating to the unit outside appropriately causing it to balance due to refrigerant pressure not being correct especially if you have a headmaster or high or low pressure sensor. 

[CoolingWizard]

@CharlesTX , what @Charleshall76 is saying is you need an HVAC Master Mechanic to come diagnose the system and determine the root cause of this problem.  What he forgets is this system is indeed a heat pump, the thermistor initiates defrost cycle and does not turn the compressor off when it engages. Residential heat pump typically do not have an ambient air temperature thermistor.  Most residential systems have a high pressure switch and not a low pressure switch. Also, pressure switches don’t open and close rapidly. And in most of the country right now you have enough ambient temperature that you’re not likely to run into too much trouble if you turn on the air conditioner instead of the heater in the cold areas or turn on the heater instead of the air conditioner in the warmer areas of the country.
Again, if you have that problem and where the ambient temperature outdoors is not sufficient, it won’t make the system turn on and off on and off rapidly, and if as he suggestion is the contactor could be going bad. When contactors go bad, they go bad one of two ways;  the electrical contact pads get very pitted and carbonized, and the power just simply doesn’t go through it. So the system won’t turn on again,  it will not turn on and off rapidly . If the magnetic coil in the contactor is going bad, you’ll either have one of two conditions, you’ll have an open wire in which it won’t close the contact at all or you will get a direct short and the resistance goes to zero and then you get a blown fuse on your control board.  Again, contractors do not by themselves cause an on off on off rapidly condition. 
Here is a simple test you can perform to isolate the problem. Turn the power off to your air handler.  Get a metal paperclip, take the Nest display off carefully, pulling straight off; not at an angle. Remove the R wire the Y wire and the G wire. Slip the bare copper end of the wires into the paper clip.  Turn the power to the air handler back on. Does the heat pump cycle on-off-on rapidly still?

AC Cooling Wizard 

[Charleshall76]

Before going and messing with your air handler and playing with your transformer which is most likely a 240 volt to 24 volt step down that's not going to be the issue causing your unit to kick on and off your control board will control that mostly it's the thermostat sensing sending the message to the control board in the air handler calling for heat or cool making contact your bounce and your compressor jump back and forth.. check your refrigerant!