Nest 4th gen compressor engages for only a few seconds and stops.

[Cheezer12]

Yesterday I upgraded from 2nd gen Nest to 4th gen.  Prior to upgrade the system worked fine.

After upgrade when Nest calls for cooling,  the compressor comes on for a few seconds and stops.

My wiring is Y1, G, Rc, W1.   Power status is "good". Vin=36.7V.  Lin:300(Y). PS:Y-300, W-35. Battery Voltage=3.951V.

I tried every troubleshooting I could think of so far,  when I reconnected my old Nest the system works fine.

[Cheezer12]

I was impatient and decided to hook up the C wire.

Yup,  C wire was my issue.  2nd gen Nest worked fine without it but 4th gen seems to need it,  Google should tell customer that there is a 99% chance they will need C wire hooked up.

I put a multimeter measuring the 24V circuit and I noticed significant voltage drop every time thermostat calls for cooling.  I suspect that the voltage drop is causing the AC compressor contactor to open and AC compressor shuts off.  As soon as I hooked up the C wire, there is no voltage drop and AC compressor stays on.  Bottom line is you need C wire with 4th gen Nest.

[Cheezer12]

To add to my explanation I think the contactor switch coil is unable to keep the 240V compressor circuit closed due to significant voltage drop (down to 14V to 16V for a second) in the 24V control circuit.  Once the compressor shuts off, Nest starts the 2 minute start delay countdown and the cycle repeats.

[CoolingWizard]

@Cheezer12 , the problem is not the thermostat then. Your Class 2 power transformer should produce not more than 29VAC and not less than 19VAC.  It should rated for 40Volt Amps. 

AC Cooling Wizard 

[Cheezer12]

@CoolingWizard Thank you for your reply.  The transformer output is 27.2V so within the range that you specified and it worked just fine for last 7 years with Nest 2nd gen.  So why doesn't it work with 4th gen?  When I put back the 2nd gen everything worked fine again, also there is no significant voltage drop when thermostat engages cooling.   Seems to me it is the problem with 4th gen Nest not functioning properly without the C wire.

[CoolingWizard]

@Cheezer12, I have a Nest 4th generation in my home that I’m in right now in and it has no C wire and it had worked as designed the day installed it. The way the Nest works without a C wire in 90% of residential systems is that there is a path to the power Common through the Y wire when the Y wire is attached to the electrical contactor coil. The Nest only draws a maximum of 200 milliamps of current over this Y wire path. Therefore it cannot create a short circuit across this path.  

You stated that the voltage dropped when the AC was turned on therefore using current to energize the contactor coil. Did that voltage drop down to 14 VAC? That’s a sign that the transformer is not performing properly to maintain the voltage. The only other possibility for this is that there’s some electrical path that’s creating an electrical short that’s drowning down the voltage from the transformer. This would have to be a pretty significant short circuit to draw down that 40 VAC transformer. 

AC Cooling Wizard

[Cheezer12]

@CoolingWizard I do understand your explanation and theory of operation however in practice my 4th Gen Nest will simply not work correctly without C wire.  I went through the exercise of swapping my 2nd gen Nest with 4th Gen nest twice and with the same result.  The 2nd gen works without C wire and 4th gen will not.

To answer your question about voltage drop, it did drop to about 16V for a split second which was enough to disengage the contactor, however with my 2nd gen there is only a slight instantenous voltage dip to about 20V.  I don't know how to explain this behavior, all I know is that my 4th gen will not function without C wire and searching the web seems to indicate that there are a lot of 4th gen users with the same problem.  Anyway thank you for your reply and your help, I solved my issue with C wire and looking forward to using Nest 4th gen.