Carrier 38BYC, vintage 1999. 4 ton. Got regular seasonal maintenance not long ago. Old but in decent shape. Last week, about 2PM, setpoint was 73 degrees as usual. Inside temp rose to 76 and we noticed it. Blower blowing warm air. Outside unit completely off. Turned the tstat up, went to get my tools. Checked the breaker, it was fine, cycled it anyway. After maybe 3-4 minutes, I ran the tstat back down then went outside to do some troubleshooting. This time the unit was running fine. It continued to do so. Then, a week later, at literally the same time, the SAME thing happened again. It's like it just didn't feel like turning on right then.. 5 minutes later it was fine.
Called my A/C guy. He said it's old and working hard, fitted it for a hard start kit. This is a highly recommended service person with a great reputation and we have used him for years. I'm an engineer and know a little and he doesn't BS the customer. Give you an idea of how well he takes care of his customers, he interrupted an install to come over and look at my balky unit, did the work, and didn't charge me!
Now, here we are 4 days later. Been working fine. Guess what, it's 2:30PM - was blowing warm air, outside unit off. Stayed like that for about 3 minutes. Went to get my tools, as I was walking over to the A/C it started up.
I do have to point out that the sun is in a position to shine directly on the unit for a few hours just before this time of day, if that matters.
The charge is ok, was checked recently. Once it starts the air is indeed nice and cold. Contactor was replaced last season. Connections are tight.
What the hell is going on?
-Bill

 

You will have to be at the unit to determine the problem when it's happening. You may have a condensor fan issue where it's slowing down or stopping.

Most outside units have a 3-5 minute delay circuit built in to protect the compressor.

 

Do you have a remote shutdown box on the unit, installed by the electric company? If so, that sounds like the high load time of the day when they cycle them.

 

There is no utility remote control, it can run whenever it wants for as long as it wants.
One of the strange things is that this is not the hottest time of day when it happens, around 2:30PM, the outside temp peaks several hours later. The other day we had a peak of 101 degrees at 5PM and company over so we cranked it down to 72 from our normal 74. It had no trouble keeping the house nice and cool though it was working hard. I would think if something was marginal it would perform worse at the time when it's cycling long and often, but I have no clue here.
All I've noticed is that the time it happens is always 2:30PM-3PM, and each time the sun has had a direct shot at it for a few hours - literally shining right at the A/C control panel (contactor, capacitor, board) cover, and it's just about to put the unit in shadow. My a/c guy is stumped.
Also, it works fine at all other times and when it does poop out it's one time, and a few minutes later it's working great. I do not know if it short cycled or if it just didn't start up. It's just out long enough for the temp to climb 4-5 degrees so we notice it. I go outside and it's not running. No hum, no buzz, just "normal off". Tstat is calling for cooling and the furnace blower is on pushing warm air out the registers. By the time I get my tools and multimeter and get out there to see what the deal is, it's running again..

 

You will have to be at the unit when it happens.... No other way to diagnose.