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My ac was making a high pitch noise that I could hear from…

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Hello, My ac was making...
Hello, My ac was making a high pitch noise that I could hear from inside. When I went outside to investigate the condenser fan was not turning. I turned the unit off and waited approx. 15 mins and restarted. The unit worked fine for a short while and then went back into the same mode, ie, compressor making a high pitch noise and condenser fan not turning. I have since replaced the fan motor and the unit worked fine for approx. two weeks and then back to the same condition. I replaced the dual capacitor (45uF/5uF) with no luck, same condition. I did notice that the cold line starts out cold and then sometime before the condenser fan stops the cold line warms up to approx. outside temp. right before the condenser fans stops. Does this sound familiar? Any suggestions would be appreciated.
Submitted: 7 months ago.Category: HVAC
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Answered in 33 minutes by:
9/30/2017
HVAC Technician: Phil, Mechanical Engineer replied 7 months ago
Phil
Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 9,371
Experience: Retired HVAC/ Electrical & Boiler contractor. Industrial
Verified

Hello, IF... that is a heat pump unit that cools in the summer and heats in the winter, it sounds like a bad defrost control board.

.

I need the full model number of the unit outside to go further.

.

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HVAC Technician: Phil, Mechanical Engineer replied 7 months ago

Thanks, the compressor and the condenser fan both get power from the SAME T1 and T2 terminal on the big relay (called a contactor) that sends power to both the compressor and fan motor at the same time.

Primary suspect is a burnt wire at either T1 or T2 on the contactor... inspect for that.

Regarding the high pitched sound. that is almost normal if the condenser fan is not running, it should make a high pitched sound with the condenser fan is running at the same time. However the unit should have a high pressure safety cut out switch that cuts off the compressor if the fan fails.

If you can take a close up. exceedingly well focused photo of the wiring diagram glued inside the units electrical controls box, I can mark it up with trouble shooting directions.

We can go from there

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Customer reply replied 7 months ago
Hi Phil, Please find attached a photo of the schematic for my ac. I noticed the picture was distorted when viewing within an email but was clear if I saved it on my computer and viewed it a jpeg viewer. I also found a sticker inside the control circuits that gives the model number and serial number for my unit. I am attaching a photo of that as well. I inspected the wires connected to the contactor and did not see anything obvious that looked burnt. The contactor button itself looked a little worn but not anything alarming. I'm thinking I will measure the voltage on the fan motor before and after stopping. I am an Electrical Engineer but I work in semiconductor testing where the power supply voltage I deal with ranges from 1.5 to 0.8V DC. I am very cautious when working around household level voltages(I am aware of how little current across the heart to start ventricular vibrillation) and I would be so embarrassed for my obituary to say EE killed by electrocution. :-)
HVAC Technician: Phil, Mechanical Engineer replied 7 months ago

Thanks, ***** ***** know how your tests go.

Here is a story for you, one of the guys working for my company about 40 years ago, wrapped the 18" long extension of my socket set in electrical tape, and taped the socket on the end, then reaching gingerly between the 480v 600 amp buss bars, reached the bolts holding the 600 amp 3 phase breaker in place , he was standing on a thick rubber mat with one hand in his rear pocket (prevents current through the heart he was told)... then when extracting the extension, there was a blinding flash, and all he got back was the completely vaporized stub end of the extension, no socket, another $3 down the drain, It didn't even blow the fuses at the pole transformer.

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Customer reply replied 7 months ago
HI Phil, I had a professor say during a lecture that he always has one hand in a pocket when working around hi voltage circuits. It stuck with me. I can't say I always do that but it made me think about the path current will take should I accidentally touch something hot. Too bad about the socket. I bet your guy had to go clean out his shorts after that. :-)
I made the measurements using my volt-ohm meter set to 300V ac setting: with power applied to the ac but before thermostat switched to cool measuring between the Fan terminal of the capacitor to the common terminal read 0V, same reading between Herm and common. After switching thermostat to cool, contactor closed and Vfc=220V, Vhc=340V (maybe higher since I am on 300V scale, but I don't have a higher setting) The unit ran with the condenser fan spinning as normal for approx. 15 mins. My upstairs unit, which is next to the one I am having trouble with, was running at the same time. Before the fan stopped running I noticed the warm line on the broken ac was much hotter than the working upstairs unit and the cold line from the broken unit never got cold enough to sweat. After about 15 minutes the fan stopped and the reading for Vfc was zero volts while the Vhc was still 340V. The contactor was solid closed.
HVAC Technician: Phil, Mechanical Engineer replied 7 months ago

Hello again, readings between herm and common a the capacitor will not produce a voltage read since they are in essentially the same line,

IF a capacitor is bad, the motor will attempt to start, get hot, then trip its internal overload... we check the amperage draw on the common going to the motor when it is first energized to determine if its attempting to start or not, it will draw its Locked Rotor Amperage (LRA) listed in the unit name plate.

If we get no amperage read on the common going to the motor in question when power is first applied, then we disconnect the wires to the motor and measure ohm reads between each of its windings, Common, Start and Run windings.

The ohm read between Start and Run will be the total of the reads between Comm and Start and Common and Run if the motor windings are good.

If we want to check the capacitor, we disconnect the wires to it, and use a multi meter with a microfarad scale to measure microfarads between comm and herm and common and fan. we check those reads against the ratings stamped on the side of the capacitor.

Good luck with it, stay in touch as needed.

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