Home AC unit problem... any ideas?

[dbot]

So a coworker is having an issue with his home AC unit. You can here the unit attempt to turn on so the thermostat is trying to activate it. However, you can here a loud buzzing noise like the unit is seized and the fan will not turn on, after a few seconds it turns off. It does this repeatedly trying to turn on and failing. This is also causing the unit to become hot.

I was thinking it might be a fan motor issue, or one of the lines running to the fan is toast.

Any ideas or anyone have the same issue?

 

[Sea Moose]

When he turns it on and it hums, might be the motor trying to start. Depending on the style / arrangement of the system, it might be a faulty capacitor.

I would tell your friend to isolate the system to avoid further damage and call a local hvac mechanic

 

[LTC8K6]

Yeah, start capacitor for the motor maybe...

 

[Sea Moose]

Or a mouse

 

[webdave]

I had a similar issue last year. It was the capacitor. Picked up a new one from a local store and replaced it and it fixed the problem. Part was only 10 bucks and very easy to replace. You should also check to make sure the fan turns freely if you try to spin it.

 

[Sea Moose]

I had a similar issue last year. It was the capacitor. Picked up a new one from a local store and replaced it and it fixed the problem. Part was only 10 bucks and very easy to replace. You should also check to make sure the fan turns freely if you try to spin it.

this could be true "siezed bearing" But usually if a motor is siezed it will blow a fuse or trip the circuit breaker.

 

[Rubycon]

this could be true "siezed bearing" But usually if a motor is siezed it will blow a fuse or trip the circuit breaker.

Lack of start electrically is the same as bearing failure. Motor will hum and overheat. Unit fuse protects entire unit from L1/L2 across the line fault or fault to ground including hermetic compressor wiring failure or winding short to ground. Even with compressor motor fail to start the klixxon snap disc will open before the branch fuse does.

The condenser fan motor has a much lower draw and won't open the fuse on locked rotor however it's also thermally protected. When this happens the motor will be very hot. (if you squirt it with water it will sizzle)

ALWAYS check the motor by turning the blades. If it's stiff then the bearings are bad. Another indicator of excessive bearing wear is play. If you yank on the fan and see the shaft move in an out 1/2" for example, your bearings are overly worn and the motor is going to need replacement sooner than later. If you can spin the motor and the blades turn several rotations before coming to a stop then it's a good chance all you need is a capacitor.

 

[Paperdoc]

Let's not forget that an Ac unit has TWO motors - one drives the fan, and the other is the compressor motor. The latter is usually sealed inside a steel case so you cannot reach its shaft to turn and check for bad bearings. But certainly do check the fan motor rotation.

BOTH motors may have capacitors mounted on their sides that are necessary to start up under load. Checking / replacing them is a good idea. But before doing that, observe what happens when the unit is trying to start. Does the fan hum and make the loud noise, or does it start up smoothly? Same question for the compressor motor - is it the item that is making the loud noise? These observations will guide you in the right direction for further work.

 

[Rubycon]

If it's cycling the high head pressure switch could be causing the unit to shut off. The fan should ALWAYS be running EXCEPT in the case with a heat pump during defrost cycle.

A faulty start capacitor on the compressor will cause the compressor to draw locked rotor amps (very high) and the klixxon will open in as little as three seconds.

 

[Linflas]

If this is the blower fan we are talking about and not the compressor fan I would go with the suggestions to replace the capacitor. This happened to our AC at the beginning of the summer and the capacitor fixed it. There is also a similar capacitor in the outside compressor unit that can cause a similar issue with the compressor.

 

[dbot]

Thanks for all the suggestions guys, the power supply cable going to the fan was getting caught up and stalling the fan.

 

[BUTCH1]

If it's cycling the high head pressure switch could be causing the unit to shut off. The fan should ALWAYS be running EXCEPT in the case with a heat pump during defrost cycle.

A faulty start capacitor on the compressor will cause the compressor to draw locked rotor amps (very high) and the klixxon will open in as little as three seconds.

Yup, when I replaced my cap a month ago the parts guy said he sells a crapload every summer, it's one of the most common failures, these caps live a hellish existence via heat high volts and vibration..