humming coming from outside unit

I noticed that half of my house was getting warn this evening and went to investigate. The air coming out of the vents int he ouse is not cold. The blower is on and is constantly running because of the high temperature (the house temp is above the stat setting). The blower will go off if I raise the stat temp setting.

The outdoor unit will not come on. When I went outside, I heard a humming noise coming from the outdoor unit. I pulled the breaker and the noise stopped. I put the breaker back in and turned the stat from cooling to off. The blower went off but the humming noise is still there. I pulled the breaker and will investigate further tomorrow. Any thoughts on were to start?

 

The humming noise could be the compressor contactor. You need to check to see if you have 240 volts at the outdoor unit. Check across the incoming power with your meter, not each leg to ground. If you have 240 volts going in, see if you have 240 volts on the load side of the contactor. Touch the compressor and fan motor bodies to see if they are hot or cool [thermally hot that is].

 

Pushed in the contactor and the compressor kicked on, but the fan stayed still. The humming seems to be coming from the fan .... that's probably the problem.

 

Yep, it sounds like you're on the right track.

 

OK, back from vacation and ready to tackle the problem. Right now, I here the thermo asking for cooling and the blower kicks on, but the contactor doesn't engage. One question regarding the fan though .... would a bad fan keep the contactor from working? That is, if there's a call for cooling from the thermostat, shouldn't the contactor work (i.e., engage) regardless of the fan condition? Is there a way to test this?

 

You have it right. The thermostat energizes the magnetic coil of the contactor. When the contactor engages. it starts both the fan motor and the compressor motor. It appears that you might have 2 problems.

On the contactor locate the wires that go to the magnetic coil. Set the thermostat to call for cooling and, with your voltmeter, check to see if there is 24 volts at the magnetic coil. Yes- replace the contactor. No- start tracing the low voltage cable back from the unit into the house. Often the cables get damaged by weedwackers, rodents, etc. If you need to replace, be sure to make a good sketch of where the wires go [or take photos] and swap the wires over one at time. Look on the label of the contactor; it will have an amperage rating like maybe 25 amps. The replacement wants to have an equal or larger rating but not lower. Example- if your contactor is 25 amps replace with a 25 or 30 but not a 20.

Fan motor- does it spin freely or are the bearings bad? Look at the capacitor, does it look bulged out? When the motor tries to start, what happens if you give the fan bllade a push?

Download this brochure, very informative.

Fasco - FHP Replacement Motors - Time Saving Motor Replacement Tips

 

Thanks for the help so far.

I am getting 24Vac to the contactor and it is pulling in. There is ~240Vac across T1 and T2 when the contactor pulls in. The compressor starts, but the fan doesn't. I gave the fan a nudge and it spins, but it doesn't seem like it is going at full speed (more like 1/3 to 1/2 speed, maybe). After a couple of minutes the compressor will stop and I'll have to reset the cutoff switch before it will start again.

I looked at the capacitors and felt some kind of oily residue on both of them. I replaced both capacitors and called for cooling. The contactor pulled in, the humming that twas there was gone, the compressor started, but the fan didn't. The fan would spin if I gave it a nudge.

I connected a new fan for giggles and no change. Is it the contactor? If so, why? There seems to be the right voltage coming out of it. Do I need to check the current?

 

what voltage is the fan motor getting? new fan? As in the same as in your unit or a random motor? Can you test the windings on the balky fan motor? See if any pair of wires from the motor is open? How does the motor spn by hand? Freely? Sometimes what happens inside a motor is the rotor spacers wear down and when the motor starts, the rotor is not properly positioned inside the stator's magnetic field.

 

will do. One other thing I didn't mention was that the motor blades will spin equally in the direction (cw or ccw) I nudge them.

 

What you are describing could be due to the motor capacitors on the compressor fan. The motor won't start, and when it does, it runs slowly. That can cause the compressor to trip.

Replace the start and run caps.
Doug

 

Thanks, Doug.

The way the unit was wired is as follows: There's a red line from T1 on the contactor to one capacitor's common. A black line goes from the this capacitor's common to the fan. A violet line goes from T2 on the contactor to the fan. Two brown lines come out of the fan (one with a white stripe) and go to separate terminals on the second (smaller) capacitor. Earlier today I noticed that both of these capacitors had some oily residue on them and I replaced both of them. Changing out these capacitors did not seem to make a difference.

 

The way the unit was wired is as follows: There's a red line from T1 on the contactor to the one capacitor common. A black line goes from the this capacitor's common to the fan. A violet line goes from T2 on the contactor to the fan. Two brown lines come out of the fan (one with a white stripe) and go to separate terminals on the second (smaller) capacitor.

The voltage across the brown terminals going to the smaller capacitor is 235Vac. The motor range says 208-230 ... is this the problem? The voltage across the violet and black lines is also 235 VAC.

On the old motor, there is continuity for the two brown lines and for each brown line with the violet one. On the new motor, there is continuity for the brown line with the white stripe and the violet and black line. There is ~ 65 ohms resistance between the two brown lines and the brown line paired with each the violet and black lines.

I don't know how to define spinning freely, but the old motor seems to spin as freely as the motor on my other AC unit.

 

One other thing .... I buckled the unit back up to end working on it for tonight (i.e., old fan, new caps,) and replaced the breaker. The motor started to hum again and it would turn at a very low speed if I nudged it, but there was no call for cooling and the contactor was not pulled in. Could this be some weird contactor problem? I think I'll replace that first thing tomorrow and see what happens.

Thoughts?

 

Ah

Corect me if I'm wrong, but the contactor has one set of contact points and the other side is a brass bar from top to bottom? Scratch the paint on the old motor down to bare metal, remove the leads and test each one [one at a time] to the spot you scratched on the motor body. I think the motor windings are grounded. There is no reading from black on the old motor to any other wire?

 

There was no continuity (and by that I mean no "beep" when I test it with my multimeter), but there was some resistance level less than 50 ohms IIRC. It was not an open circuit. If the old motor is grounded, then shouldn't a new motor solve the problem? It doesn't seem to. There is no humming with the new motor, but the fan won't start unless you nudge it (which was the case for the old motor). Could there be some other grounding issue? This all started right after one of our torrential monsoons in southern AZ.

Yes, you have it right on the contactor. There is a bar from top to bottom on one side and the other has the contact points. It just seems like the motor isn't getting enough juice to run. I thought the new caps would have fixed than, but they didn't.

 

50 ohms to the body of the motor from any lead is a bad motor. I'm thinking that maybe wiring a new capacitor to the old defective motor gomered the cap. We can't assume the fan capacitor is still good. Try installing the replacement motor with another new fan capacitor rated for the new motor.

 

On the new motor, I have two lines going into the motor body - one that connects to the T2 lead on the contactor (a violet wire) and one that connects to the common on the capacitor (a black wire). There are also two brwon lines that come out from the motor (one with a white stripe) and go to a separate smaller capacitor. Should any of the following wire combinations have any resistance?

violet - black
violet - brown with white stripe
violet - brown
black - brown with white stripe
black - brown
brown - brown with white stripe

Or am I messing this up? Should I be measuring the resistance from each lead to the motor case?

Thanks.

 

Measured the capacitance of the 5microF motor cap and it's definitely gomered .... I get no reading. The larger cap seems to be OK though. Gonna get a new motor can and try it.

 

Yep, the cap was the culprit. I bought a new one today, installed it witht he new fan, and everything works. It's good to have the entire house cool again ...

 

Success! Kudos for your perserverance and thanks for the feedback.