Hello,

The thermostate is set for 60 degrees F, inside and outside temp is well above 80 degrees F. The air conditioner turned off. About 30 minutes later, turned back on. Current surge is about 20 amps, then quickly backs down to about 7 amps. Blows cold air, noticably cold not just cool. After about 5 minutes, current starts to creep up little by little, to about 9 amps, then the compessor shuts off but fan still runs, current drops to about 0.6 amps. About 2 minutes later, compressor turns back on, blows cold air again. Surge was again about 20 amps, then back down to about 7 amps. After about 5 minutes current creeps up little by little over about 5 minutes to about 10 amps, compressor shuts off, fan still runs, about 0.6 amps current draw again.

The above keeps repeating.

What do you think is bad?

 

1) Read the label on the compressor for RLA (Running Load Amps).
If not exceeding, look for a bad current safety limit device. One of them is in the windings and almost never fails. The other one will be under the plastic cap on the compressor wires, if it has one. While you're in there, look for wire corrosion, a common problem.
If exceed, slowly rising pressures (and therefore current) are caused by failure of the condensing fan air flow.
Dirty condensor?

 

Sounds like it's overheating due to inadequate condenser cooling as @#12 has noted. Make sure the outside air is blowing too. If the blades have come loose from the shaft or if the venting has become clogged with something, the condenser coil won't get adequate cooling.

 

This is going to take a while. I just did a calculation showing that doing the job takes about 500 times as long as thinking about the job.
That's why it's called, "work".

 

Hello again,

The unit was cleaned out inside before the test as noted previously.
Coils look clean, but i noticed the evaporator coil has very very narrow slots so anything can block the air. You think that could cause a rising current like that?
The current must be too high as it is only a 5000BTU unit, about 5 years old.
Didnt see a label on the compressor though, as it was black.
It was a cheap model too.
It's been a while since i looked at air conditioners so i forgot a lot of stuff.

 

5000BTU unit, about 5 years old.

Should be about 4 amps on the compressor.
There is Freon flow. This is about accumulating too much energy (enthalpy) in the Freon, and that is only caused by lack of condensor air flow. Lack of evap air flow causes the enthalpy to decrease and thus all pressures decrease. If there was a freon flow blockage, the freon would quit flowing and that results in decreased pressures at all places in the system. Check the fan blade. Clean that thing again, and I mean pull it out of the wall and hose it. I have never seen an increasing pressure system that wasn't an air flow problem.

 

Hi,

Here is a small diagram of the system.

So you are saying that this can not happen if the evaporator fins are clogged and wont let much air through?

Everything else appears to be clean, including the fan blades.

Any way to tell for sure if the compressor is bad?

 

So you are saying that this can not happen if the evaporator fins are clogged and wont let much air through?

Too much enthalpy can not be cause by a failure to absorb more heat.
If the cold coil can't get air flow, it can't absorb heat.
(that means, "yes".)

Any way to tell for sure if the compressor is bad?

If everything else is working, the only thing left is the compressor.
The compressor is a constant volume pump running at a constant RPM locked to the power line frequency. The horsepower required to run the pump is proportional to the mass of the gas entering the intake port. If you pinched off a freon tube, the compressor would eventually arrive at nothing to enter the intake port and the current would diminish toward zero. If the machine can't get rid of heat, the gas pressures go up and up. The amount of mass entering the pump at high pressure is more than the amount of mass entering the pump when the pressures are low. High temperatures = denser gas = higher horsepower = higher current required.

I am assuming there is only one fan motor because that's how cheap 5000 BTU air conditioners are made. Therefore, the fan speed is the same for the cold side and the hot side. If the hot side fan blade isn't slipping and nothing is blocking the air flow, it should all work. If you come to the conclusion that it must be a bad compressor, you will have to remove the machine to replace it. After you remove the machine, try washing the hot coil and see if that fixes the problem. What do you have to lose by washing the hot coil?

In fact, a diagnostic method would be to flood the hot coil with hose water while it's running and measure the compressor current. If the water makes the current reduce, you have proved you have an air flow problem.

 

Too much enthalpy can not be cause by a failure to absorb more heat.
If the cold coil can't get air flow, it can't absorb heat.
(that means, "yes".)

If everything else is working, the only thing left is the compressor.
The compressor is a constant volume pump running at a constant RPM locked to the power line frequency. The horsepower required to run the pump is proportional to the mass of the gas entering the intake port. If you pinched off a freon tube, the compressor would eventually arrive at nothing to enter the intake port and the current would diminish toward zero. If the machine can't get rid of heat, the gas pressures go up and up. The amount of mass entering the pump at high pressure is more than the amount of mass entering the pump when the pressures are low. High temperatures = denser gas = higher horsepower = higher current required.

I am assuming there is only one fan motor because that's how cheap 5000 BTU air conditioners are made. Therefore, the fan speed is the same for the cold side and the hot side. If the hot side fan blade isn't slipping and nothing is blocking the air flow, it should all work. If you come to the conclusion that it must be a bad compressor, you will have to remove the machine to replace it. After you remove the machine, try washing the hot coil and see if that fixes the problem. What do you have to lose by washing the hot coil?

In fact, a diagnostic method would be to flood the hot coil with hose water while it's running and measure the compressor current. If the water makes the current reduce, you have proved you have an air flow problem.

Hi again,

Oh ok that sounds interesting. I might have to try that.
I guess you studied air conditioning systems quite extensively?
I myself have only briefly done so and forgot mostly everything from way back when.

I do remember the equation of how the total power input to the AC unit can be lower than the actual BTU rating, unlike a space heater for example
I do remember the basic parts too but not too much theory, so it's very good to hear some of this. Thanks for the briefing.

Yes, i'll have to dig into it again and see if cleaning the other coil better helps.
The side facing the room is hard to clean because the spacing between the coil fins is so narrow and i dont have compressed air available anymore.

Yes, there is one fan motor but two fan blades. The outside one connects to the same shaft as the inside one, so they both turn at the same rate, but they are designed differently. I think the outside one is a cage blower type while the inside one is just a big four blade regular fan type. Blows pretty hard though, and you can feel the cold air when it comes out the front.

I thought of the compressor first because i had first hand experience with running a compressor for an AC unit all by itself a long time ago. I took an big AC compressor out of a Cadillac and connected it to a large motor and some piping and a tank to make an ordinary air compressor for paint spraying. Worked nice, except the compressor would need constant oiling. When it started to get dry you could hear the motor start to slow down. After oiling it went faster again. That made me think maybe the compressor was drying out inside or something.
I dont know what kind of compressors they use in home AC units, but in that car they used a set of dual acting pistons. Quite a work of art as well as engineering.
Do home AC unit compressors ever dry out inside?
I also wondered the same about home refrigerator compressors.
Also, do you know if either are piston or rotary vane or whatever?

 

Blows pretty hard though, and you can feel the cold air when it comes out the front.

You still haven't said anything about the hot exhaust air outside. Is it blowing hot air out? It should be at least as vigorous as the indoor airflow.

Your unit probably also has the slinger ring that throws condensate at and through the coil to help it cool. Is your unit throwing any water out?

 

Hi,

I'd have to check. It's not being used at the moment mostly because of the problem.

 

I guess you studied air conditioning systems quite extensively?

I've had my State Certificate to design air conditioners since 1985.
No, the compressors don't run out of oil unless there is a freon leak, and the freon will escape before the oil does.
The usual compressor types for residential use are pistons and the Copeland scroll compressor.
Now, would you like to check the air flow through the hot coil?
Would you like to do the water cooling test?
Would you please quit searching the cold coil for being the source of the problem?

 

Hi,

Ohhh, do i have too ???

I'll have to take the unit apart again to test it, that might take some time.
Thanks for the ideas.

 

Before you take anything apart, check the outdoor air flow and look to see where any condensate water is going.

 

Hi,

Ok thanks. It may be a few days before i get to look at it again as it is not really mine it's a friends AC unit.
It's also out of the window now so all the testing will have to be done on a table indoors.

 

It finally sunk in that you noted 7A for a 5000 BTU unit. My new 5000 BTU uses only 3-4A. Something's amiss.

It also occurred to me that, forgetting the amp draw, you might have low freon causing the evaporator to freeze up and shut down. Amp change just coincidental as the unit runs. Probably not, but easy to rule out.

 

it is not really mine it's a friends AC unit.

That explains the delay in getting it diagnosed.

 

I

That explains the delay in getting it diagnosed.

this is typical of a worn rotary compressor

 

Totally agree - my experience