Lets take a moment to see what exactly is inside the condenser (that's an outside unit) You have a compressor, a fan and motor, a contactor, a cooling coil, and a capacitor. How does it work? The condenser is connected to the house via an electrical box called a quick disconnect I call it yank out. It is a quick way to kill power to the unit.
Quick Disconnect Power box for condenser unit
Note: this is not the breaker to the unit. That would be in the main panel by the meter. People always assume this is the only safety. It is put there by law to kill the power to the unit for service personel. Pull out the connection to kill the power before checking the unit. The first thing to look for is a swollen capacitor
Failed cap. If it swells it fails and the unit don't function too well. The next thing to look for is contactor problems. Their are small wires coming from the inside control panel that power up the 24 volt coil that yanks down the contactor.
24 volt 2 pole 30 amp contactor with cover
That coil might be bad. If it has no continuity(open) it is in need of replacement. Pictured above (white) to the right is one side of the coil, on this model the other small wire goes on the other side. So check each side for continuity (notice one side has a double spade and is a pisser to the novice. A critter could get between the contacts and fry them as well. Look on the compressor for burnt wires and loose connections (very common). The fan motor should be very easy to turn so if it is stiff then it probably is in need of lubrication. The standard oil in the HVAC world is Zoomoil.
It is hi temp resistant, will not lose viscosity and is non flammable. If a weed eater should eat up the thermostat wires going outside to that contactor nothing will work because the contactor won't pull down and engage the compressor and fan. If the contactor's 24 volt coil is open then it won't pull down and engage the compressor and fan. Note: if you have no 24 volts coming from the house check the transformer. NOTE: the transformer is gonna always be inside the house with the air handler unit. It will either be a stand alone model or mounted piggy back on a fan center.
24 volt transformerIf a critter crawled under the contacts and burnt them all to hell nothing will happen. Notice the nice cover this late model contactor has. Most older units do not have that Of course if the compressor pins are shorted to the copper piping or there is a hole in it or it has an open winding between pins it is shot and that ain't a good thang. I gotta say this though. Todays new equipment uses environment friendly refrigerant that needs smaller sized copper to work properly so just opting for a new system is pissing in the wind or tossing money into a pit my friend. It pays to replace the compressor if the unit is a newer one with serviceable cooling coils. It the system is ancient and all rusted out yeah you might want to look into a new unit. Lennox has a solar power assisted central unit. It has a condenser motor that connects to a solar panel. A very high seer rating and comes with a humidity control system to boot.
This many folk have visited as of Oct 29, 2009 Thanks