If the thermostat is correctly set to cool, and the unit won’t run, the first things to do to start troubleshooting this condenser problem would be to verify that the refrigerant pressure in the system is equivalent to ambient temperature, check the high- and low-pressure cut-outs, then verify that you have the correct line voltage at the condensing unit and correct control voltage to the thermostat.
Check the breaker, fuses, and disconnect.
If the breaker is tripped or a fuse is blown, your central air conditioning condenser problem may be a failed motor or compressor, or a shorted or grounded wire in the unit.
Turn off the power to the unit, and check the fan motor and compressor terminals for a grounded, shorted, or open winding, and check the wiring in the unit for burned, shorted, or grounded wires or connectors.
If the breaker, fuses, and disconnect are OK, open the condensing unit and verify that the main supply voltage to the unit is correct. If it isn’t, find out why and correct it.
If the main supply voltage is correct, verify that the control voltage to the thermostat is correct. If it isn’t, find out why and correct it.
Your central air conditioner condenser problem may be a failed transformer, or a blown control voltage fuse.
If the control voltage to the thermostat is correct, verify that it is reaching the thermostat. Your central air conditioning condenser problem may be a failed thermostat, or a wire in the thermostat circuit may be broken.
If control voltage is reaching the thermostat, verify that the thermostat contacts are closing when it’s turned on to cool. If they’re not, you’ve found your central air conditioning condenser problem.
If you verify that the thermostat is working correctly, turn it on to cool, and check the voltage reading at the compressor contactor. If you have control voltage at the compressor contactor coil, but the contactor does not energize, your central air conditioning condenser problem is a failed contactor.
If you don’t have control voltage at the contactor coil, take a voltage reading across the safeties. When you find a voltage reading across a safety, you’ve found the open one.
If the voltage monitor/phase protection safety is open, there may have been an intermittent problem, or the safety may have failed. If the present voltages are within the range of the safety’s settings and the safety is still open, it has failed. Verify whether or not it is a manual reset type safety.
If the high-pressure safety is open, check the condenser fan and blade, check the coil to see if it’s dirty or airflow is impeded some other way, and verify that the standing pressure is correct for the refrigerant in the system.
Measure the temperature of the condenser coil. The standing pressure on the high side should be equivalent to the temperature of the condensing coil. If it isn’t, your central air conditioning condenser problem may be that the system is contaminated with air and noncondensibles, and possibly even mixed refrigerants. If it’s contaminated, you’ll have to correct it. If you’re following up on someone else’s work, you never know what you’ll find.
If the low-pressure safety is open, check the low side pressure. If it is above 55 psi, an automatic reset safety should have already reset and closed. The safety may have failed, and this might have been your central air conditioning condenser problem.
If the oil pressure safety switch is open, check the oil level in the compressor. If it’s low, top it off to normal, and remember to keep an eye on the oil level when the unit runs again.
If the oil stayed in the system for some reason (maybe the unit was running unloaded for a long time), when it starts running at full load, the oil will come back to the compressor, and you’ll have to remove it to maintain the correct oil level in the compressor.
Another safety to check is the compressor protection module. It’s usually found next to the compressor terminals. There are several different types, and they’re designed to open the control circuit to protect the compressor from electrical failure. If this safety is open, find out why and correct it.
If the contactor pulls it but the compressor doesn’t run, verify that you have the correct voltage at the compressor terminals. If you have the correct voltage at the compressor terminals, the external overload hasn’t failed, and if the compressor won’t run, it has failed. If you don’t have the correct voltage at the compressor terminals, find out why and correct it.
If your central air conditioning condenser problem is that the condenser fan motor won’t run, first, push on the blade and see if the motor will even turn.
If it won’t turn, it’s seized and must be replaced. If it isn’t seized, check the control voltage on the relay that controls it.
If there is control voltage but the relay doesn’t energize and close, it’s failed. If there’s no control voltage, check the control circuit. There may be a failed low ambient control that is keeping the fan de-energized.
If the relay energizes, check the load side voltages and verify they are correct. Verify correct voltage is reaching the motor terminals.
If the correct voltage is reaching the fan motor terminals and it won’t run, turn off power and take a resistance reading on the windings. The fan motor may have failed.
If there’s a run capacitor, replace it and see if the motor will run.
If it does, take an amp draw on both leads on the capacitor. You should have an amp draw on both terminals of the run capacitor. If not, the run capacitor is bad, or the start winding has failed.
By the time you’ve checked all these components, if your central air conditioning condenser problem was electrical, you should have found it. Before you run the unit, check the evaporator coil, filter, blower, and ducting, just to verify that the system will run properly when you power it up.
Does your problem have something to do with the pressures, temperatures, and cooling capacity when the unit is running? Our System Evaluation Manual, availablehere, has a cycle diagram and explains the pressures and temperatures to look for.
You might also want to take a look at our seven-page Air Conditioning and Electrical Troubleshooting Chart, availablehere, which is reprinted from a U.S. military training manual.
Publication date:10/06/2008
