Troubleshooting Central Air

[Benjinkzoo]

I need some help troubleshooting my central air system. When I turn the thermostat down, the outside compressor no longer runs. The circuit breakers are all still on, so I'm assuming it has power, but haven't taken out the multimeter yet. Can someone give me a breif list of the steps that you would take to troubleshoot the potential problems? I have read on another board where someone suggested to check for a high pressure switch, but mine doesn't appear to have one.

Background info: Approx 3 years old, Goodman CPKE-18B. The filter on my air handler hadn't been changed in a while and perhaps the lack of air flow caused the lines to freeze over and cause some damage?

Also, does anyone know where to find a schematic for the above model?

[LazyPup]

there should be a schematic pasted on the inside of the compressor compartment cover.

To Troubleshoot:
Pull main power disconnect and Open the compressor compartment.

Visually examine all wires. Look for a wire disconnected or a burnt connection. DO NOT TOUCH THE WIRES- The capacitors can store a charge for a long time after the unit is off therefore you should never touch a bare wire or connection, even with the power off, until you have discharged the capacitors)

If all the wiring looks ok turn the main power disconnect on again.

Locate the main power contactor.

Measure the voltage from L1 to L2 on the contactor (should be approx. 250v but may vary a bit in different localities- a range of 220 to 250 is good)

visually check the contactor to see if the armature is pulled in and the contacts closed.

On most contactors there is a plastic tab on the top of the contactor armature that can be manually depressed to close the contacts for tesitng. BE EXTREMELY CAREFUL NOT TO TOUCH ANY METAL PARTS OF THE CONTACTOR.

Depress the contactor armature and the unit should start. If it starts when the armature is depressed, release the armature and locate the awg #18 low voltage signal wires from the thermostate at the base of the contactor. (Should be one on each side)

Measure the voltage on the signal terminals--should be 18 to 24v.
If there is no voltage the problem is in the Thermostat or wiring from thermostat to the unit.

If there is 18 to 24 volts across the contactor signal terminals and the armature is not pulled down, the contactor is bad.

If there is signal voltage and the armature is pulled down, measure the contactor output voltage from T1 to T2. Should be 220 to 250v. (should be the same as the applied voltage on L1 to L2).

If you get zero volts measure L1 to ground...should be 120 to 125v.
next measure L2 to ground..should also be 120 to 125v.

If one side shows voltage and not the other, pull the main disconnect, and measure main voltage again to insure the power is off.

Visually check the contacts. Look for burnt contacts or maybe a bug or some debris stuck in the contact preventing it from making contact.

If you find burnt contacts it is best to change the contactor.

If you find debris clean the contacts, turn power on again and try to start the unit.

The compressor and fan motors operate independant of each other, so if you are getting power to the unit, either the fan or compressor should start. The problem will then be in the circuit for the component that is not running.

If this does not locate your problem post again with additional info on where you found voltage and whether the fan or compressor started. we can then trouble shoot the compressor or fan.

[Benjinkzoo]

Great !! That is exactly what I was looking for.

I ended up finding a bug that was in the contactor. It wasn't apparent right away, but a continuity check from L1 to T1 was fine, but L2 to T2 wasn't continuous with either 24v applied, or manually pusing the contacts. I discharged the capacitor, disconnected all the wires, removed the contactor and then found what was left of a bug. I brought the unit inside to do a part number search to see if I could find a replacement unit, but since it's the Sunday before Labor Day, I didn't think I would be able to find one until at least Tuesday. As a last ditch effort, I called Lowes. The first guy didn't know what a contactor was and kept asking me if I wanted a breaker. He put me on hold to go find the "expert". During my time on hold, I took a small pocket knife and scraped the bug out and cleaned up the contacts as best I could. I then checked continuity across L2 and T2 (manually engaged) and found it worked great. I hung up on Lowes (never did find out if they have it), and reinstalled the contactor. I kept my fingers crossed, turned on the thremostat, and in a few seconds was (and still am) enjoying cold air.

It's so cool (no pun intended) when you can fix something yourself. Last night I had resigned myself to the fact that this repair was going to cost me some serious $$, but I ended up getting it done for free with your help. I can't thank you enough.

[LazyPup]

You are very welcome and thank you for the reply.

Now that you have the bug out you should have no problems, but as a followup, you may want to get an aerosol can of contact cleaner and spry the contact to remove and residue that you could not get out by scraping.

Bugs in a contactor is a very common problem. When contact make and break they produce a very small amount of ozone from the spark. For reasons unknown to me, ants and some other bugs seem to be drawn to the ozone. The get in between the open contacts, and when the contactor pulls in, the bug gets smashed, which prevents the contactor from closing properly. In those instances I often note on the service bill that i "debugged the control system" hehehehheheh.

[LazyPup]

One follow up concerning testing switches with a volt meter.

Voltage is a measurement of the difference in potential between any two points in a circuit.

A switch is simply a mechanical gap in a conductor.

WHEN A SWITCH IS ON there is a straight through path for current, therefore there is no difference in potential (The output voltage equals the input voltage) and although there is voltage on the circuit the voltmeter will indicate zero.

WHEN A SWITCH IS OFF- there is a minor physical gap in the current path, therefore the input side of the switch is energized, while the output side of the switch is at ground potential becuase it is connected to the load and ultimately to the ground. When a switch is off, if you measure the potential across a switch it will show the applied voltage.

Often people make the mistake of measuring the current across the poles of a switch, get a zero reading and assume the line is safe to work on. In that instance, if the meter says zero they erroneously assume the power is off.

When troubleshooting or servicing a circuit it is vitally important to measure the input and output side of a switch to a known point of ground. You must put the black lead of your voltmeter to the white wire or the bare wire in the circuit, while holding your Red probe to the point of voltage in question.

FOR YOU OWN PERSONAL SAFETY- GET A SINGLE PROBE VOLTAGE SENSOR.
I would strongly suggest that anyone who is not familiar with electrical trouble shooting with a voltmeter should also aquire a single probe voltage indicator. When a single probe voltage indicator is held against a wire or circuit component if that wire or component is energized an LED will flash and their is an audible signal to alert you there is power on that wire.

When servicing componets with metal enclosures or cabinets such as an AC condenser or furnace you should first touch the metal cabinet with the single prode detector to insure their is no short that is energizing the cabinet.(It is also a good idea to use the single probe sensor to chech for the presence of voltage on copper water lines before servicing them. A pump motor or electric heating element failure can energize the pipes and with the introduction of plastic water lines underground oftem water lines are not grounded.)

The single probe detectors do not measure the actual amount of energy, but they do alert you to the presence of voltage before touching the item in question.

Single probe type voltage sensors are about the size of a large ink pen, fit well in a shirt pocket and can save your life. I would not recommend anyone working on electrical systems without one.

Single probe detectors are relatively inexpensive ($20 or less) and can be purchased at any electrical supply house, hardware store or home center.

[imeduc]

That's why we tell people to bug spray around the outdoor unit now and then. Ants just love it in there. Write down the ### on the contactor and get a new one so you will have it when this goes out. Lowes and Home Depot dont have them . Try Johnstone Supply

ED

My mistakes dont define me they inform me.