Air conditioner comes on for 10 seconds or so and kicks back off again, any ideas?

[Mike]

Hi everyone.

I have a central air conditioning unit in my ranch-style home (full basement also). Yesterday the AC just kicked off randomly. I checked the thermostat and it seemed to be functioning ok (it's digital). I went downstairs to the power switch dedicated to the central air unit and turned it off, then back on again. The AC kicked back on, stayed on and started blowing cool air, then kicked back off again 2 minutes later.

I suspected there was an airflow problem of some type (filter, etc) but that doesn't appear to be case, at least not as far as I can tell.

Are there any other common causes for this type of behavior?

Thank you for any responses, Mike

[Eric]

Check the outside unit. Is the fan turning at normal speed? Anything blocking the airflow?

If the outside unit wasn't getting enough airflow, the high-pressure switch could cause it to cut out.

Also could be a problem with the compressor motor, start circuit, or capacitor, causing the thermal overload to trip.

Eric Law

[tnom]

The compressor may be drawing to much current and a bi-metallic safety switch is shutting it down or the cooling fan for the coil is not functioning and causing a over temp fault.

[Mike]

I'm not really an AC expert, so the items I see to check:

1) Fan on the compressor unit outside the house 2) Ensure nothing is blocking airflow in or around the outside unit 3) Compressor drawing too much current (any way to check this, I do have a multimeter)

[Joe]

You need an AC ammeter or the right adapter to handle 50 A more or less on your multimeter. HTH

Joe

[dpb]

Quite an adapter to put inline w/ a conventional multimeter... :)

In a practical sense, unless you have a clampon meter, no, there isn't an easy way for large currents...typical VOM's are able to handle only a few 10's of mA.

--

[Jon Danniken]

All you need is a length of wire (or a resistor) to use as a shunt, inserted in parallel with the load.

My cheapy Harbor Freight DMM measures up to 20A AC current, but most common meters usually only measure a few hundred mA of DC current.

Jon

[dpb]

Yeah, but...if needed to ask, that's not just a "just" w/ 50A...

--

[Moe Jones]

When you say that you turned off the power switch, are you talking about the

120-volt toggle switch?

If that is all you did, you had to have reset something and it could be a number of things. Your thermostat, safety control or control board. Your best bet is call out your HVAC company to have them check pout your system.

[Red]

Define "kicked off/kicked on". Do you mean the inside unit, the outside unit, or both. IIRC, all the talk about the compressor "kicking off" would be related to the outside unit only. If the inside blower was "kicking off", that would be a whole different problem than the compressor. At least that's how my residential unit works.

[Uncle Monster]

If your AC condensing unit is equipped with a low pressure cut off switch, it can make the unit cut on and off due to a low freon charge. Unless you know your way around HVAC systems and have the proper tools and equipment, you should call someone who does.

[8~{} Uncle Monster

[Telstra]

[Uncle Monster]

It's not unusual for me and my guys to add those controls to a contractor grade condensing unit. I also add a head pressure control to those AC systems installed in restaurants since the units are run year round. If there is no electronic thermostat with a built in anti-short cycle timer, an inexpensive timer module installed in the condensing unit can save an expensive compressor from those people who like to play with thermostats. It's often very difficult to convince some folks to spend money on preventive maintenance.

[8~{} Uncle Monster

[Mark Lloyd]

Some do have a 10A range (with a separate jack). I used to use that frequently to measure the current use by 120V appliances.

[David Combs]

Question: are the clamp-ons at all accurate, or are they (sometimes) GROSSLY wrong?

EG I got a radio shack 2part vom that had a plug-in clamp-on coming with it. (maybe $30 when I got it 3 or so yrs ago)

David

[DoubleG.Garcia]

If it was low on charge, why would it kick back on and blow cold air? If you want to be sure that it's not a control problem, remove the face of the thermostst and install a jumper between the R,G, and Y terminals. If the A.C. runs continually and cools effectively, replace the t-stat.

PS: Don't worry, it's only 24volts. Be careful if there is a "C or B" terminal that you don't touch them with the jumper as you could blow a transformer fuse.

[CJT]

Even the $12 ones at Harbor Freight seem to do a pretty good job.

[Mike S.]

A quality clamp on meter is VERY accurate, and they are used all the time by professionals to measure current. That's how large currents are measured. Here are some examples

(FlukeProducts) A $30 Radio Shack meter could be good or could be bad, but I wouldn't trust it personally.

[CJT]

It's easy enough to check one out with a known current or by comparison with an inline meter with a known-good calibration.