OK, I know the answer is "call an HVAC guy". However, I am wondering if anyone knows what the temperature difference between the home a/c refrigerant lines should be. I have a multimeter with a thermocouple. Should I be able to get a WAG from it? Should there be a 20-deg F drop? 30-deg F drop? Any ideas? Thanks.

According to http://hvac-talk.com/vbb/showthread.php?145014-Return-and-supply-temperature-difference it should be between 17-20 degrees F. But I don't know for myself.

As I recall, 15-20 degree F. was what you want, but check it after the system has been running for a while, i.e., don't check it when you are starting from a hot and humid house.

The reason is that at first the system's cooling coil dehumidifies the air, soaking up heat from the water that is condensing on the coil, and the air flowing over the coils doesn't get cooled as much. After the air has cooled a bit and the worst part of the humidity is gone, then you will get a more accurate reading.

Also, (if you can access it) check the air temp. just before it enters the unit, and then just after it exits off the cooling coil. Sometimes on home whole house heating/air conditioning systems, you may find small holes (may be in the duct or the fabric anti-vibration break); these would have been put there in the past to make this very check.

It is not advisable to try to check the pressure in the compressor side, even if you have the right tools and gauges, as each time you do this, you lose some of the gas (and slightly lower the pressure, and thus the ability to cool).

Also, if you think you are not getting enough cooling, make sure the system filter is clean (I buy the cheapest ones and change them every 30-60 days). The purpose of the filter is not to clean the air in the house, but to keep the fan and cooling coil from getting clogged with dust, lint etc. If the fan and/or the coil get clogged, the system will not cool properly, will run more and use more energy, and ultimately require expensive repairs.

I hope this helps----five years working summers in HVAC to pay for college taught me at least something about HVAC

chaosmagnet and bws48: Thanks for the quick responses. Also, I did a visual inspection of the compressor/coil/fan housing. A zip-tie or whatever they used to hold the flexible metal conduit failed. The cord is now lying across the fan blades preventing them from turning. Time to flip the main panel breaker and outside breaker and make it better.

My initial question wasn't that clear regarding the thermocouple. Clarification: Could I measure the surface temperature of the copper lines that enter and leave the unit in the house? Would that be a different general indication than the air temperature readings through the pinholes?

Again, thanks a ton.

I don't think so; the gas going in only cools when it expands in the cooling coil, and then picks up heat in the coil itself, cooling the air. There may be some, or almost no difference in the gas temperature before and after it goes into the coil. The general test a repairman would look at when checking the gas would be the pressure difference between the two sides. For that, you need the right tools and gauges and the specs for the unit. Even then, this would only show the compressor was not the problem---you could still not be getting proper cooling due to other problems.

What you really need to know is what is happening as the air passes over the cooling coil itself, and the test for that is the temperature drop of the air before and after it passes over the coil.

Thanks bws48. I did find the pinholes on the unit. I will wait until Wednesday to check the system. We are expecting temps near 90 here. That should give what I need. Again, thanks a ton.

Only 2c I can add is to check the switch on the compressor outside. Ours had so many ants in it that it failed and needed replacing. We now regularly spray bug spray around the electrical area.

I've been studying on my unit recently because I think the compressor motor starter windings have failed.
We'll be getting a new compressor under warranty shortly.

Some observations- when all is running the air blowing out of the outside unit should be a lot hotter than ambient. That's just sticking your hand in the airflow. (when my compressor didn't start the fan would run but without compressor there's no extra heat)
In the house the copper line to the evap coils is very warm to the touch but not too hot to touch. Cool air (but not cold) comes out the ducts.