A unit sitting on the roof does not have anything to do with it's ability to cool. When you say "relay switch", I do not know what you are talking about. A cooling unit, either a heat pump or an air conditioner, could have enough refrigerant to cool on a not so hot day. But, at higher outdoor temperature, it is inadequate.

None-the-less, possible causes of inadequate/no cooling:

1. Low air flow across the coil.

A. Dirty Coil.
B. Dirty filter.
C. Return air duct(s) to small.
D. Low blower speed.

2. Voltage drop.

A. High voltage wiring sized wrong.
B. Loose connections.
C. Weak breaker.

3. Low refrigerate. After changing out a compressor, the way to properly recharge is to weigh the refrigerate.

If a system is low in refrigerant, the coils may literally be covered with ice. If the air flow is to low, the coils again can be covered with ice.

Hope this helps.

You might also look into a problem with the evaporator metering device or cap tubes. If there is any restriction causing a higher than normal head pressure, the psi just gets up to the limit when the outdoor temps go up. Such a problem can cause a tech to overcharge a system in order to get the low psi up, yet fail to put a gauge on the high side psi.

I think that this is all good advice, by people far more knowing than me!!

One thing to think about is this - hvac systems are designed to lower outdoor temp by 20 degrees. Can't do more than it was designed for!

Ok there were two things...
the other is two part.
do you feel clammy in you home with the a/c system running?
this would have to do with relative humidity (RH).
While I don't know your location, if you are 90 degrees already..you could be living in Louisiana..like me!
But a stand alone dehumidifier would remove excess moisture
& bring RH down to 50% (ideally...and in time)and improve the comfort of you home.
Where are you located??

hope there is something in is post you can use!
best of luck with your project.

In my area, the loads are calculated to change the temp in the home as compared to outside 25 degrees mimimum difference at X humidity- (there are other factors too.)
I think what we are trying to overcome is the HEAT GAIN to the home and since the HEAT is OUTSIDE, yes- the outdoor temp has a lot to do with it.
A good delta T will be 18-24, depending on the latent load.
The SH and SC can be checked if the scale provides for Wet Bulb, Dry bulb temps and psi/temp. But the WB must be in range. A delta T will not read correctly if the latent is high.

Thanks for the responses. You answered my primary question. Which was does the temp on the roof affect the running of the compressor. I feel that the Warranty Co and the Repairmen drug their feet on this all last summer with sometimes a month between look then ineffectual repair - after all they have air conditioning and I donÂt. I think they just want me to give up and pay for it myself.

I am in the Sacramento area. Humidity is not high like in the south, but we reached temps of 116 last summer and have had days in the 90s already this year.
When the repariman was out last time the outside temp was in the 80s. The following is what he put on the reciept:
Replace sticking Relay  causing blower to run constant. Ran checked unit.
House 70/T-stat set 68
Supply 52 Â 18 split
Freon  Good
Amp Draw  Good

He also tested the temp at the ceiling and said it was the same so insulation is probably OK.

Of course he came out when the temp was low enough that the A/C is working.

I am fanatic about changing the filters every 4 wks.

Regarding the thoughts that I can only go a certain number of degrees below the outside temp. It is not that the unit is cooling, but canÂt keep up. It is that the compressor seems to quit yet the fan keeps blowing. The reason I think it is the compressor is because the fan blows hot air and there is no condensation from the drain pipe when this happens.

Thanks for the help  I will continue to research your tips further.

First of all even though it will probably cost you more money I would dump the useless Home Warranty Company techs and get a reputable contractor in there to take a look at your system. To whoever said that the temperature on the roof doesn't affect the operation of an air conditioner I say "OH Really?!!!" I got news for you pal it most certainly is affected by different ambients!
Cora2007, you stated the condenser fan has been replaced before. Knowing what kind of technicians those companies use I wouldn't be surprised if the condenser fan isn't part of the problem now. If they replaced the fan it may be the wrong # of blades or the pitch could be different then what the mfg. calls for. Or, and I've seen this more times then I care to, the fan may not be sitting in the venturi, (thats the opening on the top of the unit) properly and not pulling enough air through the coil. To high and you can't pull enough air through the condenser coil and to low and the air cannot exit properly and some of that heated air gets trapped under the cover. By calling the manufacturers when I suspect a problem like this and get the proper info and mounting dimensions I've been able to drop the head pressure 25, 30 pounds or more many times. That could make all the difference some time when the unit is really under a great load. Have someone investigate the condenser fan including the correct horsepower and motor RPM. Good luck

Yesterday the tech came out when the temp was above 90 (finally) and he FINALLY agreed that my A/C didn't work. This time he said the unit needs to be replaced and amazingly the Warrenty Co. agreed!!!! So after a year of fighting I am getting a new A/C???? So. . . I now have to move fast and I have to use the them because they are the Warrenty Co's choice. The following are the charges that I'm supposed to pay (total $1750):
$225 Transition Sheet metal
75 Gas Line DRip Leg
250 Duct Testing
325 Unit siting Curb
175 Disposal (opt)
450 Duct retest/cert by Independent Duct specialist
250 2nd Crane (2 required ??)
Home owner responsible for all permits and codes.

They say that 2 cranes are required because mine is a roof
downdraft unit. 1 to pull old unit (my expense) and 1 to
place the new unit (warrenty co). They say it has to be
done fast so the 2 cranes will sit side by side.

I know our county has a lot of rules about duct sealing etc, and as my house is old, I'll probably need work done.

Also when I checked their Contractor License had been suspended:
" License is under suspension for failure to comply with Workers Comp. A worker's comp certificate or exemption statement may have been received by Board but not
yet processed. Ask the contractor for proof of worker's comp and contact insurance company to verify coverage".

Comments about costs etc?
and the license thing?
I'm thinking about agreeing to them doing the A/C install and going somewhere else for the duct check etc?

Don't get me wrong, I'm THRILLED that they're finally doing something (after a year of fighting and games), but
that fact also makes me distrustful.

Thanks for your earlier comments, that helped me deal with them in a more informed manner.