A little AC help from those less stupid than I am.
 

So here is the deal:

The last few weeks my AC will randomly get warm, I can tell from the idle that the compressor isn't spinning. Generally this happened at lower speeds, stop and go traffic, that sort of thing. A little moderate speed, steady cruising and it would cool down again. Now it is to the point where it will only cool if the ambient temps are in the low 80s or less.

A previous owner converted to R134 but I've no idea how thorough the job was. O-rings, purging old oil and refrigerant, etc are all unknown to me.

I buy an 18oz can of R134 with oil, follow the directions to refill the system. With the AC running and the comp spinning pressure would never go above 15psi except when actively filling, which would just be a measurement of pressure from the can. When I remove my finger from the nozzle presure would drop to 15 again. Periodically the comp would stop spinning, rpms would raise, and pressure on the low side would jump to 100+. After the comp kicks back in pressure would drop to 15 or so again. I used maybe half a can with no effect.

Question is, do I just need to add more R134 or is that behavior indicative of a bad compressor, or maybe a bad low pressure switch? My AC knowledge is woefully inadequate and my experience in AC work is limited to filling, a few compressor changes, and a few R12-R134 conversions. Simple swap this part with that part sort of stuff.

Most likely they preformed a dirty conversion. In a dirty conversion, you simply drain the old mineral oil from the compressor, install either PAG or ester oil, then recharge with 134A. This works well in some vehicles, but not ones that have a small condenser. My suggestion to you is return it to a R 12 system, or delete it and save the weight.

there's 2 switches in your a/c system, a pressure switch and a temperature switch. try jumpering them one at a time and see if one of them lets the compressor engage. then go from there

With the engine running and the vent fan on high with your A/C switch on - Your Low Side should read average of 40/45psi. With the compressor off (not spinning) both your high side and low side will equalize -thus the high reading at compressor clutch disengaged.
My thought would be that you do not have enough refrigerant yet. It is always kind of "IFFY" when you are not using A/C manifiold gauges. I wouldn't do it myself, but many people do!

I'll borrow a manifold and see what it tells me. Thanks for the input...

only problem if he goes back to r12 is the price vs. 134. but, i guess if you just wanna get it done there is no reason you couldn't do that.

i second the low on refrigerant comment.