The air conditioner in a 1998 Chevy S-10 only blows warm air and when the low pressure port was checked it was getting 100 psi what would cause this?

You should probablly have the system pumped down and recharged with new freon it could be showing a good reading but still have a restriction in the system that is not allowing the gas to flow compleatly through the system. i am a certified refridgeration tech. good luck i had the same problem on my 98 s 10, in fact i just fixed it myself last night! if you have the 2.2 liter engine: they run the wires for your electomagnetic clutch on your compressor, through your intake manifold, under your airbox(the one just before your throttle body) and while it is under your airbox it is lying ON THE CYLINDER HEAD and after a few years the tape and snorkle tube protecting the wires may have come loose, and then the heat from the cylinder head melts the insulation off of the wire, and ending up in a short circuit, which blows the 10 amp. fuse (which is located in the relay and fuse box on the driver side under the hood. the first thing you need to do is start your truck and put your ac on max ac and the fan on 4 now pop the hood and go and see if the clutch on the fan is spinning (i bet you it isnt) now to check and make sure the compressor and the clutch are still good, unplug the wire that connect to the compressor, and run a hot wire from your battery to one side of the connector that is on the compressor and the other side of the compressor connector to ground. it you hear a pretty loud click and you see the clutch plate pull in and out as you add and remove power you are looking pretty good, now leave power running to the compressor and start your truck, turn on the ac and if your compressor is still good your ac should be working! take a pressure reading on the low pressure side(which is on your condensor) while you have it hot wired. it should be in the perfect range, if it is a little low like mine was , just go down to any auto part store and buy a refrigerant recharge kit( about $20 for the one with a built in guage) and fill it up to the correct pressure, im sure that you know that you have to have the truck running and the ac on full blast while reading pressure or filling it back up! ok now if everything is working properly you need remove the air box mentioned earlier and check your wires( while you are checking those you might as well check them all that run over the cylinder head there are a total of 5 sets that run that way) if the insulation is melted dont just recover it and call it good, cause the integrity of the wire is now broken down. instead cut the bad section of wire out and splice them back together, be sure to replace the diode, cause the chances of it surviving a short circuit like that are slim to none( the diode is in that small box that is clipped around the wires) after you have spliced them back together make sure that you insulate all of the wires running that way from the heat so it wont happen again. now replace the 10 amp fuse and start the truck and the ac should now be working! if so you just saved your self about $300, congadulations!oh yeah put the air box back in, and if you look inside the air box in the side that goes back to your filter, not the side that goes to your throttle body. there will be a removable plastic sleeve inside, pull that s.o.b. out it is restricting a lot of air! now put the air box back in! ! oh yeah i forgot to answer your actuall question! the reason you were reading such a high pressure was because the compressor clutch was not engaged, so your ac was not really on! which it needs to be in order to get an acurate reading. the other guy who helped you was a refriration technician, he may be able to tell you in exact detail why it works that way, i just know it does, and im just a union electrician!

Well rather than run a hotwire from your battery, a piece of copper wire to jump the connector to the switch works.

The low pressure side should only read 30 to 45 psi. You have a restriction. My guess would be an orifice tube plugged up with trash from a failing compressor.