The other day my ac started blowing hot air so I tried to add some freon. When I went to add the freon the meter on the can was in the red, between 100 and 150 psi. I'm not very familiar with ac systems so any info would be greatly appreciated.

 

The compressor wasn't running so the low side pressure was equal to the high side. Around 150. You need to figure out why your compressor doesn't want to engage. Try and find the pressure switch on your AC system. Wiggle and tap on it. If your compressor was running at the time, it sounds like a bigger problem. It's probably best that you let the pros look at it for you.

 

If the system looses a certain amount of pressure, the low pressure switch will shut off the compressor. When adding coolant, it is sometimes necessary to jump the pins in the low pressure switch plug so the compressor runs and accepts the new coolant.

 

How do I do that?

 

The low pressure switch should be on the passenger side near the fire wall on the ac line
It will have a two wire plug. Unplug it and jump the pins using a piece of wire. That should cause the compressor to engage when the engine is running/key in run position.

I do not have a pic handy so you might search the web for one of the switch location.