have an 05 mustang 4.0 my compressor failed. i replaced the compressor, the accumulator, and the valve. i am using an ac machine. pull the vacuum, it holds. cannot fill more than 8 oz of freon and when the compressor kicks on the low side pins to the highest mark then when the compressor kicks off goes back down. put 6 oz of freon and the compressor works like it should except the pressures are showing its low on freon. This is my second new compressor. could this second one also be defective?

 

You haven't mentioned anything about the high side reading. Is within normal range or is it high or low?

Are you using Manifold gauges or just a random low side gauge from a filler hose? You can pick up an inexpensive set from Harbor-freight.

This link gives a really good simple-to-follow overview of how to interpret readings from manifold guages.

http://www.agcoauto.com/content/news/p2_articleid/257

 

manifold gauge with a snap on ac machine. high side 160

 

i took out the orfice tube and put freon back into the system, cold side 70 with 31 ozs 0 pressure on the high side the ac gets cold but not that cold.

 

What brand of compressors are you using? According to your readings the compressor is bad, twice now from what you mentioned... Are you adding the appropriate amount of oil (and the correct type) when you replace it? Did you replace the the accumulator drier when you first replaced the compressor?

Did you notice any black particles in your system (Ford A/C Black death)?

Since I have no idea about what you've done before your initual post, that's why I'm asking.

I would expect the pressures to be lower without an orifice tube. That's a key component between the high and low halves of the system.