I have been a heavy equipment mechanic for over 10 years. I have done a little ac work, but I'm having problems with a 2003 1500hd chevy truck. The dealer told my brother the compressor needs to be replaced. I replaced it, new orifice tube and dryer. His complaint was the compressor would just shut off. Before I removed it I checked the compressor and it would not even turn on. It had 14 volts going to it but would not come on. After I made the repairs the clutch would keep cycling. I bypassed the low pressure switch and charged to where the pressures are at 30 and 250. 100 degrees out side. After thinking everything was ok I plugged in the low pressure switch the compressor would cut out. We replaced the low pressure switch and it is still doing the same thing. I had to bump the low pressure up to 35 psi it seems ok but I think it could be colder. How low will the pressure go on a chevy truck before the compressor will kick out? Any idea on what is wrong here or do I need more freon?

this is off gm's service site, if you need any more info or a scanner let me know I have a tech2

A/C Low Pressure Switch
The A/C low pressure switch protects the A/C system from a low pressure condition that could damage the A/C compressor or cause evaporator icing. The HVAC control module applies 5 volts to the A/C low pressure switch signal circuit. The switch will open when the A/C low side pressure reaches 138-172 kPa (20-25 psi). This prevents the A/C compressor from operating. The switch will then close when A/C low pressure side reaches 275-317 kPa (40-46 psi). This enables the A/C compressor to turn back ON.

The low pressure switch uses refrigerant pressure to open and close a set of electrical contacts. When A/C request is authorized, the switch is closed and shows normal status. During this state, the switch will show 0 volts on the A/C low pressure sensor signal circuit. When A/C request is denied due to a low pressure condition, the switch will be open. During this state, the switch will show 5 volts on the A/C low pressure sensor signal circuit.

Do you know why it would be cutting out at 30 psi on the low side

there should be a sticker saying how many pounds the system takes. i know my truck takes 2.25lbs which is 2 cans and a half i guess, i just went and did 3 cans and iv got 39 degree air and its 98 here now.

Do you know what your low side pressure was reading? was it around 35 to 39psi?

mine was at 40 aand the high side was around 200, what i did was i took out the freon and then hooked up my vacume pump to put a vacume on the system for a hour or so, then i put a can in and started it up and it came on and off untill i got it full then it stayed on, it does cycle on and off while driving but mostly when passing or heavy throttle.

It shouldn't be cutting out at that pressure, replace the switch.

Did you notice what the high side pressure was when the compressor kicked off?

high side never got over 250 that was with it charged at 30 psi on the low side and the low pressure switch bypassed. We did install a new switch and they both were doing the exact same thing.

i do hvac and have worked on alot or auto ac the pressures at 98 ambient temp should be almost 40 the supply air in the veh should be 47-49 at the coldest its colder because its under charged when ac is under charged it will cause a lower evap temp leading to freeze up try adding more freon make sure it is on max cold and at idle also if you did not pull a vacuum on the system it will make your pressures very unstable causeing your com to short cycle hope this helps