Low side pressure decrease on refrigerant ADD

[michaelwoosley]

I hooked up the manifold gauge set to my car to measure the system pressures. The car had been running with the AC on for several hours throughout the day. When I turned on the AC system to the appropriate settings and observed the gauges, my low side was sitting at 40psi and my high side was sitting at around 200psi.

The chart that I was using stated that based off the ambient 89 degree F temperature, my ideal low pressure reading would be 45-55 psi and my high side would be 250-270 psi.

So with that being said, I proceeded to ADD refrigerant to the system, but the gauges did not reflect what I expected (a gradual increase on the low side to 50 psi and an increase on the high side to around 250 psi). Instead, the low side pressure gradually decreased to 28 psi and the high side increased to around 260 psi. The high side reacted like I thought it should, but I don't have a clue why the low side showed a DECREASE in pressure with the additional refrigerant added.

What is the communities thoughts on this and I'm prepared for someone potentially telling me I don't know my butt from a hole in the ground... that is OK.

[JohnHere]

I don't think you mentioned your vehicle's make, model, and engine size. Need those for an accurate diagnosis. In addition, testing with a manifold gauge set should be done with the compressor engaged, max A/C selected, blower on high, windows down, engine at about 1,800 RPM, and vent temp noted. Is that how you did it?

Regardless, just blindly adding refrigerant doesn't tell you how much is actually in there. Sounds like it could very well be overcharged now...not a good thing. You should have the refrigerant recovered and weighed to gauge the exact state of the charge, then evacuated and recharged to the proper amount.

As mentioned earlier, you can't charge the system only by pressures. The refrigerant must be weighed-in...especially important for newer vehicles that require much less refrigerant than older cars.