A/C Pressure Test/Diagnosing in cold ambient temps

[06YarisRS]

Interesting question. I just went through this. Here is a snippet from my service manual and one from a 'generic' site. The upper 1/2 of the second image shows the pressures by temperature. It's too early in the am for me to do the math but if the relationship between temp and pressure is linear, you could extrapolate the data to get a rough idea of what pressures are required. There is no doubt that I have too much in mine as I'm getting spec pressures but at a considerably cooler ambient temp than published, so I will be testing my system again in the spring/summer and adjusting the amount of R134 accordingly. I am going to try to get my garage above 60 F with my 240V cube heater, but I'll have to pick a 'warmish' day. My garage isn't insulated - 1/2 of it is a poly film paint booth - but it's doubtful I'll be able to get it warm enough do an accurate assessment until much warmer weather arrives. The above said, I did read on one seemingly reputable website that you should never recharge your system under 60 - 65 F. Since your conditions are similar to mine, it might be advisable to just undercharge your system to keep it sealed, then adjust in summer. The other option of course is to take your car to a friend's heated garage or if your car is being serviced, ask the shop owner if you can pop in and check your pressures after it's been in there and warmed up a bit. Just make sure his back is turned if you have to release a bit of R134.