感谢车族朋友追风大老虎，x88,  MoonRiverttt , JunkYarDDo, SmallBlock, and my friend NG, 提供宝贵知识经验. AC 问题解决！Root cause 是最简单的冷媒不足. 以下是学习经验点滴：

 压缩机无法启动时加注冷媒：

There are probably at least two ways to add refrigerant when the compressor is not running; One is to vacuum your system to create pressure difference between the refrigerant can and the system. Without a vacuum pump, used 追风大老虎 曲别针大法  - a jumper to bypass the pressure checking, so that compressor can be turned on to suck the refrigerant in. This works great in my case. And of course, you must have a secured jumper. See picture.

低压端零压力，高压端低压力的情况处理：

This is a typical case of refrigerant not circulating well based on the service manual. But my case is a bit different, while the low side reading is zero, the high side reading is less 20 psi, indicating insufficient  refrigerant also.

Thanks again to my friend NG and 追风 for encouraging go on to add refrigerant; and this resolved the problem. After first 13oz can added, the pressure back to about 50psi high side; and this apparently is going over the pressure switch threshold, so removed the jumper and put the pressure switch plug back to the system, the AC on and compressor running! Added second can, and the pressure is about 14psi and 60psi now when the compressor is running, they are all still too low.

The system has capacity of 37oz, so I will need the 3^rd can refrigerant. Leave this to next month when temperature is higher; Also give me a chance to check if there is any leaking.

 工具：

Manifold Gauge Set  is a must-have tool. Plus don’t like this plastic coupler comes with the refrigerant can! It is very hard to plug onto the low side port! Ended up using my friend’s metal type coupler with the charge hose, see pictures.

You can also charge refrigerant thru the yellow hose of the MGS, but I didn’t use it because didn’t vacuum the system, so a shorter (12” vs. 70”) hose with less air in the hose, is better .

 Again as one of friend here mentioned, AC system is tricky so don’t do it until you totally understand what you’re doing, at the very least, have the tools and the service manual.

What does a leaking look like from UV light picture.