So I am trying to add some R-134a to my 2000 Chevy Metro. When I first investigated the A/C the clutch on the compressor would not engage and I had 0 PSI on the low side with the compressor running and with it off. I bought some $25 R-134a from Wal-Mart and tried to add it to the low side. Shortly after I started adding refrigerant the compressor clutch engaged. However after dumping about 1/5th of an 18 oz. can in, I was still reading 0 PSI on the low side with the compressor running. However once I turned the compressor off the pressure rose to 55 PSI. At the moment the low side reads 0 PSI with the compressor on, 55 with it off, and the air is still not cool. So what am I doing wrong?

Should the pressure be at 0 PSI with the compressor running? Should I be taking my measurements with the compressor off? If I keep adding refrigerant the pressure will become dangerously high with the compressor off so what should I do?

 

A couple of possibilities, one you had a completely empty system, in which case you don't have enough freon in the system. Second, you have a restriction in the system that is the reason for 0 pressure on the low side. Look for frosting on the lines and components, if you find any (other than the low pressure line) that could be your problem.

You also don't state what the high side readings are. Do you have a complete set of gauges hooked up?

 

I do not know what the high side is. I only have a low side guage. There is no frosting of the lines anywhere. It could be that there is not enough r-134a but if I keep adding more the pressure with the compressor off will redline. Its already yellowlined.

 

Try looking at this link and it will give you an idea of how to add the correct charge to an evacuated system.
http://wiki.answers.com/Q/How_do_you...01_Chevy_Metro
I know the year is different but the idea is the same.

 

You read the pressure only when the compressor is ON.
Your vehicle should have a sticker somewhere in the engine bay that tells you how much refrigerant the system holds. If your system was low enough to not turn the compressor on (low pressure cutoff switch to protect the compressor), you were probably close to empty. It could take something in the neighborhood of 36-48oz of refrigerant when empty. That's just a ballpark figure, it could be more, it could be less. My point is, if you added one small 12oz can, it won't be anywhere near enough.

The way it works, the compressor sucks the refrigerant from the low side, and compresses it into the high side of the system. High side pressure gets higher, and low side gets lower. When you turn the compressor off, these pressures equalize, and eventually they will be identical. That value is not important, it just tells you that there is something in the system. In fact, a fully charged system will have enough pressure in it when turned off, that it could blow up your refrigerant can! Don't let it go in the red In other words, only have the gauge connected when the compressor is running.

You need the system running so the compressor would create a low pressure area in the low side, that way the fairly low pressure of the refrigerant can you are using will have a chance of feeding refrigerant into the system. You generally get maybe 70psi out of a can, while the high side pressure is often in the 200-400psi range. For the can to work, you therefore need a lot less than 70psi in the system you are trying to fill.

0psi is normal for a system that has some refrigerant in it. Almost all of it is on the high pressure side. A completely empty system will even pull vacuum on the low side if the system controls allow it to run.
In your case it tells us that the compressor is working.

So keep filling, until your low side gauge indicates full/normal charge. Do NOT overfill, it won't get cooler. It would get warmer and it will eventually damage the compressor.

Report back and let us know how it goes. You may have a problem that needs attention. I'm wondering how it got low in the first place... maybe there's a big leak somewhere. If it was at zero pressure and there's a hole in the system, you will mostr likely have moisture in the system too. It reduces performance and corrodes the parts from inside out. You need to get all moisture out before charging, by pulling a strong vacuum in the system. Vacuum lowers the boiling point of vacuum so that it starts boiling, and the pump would then suck the vapors out. They make special pumps for that ($$$).

With small amounts of moisture in the system, it might still work, but not at 100% efficiency.