+1

Charging by weight is definitely what you what to do. Unfortunately, nobody commonly does this, at least at the shops I visit. I am having this debate with one right now, in fact, on my SL600.

I think the reasons are:

#1 It's just easier to slap a set of pressure gauges on and charge until they get where they're supposed to be, this is faster and less work, so it's just what people do.

and

#2 Many (especially older) systems have some kind of efficiency problem or another, like a clogged condenser, clogged evap, sticky expansion valve, gunked up R/D, etc., and so won't take a full charge without sending the pressure gauge shooting way up. The tech's "solution" is usually then to charge by pressure, rather than by weight, assuming it will still work as it should.

The proper thing to do is to find out the correct amount of refrigerant your vehicle should carry, charge it with that amount, and then see what happens. If the operating pressures are out of whack after you're certain you've filled the system with the correct amount of refrigerant, then you have some other problem that needs to be addressed before system performance will be normal. But simply reducing the amount of refrigerant in order to get the pressures into spec is only putting a temporary band-aid on a lurking problem.

Hence why my car stops cooling almost entirely after about 60 seconds of sitting still, even with a brand new fan clutch and the electric fans working as they should. Not to mention new expansion valve, receiver/drier, and compressor. I bet the problem all along is a plugged condenser. That should have been the first place I started, damnit! But I also bet that, if I measured by weight, the refrigerant charge is probably very low, given that even with this obvious problem going on, the pressure gauges read out within specs. The only way the tech could have NOT caught this problem is by charging using the pressure gauges, instead of charging by weight and then using the gauges as an after-the-fact checkup.

And that's the problem with not charging by weight, the system pressures can be artificially inflated by other problems that are reducing efficiency. So you *think* it's right, but it's not, and then you're still wondering why it's not cooling properly.

The bottom line is, the spec pressures and temperature curve are only valid measurements if you are already certain that you have the proper refrigerant fill. So if you charge ONLY by pressure (which most people do) then you're missing the other half of the information you need to make sure everything is correct.