It's not illegal. You must have a 609 certification to buy it.

The only way you know with certainty if the system is properly charged is to use gauges. Even with gauges, you may take less than a full can. Some folks charge with the sight glass, but that can be misleading. Often times the system is overcharged when using this technique. If you overcharge, the interior of your car will not cool well because the high pressure switch will cycle off the compressor. When the system is overcharged, you'll hear/feel the compressor cyling on and off too often.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"