W126 AC low pressure line: help!
 

On my vehicle ('88 420 SEL), apparently, previous owner retrofitted to 134a.

Which would be the low pressure line, so I know where to add refrigerant? I only see a thick line on top of left side of engine compartment, with a blue plastic covered valve. Is this the low pressure line, or is there one below it closer to the compressor?

Thanks.

Blue is LOW; RED is high. You are wise to have double checked, though. Sometimes the previous work will be "different". The thick hose where you describe is the low side. Gauges help a lot when dealing with the A/C (borrow, beg, try Autozone tool loan program).