When on full cold, the coolant flow should be off.

Any heat coming out of the vents when on full cold indicates flow.

Flow occurs when the monovalve fails to close.
Failure to close is either bad monovalve or no signal to the valve.
Default is full flow, so signal has to be present for monvalve to close.

Way to check:

Check for 12V at the valve, feel hoses to see if there is heat, which indicates flow, in the hoses.

Re A/C performance:

W124s are not fantastic A/C performers from the factory......Since your car had R134a originally, it has a better chance with the improved condenser and fual fan setup.

If you've already had several compressors replaced, there is reason to suspect that the problem lies elsewhere. Unfortunately, that means a thorough rehash is necessary.

If the compressor is providing a good high/low pressure, then I'd evacuate the system, then back flush as much as you can, to see if there is something blocking the evap or condenser tubes. Then replace the expansion valve (next to the brake booster) with a new one, button back up, evacuate, then replace the freon.

Jim

__________________
14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles