Connecting a set of gauges & checking the basic a/c system for correct pressures would be a good idea.

Does the low pressure line(large diameter) get cold?

If it does possibly the problem is related to the blend air door NOT closing.

That system doesn't use a heater valve. Would the system get cold if you blocked off(clamped) one of the heater hoses?

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator