Many of the W140 chassis cars will allow the A/C evaporator to "freeze" on long trips. The problem is with the evaporator temp sensor. Have replaced many of them for that problem.
MB part number 210-830-07-72 with list price of $51.
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)