Very common problem on 420SEls is for the vacuum hose to come off (the hose to modulator). It can be seen with the aircleaner off, down the back side of the motor. It attaches to the same area the large brake booster vacuum hose originates. It is very hard to see down there but the vacuum line to the tranny can usually be identified and then just be sure its plugged into something. I usually stir it with a 12in screw drive and see if its attached. BTW this is special hose to resist the heat from the exhaust crossover which is right there also. Don't use normal vacuum hose or you will be back soon.
The expansion valve is replaced without removing the evaporator case. It is not fun on the cars after airbags as the reinforcment is right in the way and the fitting are often real tight (if not corroded). We had one the other day that we had to replace the evaporator as the fitting was so corroded to the valve.
Bosch Master, ASE Master, L1
33 years MB technician