You could have a few things wrong
1.Low on freon and if this is the problem there are some safety devices
to protect the system,the compressor will cycle quickly also your aux.
cooling fans won't come on and you will have a high head pressure
and low cold output.
And if you get cold air when you are on the highway and warmer air in around town driving. Turn you a/c on and your aux fan or fans should come on if the don't you could have a bad switch on the receiver dryer
it's most probably red in color
Next cold be the climate control unit check the other threads about this

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Keeping 'em Running
1990 300SEL
1994 E320
1989 E300