93 300 E 2.8

On a long trip this weekend i noticed my ac started blowing a bit warm , i stopped to get gas and checked under the hood and noticed the cold side pipe on the a/c had ice on it (the larger hose) from the small divider near the strut tower back to the firewall

its a r134 system ,could i be low on a charge ? or could the evap drains be clogged? the a/c works great during the day when i make my short trips and never noticed a problem before this 3 hour trip

the a/c is virgin never been opened or any repairs on it , the sight glass is tough to see cause the abs partially blocks it

any ideas

thanks

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1985 190 2.3
1993 300e 2.8