Going down the road, air goes across the condensor, removing the heat from the system (not going to explain the entire ac process here).

Stopped, you MUST have the electric fans working to remove the heat. Start there.

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1990 300CE. Garage kept all its life. Looks & Runs Like New. Everything works. White w/Palomino int. Pioneer 12 CD player in the boot. 17" Enkei LX-F Chrome wheels w/Falken Azenis ST-115 225-45 tires. Tinted windows. Black sports grill w/Euro clear front lamps.