Low speed on fan is for AC operation. High speed is for when engine gets pretty hot.

Your problem sounds like possible clogged radiator.
Remove it and have it serviced by a radiator repair shop.

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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.