You need a new or rebuilt cruise ECU...I had mine rebuilt by General Development Laboratories (www.gdl-online.com)...current price is $185. Works like a charm. GDL was very good to do business with...highly recommended. They also provide detailed diagnostics info.

Good Luck

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1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)