I don't think an 89 should have a thermo switch, it has a temp sensor. If your switch is connecting the two pins together normally it will make the ACC control unit think the engine is overheating and run the fans on high. It will also turn off the AC compressor clutch.

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.