The switch is for the climate system, it cuts off the AC compressor. The electronic controls on the 603 are only for emissions and idle speed. If you dump those two systems it will run totally mechanically, with a 500 RPM idle (which can be mechanically adjusted in the pump).

-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.