Automatic Transmission
A speed switch mounted on transmission housing in-line with speedometer is used to sense vehicle speed. A control unit, mounted on firewall contains three parts: a control module, thermal switch and vacuum switch. It senses ambient temperature and manifold vacuum. Trucks with 383" engine have a temperature operated vacuum by-pass valve which senses engine overheat conditions. Heat causes valve to operate by-passing NOx control and normal spark advance continues until engine cools.