Spark Coolant Temperature Override System
This system operates during cold engine warm-up to improve driveability. It operates the distributor vacuum spark advance by manifold vacuum during cold engine. The CTO switch, threaded into bottom of the thermostat housing on 4 cylinder models, into left rear of block on Eagle models, into heater hose fitting on 6 cylinder models without air conditioning and in the heater valve on 6 cylinder models with air conditioning, responds to coolant temperature to allow either manifold vacuum or carburetor ported vacuum to the distributor. This switch may, on some models, be used in conjunction with other vacuum advance signal controlling devices.
With coolant temperature below 160°F (Federal 6-cylinder), 165°F (California 6-cylinder) or 120°F (4-cylinder), check ball is held against inner seat by spring pressure. Manifold vacuum enters port "1" and is applied through port "D". This allows full manifold vacuum to distributor. Above calibrated temperature, port "1" is blocked and carburetor ported vacuum enters through port "2" and connects to port "D". Ported vacuum now controls spark advance.