Vacuum Switch: Description and Operation

Vacuum Switch:

PURPOSE
The vacuum switch senses intake manifold pressure as a measure of engine load and signals the ECU to make appropriate air/fuel ratio adjustments.

OPERATION
Switch contacts are operated by a spring loaded diaphragm in a chamber. When vacuum falls below a specified amount (manifold pressure increases), spring pressure causes contacts to close. This occurs when engine speed is low and the throttle is open enough to require full load enrichment. The ECU reacts by causing the fuel system to go "open loop" while simultaneously inducing a 12% fuel enrichment.

Because the two are wired in conjunction, closed throttle position switch contacts provide the same input as closed vacuum switch contacts. Either or both switches can signal the need for full load enrichment.

LOCATION
Near right rear intake manifold runner.