High Altitude Emission Control System

The manifold absolute pressure (MAP) sensor incorporates a barometric switch which senses atmospheric pressure and thus changes in altitude relative to sea level. At high altitudes, it signals to the ECM not to record a rich fuel metering error (as long as no other faults are detected). Switching between atmospheric pressure and manifold vacuum is by means of a MAP sensor vacuum switching valve (VSV) controlled by the ECM.

MAP SENSOR VACUUM SWITCHING VALVE (VSV)

The ECM turns the MAP VSV "ON" for about one second under the following three conditions, switching the MAP sensor input signal to atmospheric pressure:
1. When the ECM is reset.
2. When these three conditions exist at the same time:
a. engine speed is less than 200 rpm.
b. battery voltage is more than 9.5V.
c. vehicle speed is less than 1 km/h.
3. When the ECM judges a high altitude condition, it switches the MAP VSV "ON" to get an atmospheric pressure reading and sets a "Barometric Map."

High altitude specification: "Barometric Map" is less than 596 mmHg absolute pressure as read by MAP sensor.