Housing Pressure Altitude Advance (HPAA)

Used on 1984-85 engines (exc. Calif.), the HPAA is used to meet emission standards at both low and high altitudes. Altitude compensation is achieved through timing changes and EGR modification and is controlled by an altitude sensitive switch.
Timing is controlled by two pressure regulators, the Housing Pressure Cold Advance, located in the injection pump, and the Housing Pressure Altitude Advance solenoid in the fuel return line.
The HPAA solenoid regulates housing pressure according to altitude. When the solenoid is activated, the glass check ball seats, regulating pressure at its calibrated value. When the solenoid is de-activated, the check ball moves off its seat, opening the fuel return line and preventing pressure regulation. It is possible for both the HPCA and the HPAA to regulate housing pressure at the same time. Likewise, it is also possible to have just the HPCA or the HPAA regulate pressure singularly. The HPCA must be energized and not regulating to allow the HPAA solenoid to regulate at its calibrated value.