Saab 9000 - Page 55

52 Technical Description
Boost pressure control
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Boost pressure control takes place by means of a
2-coil, 3-way solenoid valve which is connected by
hoses to the turbo unit wastegate, compressor outlet
and compressor inlet.
The solenoid valve is supplied with power (+54 cir-
cuit) via fuse 5 and controlled from pins 26 and 2 of
the control module. The control voltage is a PWM
signal with a frequency of 90 Hz below 2500 rpm and
70 Hz above 2500 rpm. The change in frequency at
2500 rpm is to avoid resonance in the air hoses.
The ECM adjusts the boost pressure by alternately
grounding pins 26 and 2. As a result of grounding pin
2 a longer time than pin 26 the boost pressure is re-
duced, and conversely the boost pressure is in-
creased by grounding pin 26 longer than pin 2.
The control module determines boost pressure on
the basis of throttle position and engine speed. The
control module obtains the current boost pressure
from the manifold absolute pressure sensor. Boost
pressure is reduced slightly in 1st gear. The control
module calculates the .gear position by comparing
the speed of the car with the engine speed.
If the ECM has been de-energized, a number of full-
throttle accelerations will be needed to obtain a cor-
rect build-up of boost pressure.
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