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Boost pressure control

Boost pressure control




Boost pressure control takes place by means of a two-coil three-way solenoid valve which is connected by hoses to the turbo unit wastegate, compressor outlet and compressor inlet.

The solenoid valve is supplied with +54 voltage via fuse 24 and is controlled from pins 26 and 2 on the ECM. The control voltage is a PWM which is 90 Hz below 2500 rpm and 70 Hz above 2500 rpm. The purpose of the change in frequency at 2500 rpm is to avoid the phenomenon of 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 reduced, and conversely the boost pressure is increased by grounding pin 26 longer than pin 2.

The ECM determines the boost pressure depending on throttle position and engine speed. The ECM reads off the current boost pressure using the pressure sensor for the intake manifold. The boost pressure is reduced slightly in 1st, 2nd and reverse gears. The ECM calculates the gear position by comparing the car's speed with the engine speed.

If the ECM has been de-energized, a number of full-throttle accelerations are required to obtain a correct build-up of boost pressure.


If knocking occurs, despite reducing ignition and making the fuel/air mixture richer, the boost pressure is also reduced.

If there is a discontinuity in any of the leads to the solenoid valve, basic charging pressure is obtained.

If there is a break in any of the hoses or if there is a fault on the solenoid valve, excessively high boost pressure may be obtained. The ECM will then stop fuel injection at approx. 1.15 bar.


Cars with automatic transmission

When the selector lever is set to a position other than P or N, Batt+ is applied to pin 14 on the ECM and the ECM is programmed for automatic transmission. Cars with automatic transmission have reduced boost pressure.

Brake light switch

When the brake pedal is depressed, the ECM recieves Batt+ on pin 15 and the maximum boost pressure becomes basic charging pressure.

Speed limiting

Vehicle speed is limited to 230 km/h through boost pressure reduction.

Cruise control

When cruise control is activated, pin 36 of the ECM is grounded, making a reduction in boost pressure possible. This function is not activated in Trionic for the 1994 1/2 Saab 900.