Boost Pressure Control
Boost Pressure Control
Boost pressure control is effected by means of a 2oil 3-way boost pressure control valve connected with hoses to the turbo unit's wastegate, compressor outlet and compressor inlet.
The boost pressure control valve is supplied with power (+54 circuit) via fuse 5 and controlled from control module pins 10 and 33. The control current is a 90 Hz PWM signal.
The control module controls boost pressure by grounding pins 10 and 33 alternately. Grounding pin 33 longer than pin 10 lowers the boost pressure and, conversely, grounding pin 10 longer than pin 33 raises the boost pressure.
The control module determines boost pressure on the basis of the throttle position and engine speed. It obtains a reading of the prevailing boost pressure from the manifold absolute pressure sensor (-M90). On M91- the MAP sensor has been replaced by an intake air temperature sensor. In this case, boost pressure is calculated on the basis of the engine load signal from LH and corrected with the air temperature.
If the control module has been de-energized, a number of wide open throttle accelerations will be needed to obtain a correct build up of boost pressure.
Adaptation
Boost pressure control is adaptive so that fast and accurate control of boost pressure will be obtained. With adaptation, the control module learns the boost pressure control valve's PWM needs for attainment of the correct boost pressure. Adaptation takes place at definite engine rpm intervals.
Adaptation is conditional on the throttle switch's wide open throttle switch being closed at the same time as engine speed is maintained within the 2750-3250 rpm (-M90) or 2750-4500 rpm (M91-) range for at least 3 seconds.
Pressure switch operation
A pressure switch function is programmed in the DI/APC control module (fuel injection is interrupted in the event of excessive boost pressure).
Fuel injection is stopped in that 8 engine speed pulses from the DI/APC control module are omitted. Afterwards, pulses are again sent out and a fresh check performed. If boost pressure is still too high, the engine speed pulses are again omitted after 8 pulses.
This sequence continues until boost pressure is below the programmed pressure switch level.
Torque limitation
Brakes active
When the brake pedal is depressed and the brake light comes on, battery positive voltage (B+) is applied to pin 32 of the DI/APC control module and basic boost pressure is obtained.
Cruise control active
When driving with the Cruise Control engaged, positive battery voltage (B+) (-M90) or ground (M91-) is applied to pin 21 of the DI/APC control module (not applicable to LH 2.4.2 or ETS) and basic boost pressure is obtained.
Low gear (M91-)
Cars with automatic transmission and turbo have lower boost pressure in 1st gear. The gear position is calculated on the basis of the car speed signal applied to control module pin 34 and engine speed.
Modifications, M91-
Pressure sensor discontinued
The pressure sensor has been replaced by an intake air temperature sensor. Boost pressure is calculated using the engine load signal from LH and corrected with the intake air temperature.
LH 2.4.2 and ETS
LH 2.4.2 or ETS sends a PWM throttle position signal to pin 31 of the DI/APC control module. The signal is a 100 Hz PWM signal with a pulse ratio of 5 % at closed throttle and about 85 % at wide open throttle. Boost pressure is controlled so that higher boost pressure is allowed with a larger throttle opening. When driving with the Cruise Control engaged there is no limitation of boost pressure.
LH 2.4
An LH 2.4 with no throttle position sensor (potentiometer) cannot send a throttle position signal. On these cars the Cruise Control's vacuum switch is connected to pin 31 of the DI/APC control module so that basic boost pressure will be obtained when driving with the Cruise Control active.
Increase ignition retard
Apart from regular boost pressure control in connection with knocking, retardation of the ignition is also introduced in connection with:
- activation of the brake pedal when boost pressure is above 105 kPa and engine speed below 3000 rpm (automatic transmission)
- fuel shut-off (engine braking with closed throttle).
Automatic transmission
Cars with turbo and automatic transmission have lower boost pressure in 1st gear. The gear ratio is calculated on the basis of car speed and engine rpm.