Brief description of the main instrument unit
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Brief description of the main instrument unit
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The purpose of the main instrument unit is to provide the driver with information and also to serve as a communications link between components/control modules connected to a bus and components/control module not connected to a bus.
The main instrument unit has its own diagnostics function and sets a diagnostic trouble code when a fault occurs.
The main instrument unit incorporates the following functions:
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Coolant temperature gauge
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Turbo pressure gauge (cars with turbo)
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Selector lever position indicator (cars with automatic transmission)
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Instrument and display lighting
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Warning and indicator lamps
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The main instrument unit's control module receives information from other systems as well as from its own sensors. It processes the information and uses it to control instruments and lamps. Some information is send out on the bus.
The car is equipped with an electronic speedometer which shows the current speed. The left rear wheel sensor sends speed information to the ABS control module which processes it and sends it on to the main instrument unit.
The main instrument unit's control module stores the distance covered and shows the mileage on the odometer display. The odometer display has a six-digit counter which the control unit continuously updates from 000000 to 999999 km (or miles). The trip meter has a four-digit counter which starts at 000.0 and goes up to 999.9 km (or miles).
The tachometer is electronic and shows the current engine speed. It receives control pulses from the relevant engine management system. When engine speed is below 1100 rpm, the display is damped to give a steadier reading.
The fuel gauge gives the driver information on how much fuel is left in the tank. It consists of a fuel gauge in the main instrument unit and a level sensor in the tank.
When there is less than 10 litres of fuel in the tank, the main instrument unit's control module switches to measuring fuel consumption. It obtains the necessary information from the relevant engine management system. This gives a more accurate reading of the fuel level. The fuel reserve indicator lamp comes on when the amount of fuel remaining is less than 10 litres.
Low fuel level (OBDII)
This function is used only for OBDII diagnostics.
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Tank venting diagnostics (OBDII)
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The Trionic sends a signal to the main instrument unit on the diagnostic status of the tank venting system. If there is leakage in the venting system, the main instrument unit sends the information "Tighten fuel filler cap ON" on the bus. The SID unit then displays the requested message.
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Coolant temperature gauge
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The coolant temperature gauge informs the driver of the temperature of the coolant. The main instrument unit obtains this information from the coolant temperature sensor in the cylinder head and sends it out on the bus.
The car has an electronic turbo pressure gauge which shows the current turbo pressure. Information on the pressure in the intake manifold comes from the Trionic. The main instrument unit controls the turbo pressure gauge so that its reading is proportional to the pressure in the intake manifold.
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Selector lever position indicator
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Cars with automatic transmission have an indicating display which shows the selected gear position by illuminating the appropriate symbol (PRND321). The main instrument unit obtains information on the selected gear position from the selector lever position sensor.
The main instrument unit receives the A/C request information on the bus. The A/C request signal is passed on to the relevant engine management system.
The MIU is connected to the same lead as the one the engine management system uses to control the A/C relay. When the engine management system grounds the lead, the MIU sends out an A/C relay ON signal on the bus.
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Idle speed compensation, radiator fan
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To maintain the idling speed constant when the charging circuit load increases (when the radiator fan starts), the DICE sends out an electric load signal on the bus.
The main instrument unit processes this information and sends a signal to the Motronic which then compensates for the load increase in its idle speed control circuit.
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Warning and indicator lamps
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The main instrument unit contains the following lamps:
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Indicator lamp INFO DISPLAY, lights up when a message is shown on the SID unit.
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Central warning lamp, lights up when a fault occurs in a system having to do with the car's safety.
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Oil pressure warning lamp, warns the driver when engine oil pressure is too low.
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Charge warning lamp, indicates to the driver whether or not the generator is charging.
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AIRBAG warning lamp, warns the driver when a fault has occurred in the airbag system.
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Brake fluid warning lamp, warns the driver when the fluid level in the reservoir is too low and when a fault has occurred in the anti-lock brake system (ABS).
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Handbrake warning lamp, informs the driver that the handbrake is applied.
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ABS warning lamp, the lamp lights up when there is a fault in the ABS system.
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SPORT indicator lamp, on cars with automatic transmission indicates to the driver that the SPORT mode is active.
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WINTER indicator lamp, on cars with automatic transmission indicates to the driver that the WINTER mode is active.
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CHECK GEARBOX indicator lamp, warns the driver when there is a fault in the automatic transmission.
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Fuel reserve indicator lamp, the lamp lights up when the fuel level is below 10 litres.
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CHECK ENGINE indicator lamp, warns the driver that there is a fault in the engine management system.
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Direction indicator repeater lamps, right and left. Show when the direction indicators are on.
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CRUISE indicator lamp, on cars with cruise control the lamp lights up when cruise control is engaged.
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SHIFT UP indicator lamp, the lamp lights up when a higher gear should be engaged (certain markets).
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Lights-on indicator lamp, indicates that the headlamps are on.
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Rear fog light indicator lamp, indicates that the rear fog light is on.
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Main beam indicator lamp, indicates that the main beam is on.
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Selector lever position indicator, indicates the selected gear position on cars with automatic transmission.
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Door indication, indicates if any of the doors or the tailgate is open.
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Instrument and display lighting
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The main instrument unit's control module controls the instrument and display lighting according to the light intensity in the cabin in combination with the selected rheostat setting. This gives a good level of lighting for day or night driving and optimum bulb life.
P bus and I bus in the Saab 9-3
On the Saab 9-3, not as many control modules as on the Saab 9-5 are connected to a bus system.
The two power train systems, EDC 15 and the diesel pump, are not connected to the I bus, however. These systems need
much faster
communication so that no delays will be noticeable, such as when EDC 15 requests a certain amount of fuel for injection.
Therefore, they are connected to a separate bus called the P bus (Powertrain Bus). Communication on the P bus is ten times faster than on the I bus.
In addition, the P bus is connected to the MIU. The MIU ensures that information which is available on one bus is also available on the other.
