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Heads Up Display Unit: Description and Operation



Head-Up Display

Head-Up Display
The Head-Up Display is used to display information relevant to driving the vehicle (e.g. navigation instructions, driving speed) in an ergonomically favourable position on the windscreen in the field of view of the driver.
The Head-Up Display with integrated control unit is installed immediate behind the instrument panel in the dashboard.

Displays in the instrument panel and in the central information display
Information relevant to driving the vehicle still appears in the customary manner in the instrument panel and in the central information display.
The information displayed on the Head-Up Display is dependent on the vehicle equipment. You can select the information using the controller on the central information display (CID). The selected information is stored in the Personal Profile in the HUD control unit.
The following information can be shown on the Head-Up Display, for example:
- Check Control message
- Cruise control
- Navigation instructions
- Lane departure warning
- Driving speed

The information appears as a virtual image floating approx. 2.4 metres away from the user at the end of the engine compartment lid. The image size of the virtual image is approx. 20 x 10 cm (width x height).
The key components of the Head-Up Display are:
- 2 mirrors
The mirrors ensure that the content of the projection display appear at the distance and size required, and that deformations caused by the windscreen are largely compensated for. The curved mirror and the flat mirror determine the shape and size of the projected image.

- Projection light source
The projection light source is the backlighting of the projection display. This light source is a number of colored LEDs laid out in rows over a certain area. The HUD control unit activates the projection light source and adjusts the brightness of the display content.

- Projection display
The projection display with TFT technology (TFT = thin film transistor) is used to show the display content. The HUD control unit activates the projection display.

- HUD control unit
The HUD control unit has the following tasks:
- Activation of the projection light source
- Protection of the Head-Up Display against overheating The HUD control unit has an integrated temperature sensor that protects the Head-Up Display against overheating. The projection light source is switched off if the temperature exceeds approx. 105 °C.
- Evaluating and processing incoming image information
- Display of the image content created by the instrument panel (KOMBI)

Functional description
The following functions of the Head-Up Display are described:

Operation of the Head-Up Display
The Head-Up Display is controlled using the following operating elements:
- Button for switching Head-Up Display on and off
The button in the operating facility for assist systems switches the Head-Up Display on and off as of terminal 15 on. The operating facility for assist systems is connected via LIN bus to the footwell module (FRM). A bus signal from the footwell module to the instrument panel (KOMBI) indicates the button has been pressed.
The instrument panel activates or deactivates the transmission of image data to the Head-Up Display via the LVDS data line. The instrument cluster transmits a bus signal to request that the backlighting in the Head-Up Display is switched on or off.
A bus signal from the footwell module to the HUD control unit indicates the button has been pressed.
The button has 2 functions that are activated by pressing the button for different lengths of time.
- Short press of the button, under 10 seconds
Switches the Head-Up Display on or off

- Long press of the button, more than 10 seconds
The test function is started. This mode is intended for service (e.g. system test). As soon as the first test function is displayed, other test functions can be called up by briefly pressing the button.
Pressing the button again for longer than 10 seconds terminates the test function.

- Dimmer for instrument lighting
The knurled wheel for the instrument lighting is part of the light switch. The LIN bus connects the light switch with the footwell module (FRM).
The footwell module provides information regarding the dimmer setting for brightness control of the instrument panel (KOMBI), the central information display (CID) as well as the Head-Up Display (HUD).

- Controller for selecting brightness of head-up display in central information display (CID). Also for adjusting the position of the virtual image and for selecting the displayed information.

Characteristics when switching on
The HUD control unit receives the status terminal 30B ON via the K-CAN. To protect the projection display against direct solar radiation when parked, one mirror is swivelled away while the vehicle is going to sleep. As of terminal 30B ON, the mirror is swivelled from the parked position into the beam path.
The Head-Up Display is partially operational when terminal R is switched ON. That means:
- The projection display is initialized but information is not yet displayed.
- The Head-Up Display is able to communicate with other bus users via the K-CAN data bus.
- The LEDs are off.

The Head-Up Display is operational when terminal 15 is switched ON. That means:
- If the status "Head-Up Display active" is selected in Personal Profile, the Head-Up Display is switched on automatically.
- The projection light source is switched on or off when the button for switching the Head-Up Display on and off is operated.
- The automatic adaptation of the display brightness is activated.
- The image orientation of the virtual image can be height-adjusted.
- The information to be displayed can be selected.

Projection of virtual image in front of the vehicle
The Head-Up Display projects a virtual image. In other words, this means that an image is created that appears to be projected onto the air, without the use of a display surface for the image (e.g. a projection wall or similar). The image is generated on a projection display in the Head-Up Display and deflected by the windscreen in such a way that the driver seemingly sees the image floating above the engine compartment lid. (The effect of the virtual image is the same as when looking in a mirror: you do not see yourself on the mirror, rather behind it.) The size of the virtual image is approx. 20 x 10 cm (width x height). The virtual image can only be seen from a defined area from the driver's side.

Automatic adaptation of display brightness
The automatic adaptation of display brightness prevents major sudden changes in the brightness of the Head-Up Display when the vehicle moves between different ambient lighting conditions (e.g. from light to dark and from dark to light when driving through a tunnel).
The automatic adaptation of the display brightness depends on the ambient brightness, the brightness adjustment and the dimmer setting of the instrument lighting (only with light ON) or the brightness setting via iDrive.
The ambient brightness (day, night, dawn/dusk, sunshine, etc.) is picked up by the rain/light sensor (part of the rain, light and precipitation solar sensor). This also picks up the brightness in the area of the virtual image to ensure optimal brightness control.
The value of the brightness adjustment (difference between basic setting and individually set display brightness) is stored in the Head-Up Display. When the lighting is switched on (terminal 58g, lights ON) the brightness of the Head-Up Display is also influenced by the dimmer setting of the instrument lighting of the brightness setting via iDrive.
The brightness adjustment takes into account the individual brightness setting for the Head-Up Display, deviating from the basic setting. The individual brightness setting is made on the CID (Central Information Display) using the controller. The set value is stored in the Personal Profile in the HUD control unit.

Prioritization of displays, switch-on characteristics
The information in the Head-Up Display is classified according to priority (= importance for the driver). Displays with low priority are overwritten by displays with higher priority.
- Priority 1:
Warnings of cruise control

- Priority 2:
Check Control messages

- Priority 3:
Test function (= auxiliary materials when troubleshooting without diagnosis system)

- Priority 4:
Navigation instructions

Image generation
The picture signals for the screen mask are transferred from the instrument panel (KOMBI) via the LVDS data line to the HUD control unit. The switch-on and switch-off signal as well as the information for the screen mask are output by the corresponding control units (e.g. headunit) as bus signals. The HUD control unit evaluates the bus signals received via the K-CAN and forwards the processed image information to the components of the Head-Up Display.
The background color of the Head-Up Display is black. The contents of the Head-Up Display are displayed as follows:
- Orange as standard color
- Red or yellow for warning messages
- Green for set speed
- as well as levels of grey of the above colors

The 4 colors orange, red, yellow and green are generated as follows:
- Special layout of the LEDs
- Corresponding activation of the LEDs by the HUD control unit.

The size of the indication range of the virtual image is approx. 20 x 10 cm (width x height). To make the contents of the display easier to read, the display area is divided into an upper section and a lower section. Navigation instructions and Check Control messages are displayed in the upper section. Speed-related information is displayed in the lower section. Which display contents are shown is set in the central information display (CID) using the controller.

Setting the display area of the Head-Up Display
The virtual image can only be viewed completely and properly if the position of the eyes is within a certain area on the driver's side. Outside this area, it is not possible to view the virtual image completely. The height of the position of the virtual image can be shifted (± 5 cm). In this way, the user can alter the determined eye position area by horizontal height adjustment and adapt it to his/her requirements (via CID and controller).

Position of the virtual image
It is possible on the CID (central information display) to use the controller to adjust the position of the virtual image to the seat position of the driver. The set value is stored in the Personal Profile in the HUD control unit.

Switching off the Head-Up Display
The Head-Up Display is switched off either using the button for switching the Head-Up Display on or off, or at terminal 15.







Structure and inner electrical connection
The HUD control unit is connected via an 8-pin plug connection to the vehicle electrical system and the K-CAN.
The LVDS data line is used to transmit the image information from the instrument panel to the HUD control unit.
The HUD control unit is supplied with terminal 30B via the front power distribution box in the junction box.







Pin assignments
The graphic above shows only the supply and bus connection. The current pin assignment is recorded on the wiring diagrams in the ISTA diagnosis system (Integrated Service Technical Application). Click on the component code in the wiring diagram to activate the "Installation location" and "Pin assignment" tabs.

Nominal values
Observe the following setpoint values for the HUD control unit:

Size Value
Supply voltage 9 to 16 V
Overheating protection (switch-off) Above 105 °C
Temperature range -40 to 85 °C

Diagnosis instructions

Failure of the component
If the communication to the HUD control unit fails, run the standard checks (global testing procedure). If there is an internal control unit fault, the following Behavior is to be expected:
- Fault entry in the HUD control unit

Test function
The test function is an auxiliary material for troubleshooting without the diagnosis system. The test function is shown in the Head-Up Display.
With terminal 15 ON, pressing the button to switch the Head-Up Display on or off for a longer period activates the test function. The following procedure must be used:
1. When the Head-Up Display is active after terminal 15 ON, switch off the Head-Up Display.
2. Press the button for a longer period (longer than 10 seconds)
a. The Head-Up Display is switched on, keep the button pressed.
b. After approx. 10 seconds, the display in the Head-Up Display switches to the test function. The information displayed up to that point is hidden.

3. Short presses of the button now switch between the test card.
The test card contains e.g. displays of full screens of the colors supported in the Head-Up Display as a function check of the LED light elements. There are also images with grid lines to enable identification of any image distortions that occur (e.g. that arose due to installation errors).

The test function can be terminated by pressing the button again for a longer period (longer than 10 seconds) or by means of a terminal change.

Test function replaced when driving
At driving speeds greater than 5 km/h, a warning given by the Active Cruise Control or a Check Control message will replace the test function.
There is a service function in the diagnosis system to correct image distortions in the Head-Up Display. The geometric representation of the Head-Up Display can be adjusted with the service function.
Path: Service function > Driver assistance > Head-Up Display -> Adjust the Head-Up Display
We can assume no liability for printing errors or inaccuracies in this document and reserve the right to introduce technical modifications at any time.
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