HVAC control unit roller wheel illumination

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HVAC control unit

The modification of the HVAC control unit roller wheel illumination takes on one of the BMW 8 Series' design flaws; the inconsistent illumination of certain controls and instruments. The roller wheels of the HVAC control unit (also know as IHKA control unit) are barely visible at night. This article is a step-by-step guide for those who want to fix this issue with a do-it-yourself (DIY) approach. There is not much skill required for this modification, but if you worry about your own abilities or simply lack the time for it, consider one of the alternative fixes.

Removing the HVAC control unit

Unlike the radio and multi-information display (MID), the HVAC control unit cannot be removed from the front. It is screwed in the center console from the rear. Thus to remove the control unit the center console must be taken out too.

Removing the center console sounds worse than it actually is – unless really clumsy it should take less than ten minutes. The location and amount of the screws that hold the center console differ with the style of center console and the car's transmission. On a center console with wood trim three screws are hidden inside the opening where the multi-information display is installed. To reach those the MID must be removed. The regular plastic center console has two screws behind blind caps just above the MID location. Be careful when removing these caps to avoid damage to the plastic. Although not strictly required for the removal of the plastic center console, it is recommended to remove the MID here too. It makes handling the center console a lot easier.

The multi-information display is secured in the center console with retaining tabs – three small ones on top of the unit and one wide tab on the bottom. The bottom tab must be pressed to remove the MID. To gain access to this tab it is necessary to remove the radio. Once the radio is out and disconnected, reach in the opening and firmly press the retaining tab on the bottom of the unit while simultaneously pushing the MID towards you. Usually it slides out without much effort but in some cases the top retaining tabs block it. Just push harder and carefully wrench the unit out. The MID is connected with a single cable. Release the connector's locking lever and unplug it. All connectors behind the center console fit in one direction only and the shape or color makes clear where they should connect to. There is no real need to label the connectors although it never hurts of course.

The remaining screws are underneath the shifter's leather boot. Grab the boot on both sides and pull up. It just clicks softly in the center console and should come off quite easily. Then grab the shift knob with both hands and pull hard and straight up. This will require some force! Avoid accidently hitting something – like your face – when the shift knob comes off the shift lever. In cars equipped with manual transmission there is only one screw in the opening – undo it. In case of an automatic transmission, the shifter plate must be removed as well. There is a fragile pin through the shift lever for the older automatic transmission without Steptronic. The pin is held in position by a clip on the driver's side. Press the clip upwards until the pin can move. Newer Steptronic transmissions do not have this pin. Now undo the two remaining screws holding the shifter plate and lift it. Unplug the connector attached to the transmission switch and remove the shifter plate from the center console. The metal shift lever has sharp edges – wrap a small soft towel around it to avoid damaging other parts.

Make a picture of the center console or write down the location of each switch surrounding the shifter for the installation later on. Normally the wiring harness points out what goes where but a photograph or drawing removes all doubt. Lift the center console a bit and push the center console switches out from underneath. Press the small retaining tabs on each connector and pull to unplug.

If the car is equipped with the factory hands-free mobile phone, its microphone must be removed before the center console can be taken out. The microphone is the small grate left from the multi-information display. Reach in through the MID opening, push the microphone out of the center console and unplug it. If you did not remove the radio and MID, lift the center console until you can reach behind it and push the microphone out that way. Then flip the center console and disconnect the radio and MID. The last thing that ties down the center console to the car is the wiring harness to the HVAC control unit: A large blue connector with a locking lever and a small connector just above that. Unplug both and remove the center console from the car.

The HVAC control unit is secured to the center console from the rear by four screws – two on the bottom and one on each side. Undo all four screws to remove the unit.

Dismantling the HVAC control unit

Once the HVAC control unit is out of the car, it is time to take a closer look at the failing illumination. This means dismantling the unit completely. Even if the cause cannot interest you, the fix requires taking the control unit completely apart. Always put the unit on a clean and soft towel when working on it – the front scratches very easily. Work very carefully. If something seems to be stuck, check twice if you overlooked something instead of using brute force. The risks involved with this modification are very low, but a careless person will manage to break everything...

Start with the removal of the button caps. The best and safest way to do this is by using an extraction tool for keys. This is a simple tool which hooks behind the key cap on both sides, providing grip to pull it off very gently. One that works for a computer keyboard should also work for the HVAC control unit. Alternatively you can use a small screwdriver or knife.

Removal of the button caps with an extraction tool

On the bottom of the HVAC control unit are the three axles visible around which the roller wheels turn. Each axle has a small lip. Turn all three roller wheels to their center position – all lips should now point to the front of the unit. Remember this as it will aid the assembling later on. Grab the lip with long-nose pliers or needle-nose pliers, push it slightly towards the center of the axle and then pull straight up with a bit force to remove the axle. Remove all three axles.

Unplug the fan's connector on the rear side, remove the four screws and take the backside off. Be careful when handling the unit at this point. The roller wheels are now loose, but they can't be removed yet due to the existing illumination. Pressing the wheels or trying to remove them may damage the illumination.

Rear side of the HVAC control unit with the internals exposed

In one of the corners of the printed circuit board (PCB) is an incandescent light bulb. Turn the bulb a quarter counterclockwise and pull it out. The PCB is mounted with two screws. Remove these and lift the board. The white ribbon cable may need to be bend a bit to lift the PCB. Below the board is yet another PCB, also mounted with two screws which must be removed. Take both circuit boards out of the unit. On the bottom PCB is a black plastic tube – the car's interior temperature sensor. It may have collected a lot of dust over the years so now may be a good time to clean it with compressed air. Be careful though, as the wires from the thermocouple are very thin and you don't want to damage it.

With the circuit boards removed the cause of the bad roller wheel illumination comes in sight. The bulb's light is distributed to the wheels by a three legged plastic light bar. It's a very optimistic construction and practice shows it doesn't work. Remove the two screws that hold the light bar and then carefully remove the roller wheels along with the light bar.

Disassembled HVAC control unit

In theory the light bar could work, but it would require a much brighter light source. There is simply too much light lost in the construction. However, a much brighter light bulb might melt the plastic.


The goal is to get more light into the roller wheels. A few options are possible. For example one could replace the original light bulb with a light-emitting diode (LED). Apart from high-power LEDs, LEDs do not have issues with heat. However a single LED would probably not be bright enough or have a too narrow viewing angle, but there is not enough space in the light bar's bulb opening to fit multiple LEDs. Another approach is to cut-off the bulb holder from the light bar and attach an LED on each remaining bar. This way the LEDs do not require a large viewing angle. But the light bar must be damaged to perform the modification and non-destructive methods always get the preference. The best solution is to remove the light bar entirely and make the LEDs illuminate the roller wheels directly. The latter is also the method used in this article.

Choosing the right LEDs

LEDs are available in many different packages, colors and intensities. It's important to pick the right LED for this application. To match BMW orange illumination, the LEDs' wavelength must be around 605 nm (600–610 nm). Anything outside that range will look out of tone in the 8 Series interior.

LEDs emit monochromatic light which may be an issue. Each roller wheel has white printing, but the temperature roller wheels also have red and blue indicators for hot and cold. In daylight – without the instruments illumination on – the roller wheels will appear just like that; white with red and blue indicators. But at night with the illumination on, the white becomes BMW orange with only a very subtle difference to the red indicators. The blue indicators will be dark and almost invisible. That's because monochromatic light doesn't work well with color filters and the HVAC control unit was originally designed for an incandescent light. Some people prefer the all-orange look, while others may want to retain the distinct red and blue colors. In case of the latter white LEDs are the only option as these are not monochromatic.

The rest of the LEDs' specifications depend on the mounting type:

Round LEDs facing down

The simplest method is to use standard round LEDs. Standard 5 mm round LEDs are too big to fit inside a roller wheel, but 3 mm will do fine. Ideal would be forward-facing LEDs, but that requires a large viewing angle – which is hard to get in BMW orange round LEDs. LEDs with a narrow viewing angle would cause the middle of the roller wheels printing to be brighter than the sides. Facing the LEDs down is a lot less efficient, but round LEDs feature an even light distribution around their axis which is perfect for the roller wheels. The viewing angle must be 60° or better and the luminous intensity 40 millicandelas (mcd) or better. The larger the viewing angle, the less bright the LEDs need to be. Viewing angles narrower than 60° are to be avoided – most of the light is lost.

SMT LEDs facing forward

A large viewing angle is a necessity for an even illumination with forward-facing LEDs. Other applications where a large viewing angle is essential too – like display backlights – normally use surface-mount technology (SMT) LEDs. These are thus readily available in large viewing angles. The viewing angle is preferably over 120° and a luminous intensity of 15 mcd is sufficient. The LEDs' size should not exceed 3 mm in any dimension. Small SMT LEDs may not emit enough light for this application, but a simple solution to this is putting two LEDs in parallel for each roller wheel. Putting LEDs in parallel is not recommended practice, but due to space restrictions it is a lot easier in this case. When doing so, don't forget to double the current in the calculations later on. An excellent choice for this modification is the Avago Technologies HSMD-C170 LED:

Avago Technologies HSMD-C170

  • Package: 2 mm SMT 0805
  • Wavelength: 605 nm BMW orange
  • Viewing angle: 170°
  • Luminous intensity: 8 mcd
  • Forward current: 20 mA
  • Forward voltage: 2.2 V
Avago Technologies HSMD-C170: Welcome to the world of miniature SMT components

Another good choice would be the Avago Technologies HSML-A101-S00J1 LED. Although its package is larger than that of the HSMD-C170 and features a less wide viewing angle, it should still be sufficient. Since it's a lot brighter, only a single LED is required for each roller wheel.

Avago Technologies HSML-A101-S00J1

  • Package: 3.2 mm SMT PLCC-2
  • Wavelength: 605 nm BMW orange
  • Viewing angle: 120°
  • Luminous intensity: 220 mcd
  • Forward current: 20 mA
  • Forward voltage: 1.9 V
Avago Technologies HSML-A101-S00J1

Brightness control

It's very unlikely the LEDs have the correct brightness out-of-the-box. The brightness must be adjusted to match the rest of the illumination. The easiest way to adjust a low-power LED's intensity is by changing the current through it. The current through the LED is determined by the current-limiting resistor. One could simply test fit different resistor values until the correct intensity is achieved, but that may be time-consuming. An option is to replace the resistor with a variable resistor – a potentiometer. But standard potentiometers can handle only very low powers and may get damaged while adjusting. A better approach is to add an extra transistor step. Transistors can be driven with very low currents, yet at the same time switch or regulate much higher currents.

Emitter follower circuit diagram

The dimmer is built around a so-called emitter follower or common-collector circuit. In an emitter follower the voltage at the emitter (E) always equals the voltage at the base (B), minus a small voltage drop over base and emitter. When adjusting potentiometer P1, the voltage at the base changes. Transistor T1 then adjust the current through emitter and collector (C) – and thus also the load – until the voltage at the emitter is back equal to the base voltage – Ohm's law in practice. By replacing the load with a LED and current-limiting resistor the circuit becomes a dimmer. Adjusting potentiometer P1 increases or decreases the current through the LED and thus controls its brightness.

The efficiency of the application can be increased by putting multiple LEDs in series, increasing the voltage drop over the LEDs while lowering the drop over transistor and resistor. The lower the voltage drop over the transistor and resistor, the lower the power dissipation and heat lost in these components. It may seem interesting to put as many LEDs in series to get the total forward voltage over the LEDs as close as possible to the source voltage, but while this increases the efficiency considerably it also dramatically increases the circuit's sensitivity to variations in the source voltage. This is important in a 12 V automotive electrical system where the voltage is not a constant. To keep the application's voltage sensitivity within reasonable margins, it is not recommended to create a total forward voltage of over 60 % of the source voltage – which translates to just under 9 V calculating from a 14.5 V peak voltage. With a forward voltage of 2.2 V for the mentioned Avago Technologies HSMD-C170 LED this would mean four LEDs can be put in series. Only three are needed since there are only three roller wheels in the HVAC control unit. When opting for the HSMD-C170 LED it would actually mean six LEDs in total because each wheel needs two LEDs in parallel to provide enough brightness, but a parallel setup does not increase the forward voltage – it only doubles the total current.

The circuit can be used in other applications where LEDs need to be dimmed, but due to its limitations and drawbacks it is only recommended to dim a relatively small amount of low-power LEDs.

Circuit diagram and PCB layout

Circuit diagram and PCB layout. Caution: the solder side layout (top) is mirrored!

Parts list

  • R1: See text
  • P1: Trim potentiometer 100 kΩ linear (top adjustment)
  • D1..D6: See text
  • T1: Transistor BC547 or equivalent

R1 limits the current and protects the circuit against exceeding the specifications of the LEDs. Its value can be calculated as follows. First calculate the total forward drop over all LEDs. Datasheets usually refer to a LED's voltage drop as forward voltage VF.




The LEDs' forward current can also be found in its datasheet. A forward current of 20 mA is very common. For the six-LED circuit this number has to be doubled, but practice shows a current this high is not needed with the Avago Technologies HSMD-C170 LED. At 10 mA each (thus 20 mA total) they already emit more than enough light for this application. Besides, currents much higher than 20 mA would increase the power dissipation in the resistor, requiring a bigger resistor with a higher power rating. But space is limited inside the HVAC control unit and lower currents result in a higher efficiency of the circuit, so small low-power resistors get the preference.



  • R1: Resistance [Ω]
  • V: Battery voltage [V]
  • VBE: Transistor base-emitter voltage [V]
  • VD: Total LED forward voltage [V]
  • ID: Total LED forward current [A]

Note the calculation uses 14.5 V instead of 12 V for the battery voltage. Cars are indeed equipped with 12 V lead-acid batteries, but to charge such batteries a slightly higher voltage is required. That's why the alternator and voltage regulator deliver 13.8 V (in practice 13.5–14.5 V). The voltage of a car's electrical system is thus higher when the engine is running. That's an important fact because when the resistor was dimensioned for a true 12 V system, the LED may operate outside maximum current specifications while the engine is running! When dimensioning circuits, always start from the worst-case scenario – in this case 14.5 V.

Not only the LEDs cause a voltage drop that has to be subtracted from the battery voltage. The transistor has a small voltage drop over its base and emitter – VBE. Because of this drop, the voltage at the emitter can never reach the battery voltage. For the used BC547 transistor this voltage is 660 mV.

The value of 362 Ω does not exist in the popular E12 series. The E12 series offers only a limited set of twelve values and their tenfolds. The E12 base numbers are 1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, and 8.2. Unless really close to a lower value, always pick the next higher E12 value. A lower value means a higher current which could cause the LEDs to operate outside specifications. The closest E12 value for 362 Ω is 390 Ω.

By limiting the current through the LEDs, the resistor also consumes power – power that's dissipated into heat. It's important the power dissipation is within specifications too. A standard resistor can handle only up to ¼ W.


At 134 mW maximum power consumption, the resistor is well below the permitted 250 mW. All information to build the brightness control is now gathered.

The brightness control is simple and fits on a very small piece of prototyping board with single islands and standard 0.1" pitch. Normally prototyping board is meant for circuits in development and not final designs. It doesn't look professional and is more time-consuming to solder because all traces must be soldered by bridging islands. However, prototyping board is readily available in electronic components stores and does not require special tools to develop printed circuit boards (PCB). When built as described in this article, the dimmer can be mounted inside the HVAC control unit and can be adjusted from the rear through the lamp opening.

It's best to start with cutting the board to the correct dimensions. The material can easily be sawn with a hacksaw. The rough sides can be worked away with a smooth flat file. The final board should be 3 x 9 islands large. Next use a 2.5 mm (3/32") metal drill bit to make the mounting hole. Take a look at the print layout for its exact location. Not too close to the edge of the board but certainly not too close to the trim potentiometer. The screw head should under no circumstances touch the potentiometer's connection leads. If both leads make contact via the screw head, a short circuit occurs. It's recommended to put a small piece of insulating plastic foil between screw head and potentiometer. Note that a regular top adjustment trim potentiometer covers five islands while the print layout only has room for four islands. Bend the potentiometer's leads on the side with two leads so it fits on four islands. This way the distance between connection leads and screw is increased. Moving all components one island to the right wouldn't work as the potentiometer must be in this exact location to be adjustable from the rear of the HVAC control unit.

Built-up brightness control

Positioning the LEDs

Take one of the roller wheels for a closer inspection. Two white rings with a cut-out in the center position are visible when looking into the heart of the roller wheel. To provide a good illumination of the wheel, light must be distributed between these two white rings.

Round LEDs facing down

The depth at which a round LED is placed inside the roller wheel is critical. Most of the light is emitted downward and thus lost. Therefore it is important to get the LED in exact position to get as much light as possible between both white rings. The epoxy lens case of round a LED is slightly thicker at the base – as if there's a ring on it. The best location for the LED is when this thicker ring at the LED's base is at exact the same height as the top white ring inside the roller wheel. That's 8 mm (0.315") deep in the wheel. Bend the LED's leads in a right angle at this distance from the base. Don't forget LEDs have polarity. To avoid confusion or wiring issues later on, bend the leads of all three LEDs in the same direction. Each LED should be placed so it just doesn't touch the top white ring in the roller wheel. Too much to the front and the LED may grip in the center position cut-out of the white ring which will give the wheel an annoying audible click around the center position. Too much to the rear and the axle will no longer fit in the roller wheel.

Round LED positioned inside a roller wheel

If – even after carefully measuring – the LED isn't in the correct place after mounting, its position can still be changed a bit by somewhat bending the leads.

SMT LEDs facing forward

The position of SMT LEDs inside a roller wheel is less critical as is the case with round LEDs. Thanks to the SMT LEDs' large viewing angle and even light distribution anywhere between 9 mm (0.36") and 14 mm (0.55") deep in the roller wheel is fine. Unlike standard round LEDs, SMT LEDs do not have connection leads that can act as a support to position the LEDs inside a roller wheel. Some sort of support must be constructed. The picture below shows an example of a support made of leftovers – a small piece of prototyping board and a tall 2-pin header. You are of course free to use a different support. For example, stiff wires make a good support – much like the round LEDs' stiff leads.

SMT LEDs on a support made of leftovers. A pair of stiff wires would make a good support as well. Two LEDs in parallel are only required when using very dim LEDs like the Avago Technologies HSMD-C170

When using a similar setup, keep the pins of the header on the solder side of the board as short as possible to avoid touching the potentiometers on the HVAC control unit PCB. Space is limited inside the enclosure. When working with the Avago Technologies HSMD-C170 LEDs each support should have a pair of LEDs put in parallel. For the larger and brighter HSML-A101-S00J1 a single LED per support is sufficient. Always pay attention to the polarity. All LEDs should be soldered in the same direction.

SMT LEDs positioned inside a roller wheel

Building the LED bar

Mounting the LEDs and then soldering the wiring inside the HVAC control unit's enclosure is not practical. It's far more practical to wire and solder everything before mounting the LEDs inside the unit. Solder the wires in a way that provides some play in the distance between the LEDs. Don't forget the wires to the left roller wheel (the right one seen from behind) have to make a detour around the temperature sensor's tube. Take the old plastic light bar as guidance when wiring the LEDs. Keep the wires together with tape or – even better – heat-shrink tubing.

LED bar with round LEDs
LED bar with SMT LEDs

Mounting the LED bar

This is the tricky part... Put the roller wheels back in the enclosure together with the LED bar. The roller wheels protrude a few millimeters on the front side. In other words, if the enclosure is facing down on a flat surface, the roller wheels are not in the correct position. Pay attention to this! When everything is in correct position, use a few drops of hot melt adhesive to fixate the LED bar in the enclosure. Hot melt glue has several advantages: It creates bonds more than strong enough for applications like this, yet it can be removed easily – if ever needed – when applied in blobs. Thin layers are more difficult to remove, though. Another advantage is the hardening time. Within a few minutes at most the glue is hardened enough to let loose and it reaches full strength in less than ten minutes. A disadvantage of hot melt adhesives is stringing, but the strings are very thin and cool down rapidly without sticking. The strings can easily be removed.

The brightness control can be mounted once the glue is hardened. There is a screw hole near the location where the incandescent light bulb was placed before. Use one of the screws from the original plastic light bar to mount the brightness control in this hole. The screw hole was previously unused, thus driving a screw in may require some force. The circuit board should be mounted tightly – it may not be able to move around and rattle. Double check the screw head and trim potentiometer leads don't touch each other. In case of doubt, put a small piece of insulating plastic foil between screw head and trim potentiometer.

LED bar and brightness control installed in the enclosure

Reassembling the HVAC control unit

Reassembly of the HVAC control unit is very much the same as the disassembly, but in reverse direction. Put the lower PCB back into the HVAC control unit's enclosure. The importance of the brightness control's location and dimensions is now clearly visible – the dimmer fits exactly in the lower PCB's cut-out. Verify the LEDs or the LED supports do not obstruct the three potentiometers on this PCB. If all is well, tighten the lower board with the two screws. Turn the roller wheels back to the center position and put the axles back in place. The axles' lips should point to the front of the unit. Operate the roller wheels and verify everything works smoothly. If a notch can be felt around the center position, remove the axles and lower PCB again and adjust the LED's position a tiny bit.

Installed brightness control. Note the close fit in the cut-out area of the lower PCB

Although the brightness control and LED bar are already mounted, they still need power to actually work. The most logical power source is the light bulb's socket – the tin-plated surfaces surrounding the bulb opening in the top PCB. Unlike LEDs incandescent light bulbs do not have polarity, so there is no indication on the PCB what the positive or negative terminal is. The tin-plated surface closest to the edge of the PCB is the negative terminal or ground (GND). The tin-plated surface on the opposite side of the opening – where text is printed on the PCB – is the positive terminal (+12V). The terminals are available on both sides of the circuit board, but it is nicest to solder the wires to the brightness control on the bottom side of the top PCB. This hides the wires and solder joints from the eye when looking at the backside of the fully assembled HVAC control unit.

Next put the top PCB back in place and make sure the wiring to the brightness control does not obstruct access to the trim potentiometer through the light bulb socket. Secure the board with the two screws and put the backside on with the four screws. Don't forget to connect the small fan on the rear. The HVAC control unit is now fully assembled. If all is well, the trim potentiometer should be easily accessible from the rear through the lamp opening.

The trim potentiometer is exactly below the lamp opening making it possible to adjust the brightness without disassembling the unit

Adjusting the brightness

Turn the trim potentiometer all the way to the left (counterclockwise). By doing so the transistor's base is connected to the ground and no current can flow. This is just a safety measure even though the LEDs should be protected by the current-limiting resistor – if properly dimensioned.

Connect the HVAC control unit back to the E31 wiring harness. No need to install it yet! Just inserting the large blue connector is enough. Even the center console may be left out of the car if it was previously removed. The MID, radio, microphone and the buttons surrounding the shifter do not need to be connected.

Insert the key in the ignition and turn it to position I. Turn on the lights. The E31 instrument illumination should now light up. The roller wheels on the HVAC control unit remain dark – that's normal; the dimmer is at its minimum position. Now turn the trim potentiometer until the illumination of the roller wheels is equally bright as the buttons on the HVAC control unit. It's very likely the potentiometer needs to be turned at least half way before the roller wheels light up. To avoid accidentally short circuits while adjusting in the dark, use a plastic screwdriver or anything non-conductive that fits.

HVAC control unit with modified illumination for buttons, button indicators and roller wheels (bottom). The brightness and color matches perfectly with the multi-information display and radio (which are modified as well)


Once satisfied with the result, it is time to prepare the car for the road again. Mount the HVAC control unit in the center console, hook up all connectors and put the center console back in place. Before you tightly screw the center console in place make sure the radio harness and MID connector can be reached and didn't accidentally get stuck below the center console. If all is fine, reinsert the radio and MID and put the shift lever's leather boot back.

HVAC control unit installed in center console (bottom)