HVAC control unit button indicator illumination

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HVAC control unit

One of the BMW 8 Series' design flaws is the inconsistent illumination of certain controls and instruments. The HVAC control unit is a good example of this: The roller wheel illumination is close to non-existing and the button illumination is dim. The modification of the HVAC control unit button indicator illumination described in this article does not address a design flaw, but is rather cosmetic. The button indicators are illuminated with green and yellow light-emitting diodes (LEDs). With almost all other instruments illuminated in BMW orange, some people may find the yellow and green indicators out of tone. In this article the green and yellow LEDs are replaced with BMW orange LEDs for an all-orange look.

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 partially dismantle the unit. 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

Unplug the fan's connector on the rear side, remove the four screws and take the backside off. If the roller wheel illumination modification was not yet performed, there is an incandescent light bulb in one of the corners of the printed circuit board (PCB). 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.

Illumination

The lower PCB reveals the LEDs for the buttons. The pinkish LEDs with flat top illuminate the button symbols in BMW orange, the single yellow LED is the status indicator of the maximum defrost button and the remaining LEDs are – despite the colorless transparent plastic lens – the green status indicators. If the car is equipped with automatic recirculated air control (UAC), the recirculated air button has two status indicator LEDs – otherwise only one.

HVAC control unit printed circuit boards

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. The original LEDs have a standard 3 mm (T1) round package, thus any regular 3 mm round LED will fit just fine. Since the LEDs are aimed towards the driver and the emitted light is only obstructed by a small transparent window in the button cap, the LEDs' intensity should be low to not to distract the driver. A luminous intensity of 10 millicandela is enough. Finding LEDs this dim may prove to be a challenge because most LEDs emit a multiple of this – up to thousands of millicandelas. The LEDs are close to the front of the button caps – especially in the depressed position. Therefore it's important the LEDs have a large viewing angle – preferably well over 60° – to get an even illumination of the indicator windows. A good choice is the LED Technology L08R3000F1 LED:

LED Technology L08R3000F1

  • Package: 3 mm (T1) round – 12.7 mm (½") leads
  • Wavelength: 610 nm BMW orange
  • Viewing angle: 74°
  • Luminous intensity: 9.7 mcd
  • Forward current: 20 mA
  • Forward voltage: 2.1 V
LED Technology L08R3000F1

The L08R3000F1 features short leads of only 12.7 mm (½") while the original LEDs have tall 25.4 mm (1") leads. In other words, the leads must be extended. An alternative that comes with tall leads is the Kingbright L-934ND. This LED makes the soldering a bit easier, but the result may not be as good because at 30 mcd the LED is just a bit too bright and the 60° viewing angle is the minimum for this application:

Kingbright L-934ND

  • Package: 3 mm (T1) round – 25.4 mm (1") leads
  • Wavelength: 610 nm BMW orange
  • Viewing angle: 60°
  • Luminous intensity: 30 mcd
  • Forward current: 20 mA
  • Forward voltage: 2.05 V
Kingbright L-934ND

The best choice is the Avago Technologies HSMD-C170 LED. With an impressive 170° viewing angle an even indicator window illumination is guaranteed. The disadvantage is that it comes in a very tiny surface-mount technology (SMT) package. SMT components do not have leads and take some practice to solder due to the small dimensions:

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

Removing the existing LEDs

The existing LEDs are soldered to the PCB. They must be desoldered to remove. Luckily LEDs have only two leads, so desoldering isn't that difficult. It's best to use a chisel-shaped soldering iron tip around 2.5 mm (0.1") wide. Both solder joints can be heated at the same time when using a wide tip. This makes removal a lot easier. Do not use soldering wick (desoldering braid) or a solder sucker to remove the solder to aid the removal. The soldering iron tip gets better thermal contact when there is a bit of solder flowing around it. In fact, applying a small bit of fresh solder may even help. Make sure the tip is clean and well-tinned for optimal thermal contact. When desoldering pay attention to the following:

  • Avoid overheating. Overheating will obviously damage the LEDs but even if you don't want to recover the existing LEDs overheating is to be avoided. A printed circuit board starts from an epoxy board with a thin layer of copper glued onto one or both sides. The excessive copper is then removed in an etching process until only the necessary traces remain. Overheating may dissolve the glue that keeps the traces on the epoxy board. Once traces start coming off you are in trouble... If a LED doesn't come loose after heating its solder joints for a few seconds, remove the soldering iron and wait a short while before taking a new attempt to allow both LED and board to cool down.
  • Do not pull a LED when its solder joints are not heated with the soldering iron. The LED won't be damaged by this, but there is a chance of pulling the traces off the PCB. When heated properly the LED should come off quite easily.

Once all green and yellow LEDs are desoldered, clean the solder islands using soldering wick (desoldering braid) or a solder sucker. Remove the three black plastic LED supports. Each support is clicked into the metal frame of the switches it is placed next to, so they may be a bit difficult to remove. Clean the solder islands on the components side of the PCB. Perfect is when you can see through all holes, but don't spend too much time on this if it won't work.

Placing the new LEDs

Depending on the chosen LEDs, the method to solder the new LEDs onto the board differs. The easiest approach is when using the Kingbright L-934ND LEDs because these are very similar to the original LEDs. Use long-nose pliers or needle-nose pliers to bend each LED's legs similar to those of the removed LEDs. Pay attention to the LED's polarity – bend the legs in the right direction! The polarity of all LEDs is in the same direction – the anode must face the side of the PCB where the cut-out is for the incandescent light bulb or the brightness control of the roller wheel illumination modification. Insert the LEDs in the plastic supports and put the support back on the PCB with the LEDs' legs through the holes. When the holes are not open, use the soldering iron to heat the islands and push the LEDs through. Do not push the LEDs all the way down onto the support. First click the supports back in place, then solder the LEDs pushing them down for a tight fit on the supports.

The original green and yellow LEDs replaced with BMW orange round LEDs

When using the LED Technology L08R3000F1 LED with short leads or the Avago Technologies HSMD-C170 with no leads at all, a different approach is necessary. Shorten the leads of the L08R3000F1 LED to around 5 mm (0.2"). The tallest lead of a LED indicates the polarity. By cutting both leads to equal length this polarity indication is lost. But don't worry, the L08R3000F1 has a flat spot on the edge indicating the cathode and even though the L08R3000F1 has a diffuse plastic lens, its internals are clearly visible showing the large cathode and small anode. Apply a thin layer of solder over the remaining leads – this will aid the soldering later on. When using the SMT LEDs it is also recommended to tin the terminals. This may prove to be a challenge with LEDs this small. The LEDs are way too small to keep in position with a finger while soldering and weigh so little they will stick onto the soldering iron's tip. Use some creativity to aid the soldering. For example, wrap a rubber band around the hand grips of long-nose pliers or needle-nose pliers so they stay shut and clamp an LED between the tips. Or paste the LEDs upside down on double-sided tape. Tweezers to handle the LEDs are no superfluous luxury.

Cut new leads from solid wire stripped from its insulation. The wires must be around 30 mm (1.18") long. Bend all wires just like the original LED legs. Apply a thin layer of solder over the straight part where the LED will be soldered onto the wires. That will help the soldering later on. Put all wires in the plastic supports. Perhaps use tape to keep the wires in position. Then put the support back onto the PCB with the wires through the holes. Again, when the holes are not open, use the soldering iron. Once all wires go through the PCB, click the supports back in place and solder the wires.

The wires should protrude a few millimeters above the plastic supports. When using 3mm round LEDs, cut all wires exactly to the height of the plastic supports. When using SMT LEDs cut the wires about 1 mm (0.04") above the supports. Solder the LEDs in place. Thanks to the tinning of the wires and LEDs, it should be fairly easy. Pay attention to the polarity! The anode must face the side of the PCB where the cut-out is for the incandescent light bulb or the brightness control of the roller wheel illumination modification.

The original green and yellow LEDs replaced with BMW orange miniature SMT LEDs

The picture above shows the HVAC control unit PCB with SMT LEDs. Thin heat-shrink tubing keeps the LEDs in position, but that's optional and probably not required.

Reassembly and installation

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. 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. Next put the top PCB back in place and secure the board with the two screws. 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.

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. Test all buttons.

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)