HVAC control unit button illumination

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HVAC control unit

The modification of the HVAC control unit button illumination takes on one of the BMW 8 Series' design flaws; the inconsistent illumination of certain controls and instruments. The HVAC control unit roller wheel illumination – or rather lack of illumination – is a good example of this. The buttons are better lighted, but after modifying the roller wheel illumination a brighter illumination of the buttons may be desired. This article is a step-by-step guide for those who want to fix this issue with a do-it-yourself (DIY) approach. The required skill level is very low – only basic soldering experience is needed.

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.

Opening the HVAC control unit

Once the HVAC control unit is out of the car, it is time to open 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...

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 modification of the roller wheel illumination 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. Do not remove it. It's held in place with two screws, but also by the three axles of the roller wheels. If the board is lifted without removing the axles first, the potentiometers on the board may get damaged. But, the modification can be performed without taking the board out.


HVAC control unit button illumination circuit diagram

Reverse engineering of the HVAC control unit illumination shows there are three groups of each three LEDs in series with a 2.2 kΩ current-limiting resistor. As their name implies, the resistors limit the current through the LEDs. The brightness of a LED is related to the current flowing through it. Thus, to adjust the brightness of the LEDs the current-limiting resistors must be replaced with different values. There are two potential issues, though. It may be impossible to increase the current through the LEDs if these already operate at maximum current specification, and one of the resistors is located underneath the buttons making it very difficult to replace.

The actual current through the LEDs can easily be calculated, but the calculation requires the forward voltage over a single LED. This can be looked up in the LED's datasheet. However, the exact type of LED used by BMW is unknown. The forward voltage can also be measured, but orange LEDs feature a forward voltage typically between 1.8 V and 2.2 V. The lower the forward voltage, the higher the current through the LED for a given resistance. Always use the worst possible conditions in equations to make sure the circuit never operates outside specifications. In this case that means a forward voltage of 1.8 V. The current through the LEDs is:


Note the calculation uses 14.5 V for the 12 V automotive electrical system. 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! As said earlier, always start from the worst-case scenario – in this case 14.5 V.

A current of 4.14 mA is very low, but some high-efficiency low-power LEDs have a maximum current rating of only 5 mA. Increasing the current over the maximum rating will shorten a LED's operating lifetime or even kill it at once. However, during the days the HVAC control unit was designed there were no affordable high-efficiency low-power LEDs. Most if not all LEDs had a maximum current rating of at least 20 mA. There is no reason to assume the LEDs in the HVAC control unit can't take 20 mA, but the author only tested up to 10 mA. It is recommended to keep the current below 10 mA too, just to be sure.

Experiments showed the illumination is perfect with a current-limiting resistor between 1 kΩ and 1.2 kΩ. The brightness then equals the illumination of the seat heating button – which is one of the best illuminated buttons in the E31 and is thus a good reference. But as said earlier, one of the resistors is in a difficult location underneath the buttons. It can be replaced, but that would involve desoldering the buttons. That's a very time-consuming job and if not experienced with desoldering the PCB may get damaged. A better approach is to decrease the resistance by putting a resistor in parallel over the existing resistor. The parallel resistor can be soldered on the opposite side of the PCB – onto the leads of the existing resistor – so there is no need to remove the buttons. When a resistor is put in parallel with another resistor of exactly the same value, the parallel resistance is cut in half. In case of two 2.2 kΩ resistors in parallel, the total resistance is thus 1.1 kΩ – which is exactly the wanted resistance:


The current through the LEDs is:


The table below shows the relation between the parallel resistance and the current through the LED. A selection of E12 series resistors is put in parallel over the existing 2.2 kΩ resistor.

Relation between parallel resistance and current
R1 R Rparallel ID
2200 Ω 8200 Ω 1735 Ω 5.25 mA
2200 Ω 6800 Ω 1662 Ω 5.47 mA
2200 Ω 5600 Ω 1579 Ω 5.76 mA
2200 Ω 4700 Ω 1499 Ω 6.07 mA
2200 Ω 3900 Ω 1407 Ω 6.47 mA
2200 Ω 3300 Ω 1320 Ω 6.89 mA
2200 Ω 2700 Ω 1212 Ω 7.51 mA
2200 Ω 2200 Ω 1100 Ω 8.27 mA
2200 Ω 1800 Ω 990 Ω 9.19 mA
2200 Ω 1500 Ω 892 Ω 10.2 mA
2200 Ω 1200 Ω 776 Ω 11.7 mA
2200 Ω 1000 Ω 688 Ω 13.2 mA

The picture below shows the exact locations where the three parallel resistors should be mounted on the lower PCB. The existing resistors are in the same location but on the other side of the board. The parallel resistors are to be soldered onto the leads of the existing resistors. Use standard ¼ W resistors for this application.

The exact locations of the three parallel resistors on the PCB

Cut the leads off from the parallel resistors at around 3 mm (0.1") from the resistor. Bend the remains of the leads back. One of the problems frequently occurring when soldering is that three hands are needed; one to keep the component in place, one to apply solder and one to handle the soldering iron. To make this a bit easier, apply extra solder to the existing solder joints (where the parallel resistors will be soldered onto). Now grab a parallel resistor with long-nose pliers or needle-nose pliers and hold it in position. Heat one of the leads and the solder joint until the solder flows and attaches to the lead. Wait for the solder to cool down and fixate, then repeat for the other lead and the other two resistors. Be careful not to touch the roller wheels or plastic enclosure accidentally with the soldering iron. Use soldering wick (desoldering braid) or a solder sucker to remove excessive solder if necessary.

Once ready, put the top PCB back in place, 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 again.

Adjusting the brightness

The title of this chapter is misleading. The brightness cannot be changed without replacing the resistors. But the roller wheel illumination is not affected by this modification. So after increasing the brightness, the roller wheel illumination will no longer match. It needs to be adjusted. If the modification of the roller wheel illumination is already performed, use the trim potentiometer in the light bulb opening to adjust the roller wheel brightness. Otherwise see what alternatives are available.

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 buttons on the HVAC control unit should be brighter than before. Adjust the roller wheel illumination until buttons and roller wheels match. If not happy with the button illumination, replace the parallel resistors with different values until satisfied.

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)