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Thread: LKM Modification For LED Exterior Lighting

  1. #1
    Join Date
    Sep 2017
    Vienna, VA
    My Cars
    1995 525i E34 6-speed

    LKM Modification For LED Exterior Lighting

    First, I should explain my reasons for doing this mod. I wanted to lower the load on the alternator while also improving the light output of my car. I also did not want to constantly be told that all of my bulbs were burned out. This meant that the normal solution of installing a load resistor to suppress warnings caused by LED bulbs would not work because it would still draw all of the power of a normal bulb. I am aware that there are a few great guides on how to cancel low beam warnings, but I wanted to cancel ALL warnings.

    How the LKM works:

    The BMW E34 and E32 chassis are equipped with a light control module that contains the relays for the high and low beam headlights as well as the front and rear (if equipped) fog lights. Power for all of the rest of the exterior lights passes through this module as well. This is because this module is responsible for informing the driver of any bulb failures or malfunctions. Bulb failures are detected using two different methods: a cold test and a hot test.
    The cold test is always running and works by applying a small amount of power to the positive side of the bulb being tested. A comparator chip looks for a flow of current to determine if the bulb is burned out; if there is no flow, the circuit must be open, so it is assumed the bulb is burned out and this is reported to the driver. If there is current flow, the circuit must be closed and so it is assumed that the bulb is still ok. It is important to note that this test is being run even when the car is turned off, even if the key is not in the ignition.
    The hot test is only done when the bulb being tested is turned on (or at least, when it is supposed to be on). The test works using a comparator chip and a resistor of a known value. In our case the resistor happens to be a large trace on the circuit board instead of a component mounted to the board. When the bulb is turned on, all of the power going to it must first pass through the resistor; the comparator chip tests the voltage of this power just before and just after the resistor. This allows the chip to calculate the current draw (amperage) of the bulb. This value is then compared to the allowable range hardwired from the factory. If the amperage is outside the allowable range, the bulb is reported as failed to the driver.

    Lights that are cold tested:

    • Left low beam
    • Right low beam
    • Front fog lights
    • Rear fog lights (?) (euro)
    • Front marker lights

    Lights that are hot tested:

    • Low beams (individually)
    • Front markers (one of the filaments in the turn signal bulb)
    • Front fogs
    • Rear fogs (euro)
    • Tail lights
    • Brake lights (individually)
    • Third brake light
    • License plate lights

    Lights that are NOT tested by the LKM:

    • Turn signals (double
    • Reverse lights
    • High beams
    • Bumper “reflector” lights (US equipped, probably burned out)

    The LKM also has a test function for the brake light circuit, and check control error.

    Last thing before starting the modifications is to identify the components on the circuit board.
    ///Figure 1///

    The two chips circled in blue are serial communication chips that talk to the instrument cluster and because they are surface mount chips (on my board, other versions are not the same) I did NOT be mess with them.

    The three spaces (forgot to take before pics, there are supposed to be chips there) outlined in red are the comparator chips. They were AD22001N, and appear to be common on all LKM revisions that have a single board (early LKM’s have two boards with a ribbon cable between, see Shogun’s LKM fix guide). For the purposes of this guide, I will refer to the comparator chips starting from the left as Chip 1, chip 2, and chip 3. (only applies with pcb flat on table and connector towards you, see photo above)

    I chose to modify the comparator chips. The summary is that I tricked the serial chips into thinking that everything was ok by hardwiring the “all clear” output from the comparators.
    Required tools:

    • Soldering iron
    • Something to hold the board securely
    • Multimeter

    Required supplies:

    • 1 LKM (I recommend using one from pick-a-part over the one in your car, so you can always undo the mod)
    • Scrap wire (solid breadboard jumper wire works great)
    • Solder (the lower the melting temp the better, less stress on the pcb)


    • Additional LKM (used for OEM rear fog mod)
    • 1 turn signal relay (used for LED turn signal mod)

    Step 1:
    Open up the LKM you have chosen to modify, I would highly recommend against using the one that came with your car because if you damage it you will have NO exterior lights. There are 5 tabs in the plastic housing that hold it shut, try not to break them.

    Step 2:
    Remove the three AD22001N chips, if you cannot locate three, you either have the base version of the LKM (euro only) or you have too old of a LKM for this DIY. I’m pretty sure that they are all interchangeable, but if someone knows otherwise… don’t burn down your E34.

    Step 3:
    Using the pinout for the AD22001N chip, locate pins 3, 7, 12, 13, and 18. These are the outputs of the comparator chip. To pull a pin low, solder it to the ground plane under the chip, or to pin 20. To pull a pin high, solder it to pin 2, this is the power supply pin for the chip.

    Step 4: Make connections as follows:

    • Chip 1 (left)

    -pin 3, no warning, pull high
    -pin 7, F/Fog Light, pull low
    -pin 12, not connected
    -pin 13, Lic plate light, pull low
    -pin 18, brake light circuit, pull low

    • Chip 2 (center)

    -pin 3, check control error, pull high
    -pin 7, R/Fog light, pull low
    -pin 12, no warning, pull low
    -pin 13, parking light, pull low
    -pin 18, 1 brake light, pull low

    • Chip 3 (right)

    -pin 3, low beam + brake light circuit, pull low
    -pin 7, low beam + tail light, pull low
    -pin 12, no warning, pull low
    -pin 13, low beam, pull low
    -pin 18, brake light circuit, pull low

    Step 5:
    Disable the cold test circuit by removing every SMD resistor that has “332” on it. If you do not do this power will ALWAYS be flowing through any bulbs that are cold tested. It is not enough power to light up a filament, but it will light up or flash an LED (even when the car is off).

    Step 6:
    Here in the US you cannot have your fog lights, low beams, and high beams on at the same time. To achieve this, the LKM grounds the fog light relay coil through the output of the high beam relay. With halogen bulbs this is fine, but with LED’s I found that turning on the fog lights would cause the high beams to flicker or come on dimly. So, to avoid being pulled over, I removed the diode making this connection and gave the fog lights a proper ground. Doing this isolates each relay and places the responsibility of not blinding people in the hands of the driver. Please use this power respectfully.
    ///figure 2 and 3///

    Step 7 (Optional):
    Using second LKM, remove one relay and one SMD resistor labeled, “561”. Attach these two components to the modified LKM as shown.
    ///figure 4, 5, and 6///

    Step 8 (optional):
    Use the following link to modify the turn signal relay to allow for LED bulbs.

    Step 9:

    Attachment 626791Attachment 626792Attachment 626793Attachment 626794Attachment 626795
    figures 1 through 5 starting from the left. (figure six will be in separate post)

    Shout out to Shogun for his posts on this topic, and the many other contributors who have spoken on this topic, it would take me forever to track them all down!
    Attached Images Attached Images

  2. #2
    Join Date
    Oct 2014
    Prescott, Az
    My Cars
    '95 540i/6
    Great write up, thank you!

  3. #3
    Join Date
    Jan 2005
    My Cars
    11/88 E32 750iL+98 E36M3
    LKM-L and LKM-B pinout

    1 High Beam stalk at steering wheel
    2 terminal 30 (continious power)
    3 switch low beam/dip light
    4 data to CCM = check control module
    5 data to CCM = check control module
    6 F1 terminal R, IC supply
    7 from k7 P2 - tail light right

    8 tail light right
    9 tail light left
    10 parking light right
    11 parking light left
    12 from brake light switch

    13 brake light left
    14 switch licence plate light on
    15 3rd brake light
    16 fog light front left
    17 F7 15A, terminal 30
    18 fog light switch
    19 rear fog light switch

    20 high beam via fuse F13, F14
    21 low beam left F10
    22 low beam right F11
    23 check via CCM + CC + OBC (check control module-check control-on board computer)
    24 ground
    25 data to CCM

    26 from K7P6 tail light left
    27 from K7P9 parking light right
    28 from K7P5 parking light left
    29 brake light right

    30 licence plate right
    31 licence plate left
    32 fog light right, front
    33 fog light right, rear
    34 fog light left, rear
    35 F8 15A for rear fog light

    Note: this is the ECE/EU version, some pins might be missing in other versions. But the set - up will be the same. So for the US LKM probably the pins for rear fog light might not be there respect. not connected internally.
    Depending on year of production and the makers of the LKM the internal parts look different, but the pins are always for the same function.
    The pins in one row are 1 - 19 and the other row 20 - 35

    and here is the CCM, and see the brake light test switch pin number

  4. #4
    Join Date
    Sep 2017
    Vienna, VA
    My Cars
    1995 525i E34 6-speed
    Attachment 626812Attachment 626813Attachment 626814Attachment 626815Attachment 626816
    from ltft to right, the pinnout, figure six, and then the modifications to the chips (1-3)
    Attached Images Attached Images
    Last edited by shogun; 04-12-2018 at 09:28 AM.

  5. #5
    Join Date
    Sep 2017
    Vienna, VA
    My Cars
    1995 525i E34 6-speed
    So as can be seen in the photos, in the case of the LKM that I worked on, the board would be the same for US and Euro cars, it was just populated different. I will post pics of internals of other LKM's but I think that most LKM-L modules share the board and that they are populated different for different markets, as oppose to having a different board for US vs Euro markets. However, if someone has evidence otherwise, I would love to see it! I really enjoy learning about the tiny little revisions and updates made to these cars during their production cycles. Plus it can only help future diy'ers.
    Last edited by shogun; 04-12-2018 at 09:27 AM.

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