The word “hack” as many meanings, but In this blog, I’m focused on the most useful one. ¬†ūüôā ¬†In today’s world of ultra productivity and always connectedness, for any activity, the pressure is always there to find ways to make your projects and life more efficient. The word “hack” in this blog, is defined as a noun that describes an activity or project that can be done more efficiently. For example, adding a hands-free capability to the BMW E36 radio using a Bluetooth speaker so I can listen and use my iPhone while driving to work. Or to add a Lawn Light controller that will send me a notice when one of my lights is not working so I don’t have to remember to inspect them. ¬†In this post, I’ll show a simple fit to your BMW M3 E36 Licence Plate Lights when one is not working because only ONE wire has broken, saving hours or rework to find the exact location of the failure. ¬†If your E36 has the same or similar problem, then this hack is for you.

 

Recently, my BMW computer gave me an error that my rear license plate light had failed, So I went ahead and replaced the light. Not surprisingly, this didn’t work, or I would not have to write this blog. I went on line and found the schematic of the E36 to help with the debug.

BMW E36 M3 License Plate Light Schematic
BMW E36 M3 License Plate Light Schematic

In my case, the right License Plate Light was not working and from the schematic, the issue was either the gray/black/yellow power wire (pin 2 of connector X710) or the brown ground wire (pin 1 of X710).    After some measurements, I determined the issue was with the brown ground wire.

As it turns out, for the BMW E36, all the lights attached to the rear trunk, including the license plate light, have a common failure point without an easy way to fix it.   For each opening and closure of the turn, the rear light wiring harness, where all the wires to the lights are bundled, will bend at one location.   Eventually, this is where the wires will fail.

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Wiring harness bend and eventual failure point due to trunk open and closure

As you can see my nephew already repaired this several years ago and I did not want to go hunting again in this area for the license plate wire failure.

Using my multimeter, I verified that the ground wire going to the left license plate light had failed.    After removing both rear tail lights, I turned on the car light switch.  I measured the voltage across the good license plate light and recorded 11.75v.    Then I still had one wire of the multimeter connected to the ground wire of the good license plate light and measured the voltage to the bad license plate light.  This also measured 11.75v.    So most likely the ground wire of the bad license plate light was faulty.   To verify, I turned OFF the lights to the car and then measured the impedances between the two Power connections of the rear license plate lights.   The impedance was very low <10ohms.  Then I measured the ground wires and measured an ultra high impedance, confirming the issue with the ground wire.  Since both ground wires are in the same location to the rear trunk, after checking the current capability of the wire, I simply used the good license plate light ground wire and wired it to both lights.

Some simple calculations were needed to confirm this fix would not cause any issue with the car’s safety. ¬† The Schematic shows that the ground wire has an area of 0.75mm2. or 1480 circular mils. ¬†This is close in size to at 19 AWG wire gauge.

AWG wire gauge.png
The ground wire is larger than 19AWG.   19 AWG has a resistance of 9 ohms for each 1000 feet at 50 degrees C.

The ground wire is attached to the chassis behind the Left rear passenger seat, so the length is close to 10 feet and the size will have no issue carrying the return current of each light.     19 AWG has a resistance of 9 ohms for each 1000 feet.   So the resistance of this ground wire is 9/100010 = 90mOhms.   The license plate lights have a rating of 10W at 12v, giving a current of 0.83Amps.   By doubling up the current, the round wire will have 1.667Amps and a power dissipation of 1.667^209 = 25mWatts.   The heat rise will be tiny since it is distributed over the 10 feet of wire length.      The chassis ground location is behind the rear passenger seat.  Looking at the size of this ground splice connection, it also can easily handle the additional ground wire current.

I expect that over time, I will need to re-fix the complete harness, but with only one bad wire, this was a 20-minute fix.

Let me know if you have any questions and please subscribe.  If you have any ideas of other hacks also let me know in the comments below

References:

  1. See the surfncircuits.com youtube channel: surfncircuits youtube channel
  2. Wiring harness Schematic.  See link at BimmerForums 
  3. Location of the Chassis Ground Connection.  Youtube video