Headlight (and Fog/Driving Light) Wiring Upgrades

While the electrical system of the 2002 is no Lucas catastrophe, it's still 60's level technology and has its fair share of shortcomings.  I'll refrain from labeling it 'poorly designed,' but the headlight circuit is one of the primary 2002 electrical systems that has lots of 'room for improvement.'  Also, with the popularity of adding additional fog and/or driving lights to these cars, this makes for the perfect time to rewire and improve this whole system while adding such upgrades.

A quick note on 2002 wiring

Generally speaking, Germans are clever, organized people, and this shows in the 2002 wiring.  Here are some of the more subtle but clever points to keep in mind when looking at the 2002 wiring diagram:

1.) The SOLID RED wires all go straight to the battery.  These are the ones to be careful with.  Don't short one and blow up your battery! Everything electrical component on the car can be traced back to one of these red wires for its ultimate power supply (usually via the ignition switch).

2.) All SOLID color wires are UNFUSED.  Screwing up and shorting one of these wires will likely damage something.

3.) The SOLID GREEN wires go to the ignition switch and are hot (+12V from the battery) when the key is in the 'Start' and 'Run' positions.

4.) The SOLID PURPLE wires go to the ignition switch and are hot (+12V from the battery) when the key is in the 'Accessory' and 'Run' positions.

5.) All 2-color or STRIPED wires are FUSED and therefore safer; shorting one of these will just blow the corresponding fuse.

6.) The SOLID BROWN wires are always ground. Every electrical component on the car eventually terminates to ground via one of these brown wires (You probably already knew this one!)

The Problem:

A relay is a device used to switch a high-current load (e.g. a big bright headlight) based on a much lower-current input signal (often a switch).  The early roundie 2002 low beams use a relay like this, however the high beams have no relay and draw their power instead through the light switch on the dashboard; something that is NOT good for the longevity of that switch.  Plus there's also the safety risk associated with having that high current path in the interior cockpit.  Here's the 'proper' current path for the low beams, traced in blue, from the battery, through the relay, to the fuse, to the headlight, and finally to ground:

In comparison, here's the path for the high beams from the battery, through the switches, to the fuse, to the headlight, and finally to ground:

Not as pretty, right?  All that high current for the high beams goes through the ignition switch, main light switch, and high beam/turn signal stalk!  Note that on later (I *think* all square-tail) models, even though they added and additional high beam relay, BMW still just routed both the high and low beam current through the switches in a similar manner! Why the Bavarians didn't just use a second relay in the same manner as the roundie low beams were done I'll never know, but no matter, that's what the FAQ is here for!

The Solution:

No that we know what we do and don't want, here's how to go about fixing things.  The simplest method, and what I recommend to everyone whether upgrading anything else or not, is to add in a relay for the high beams.  This can be done easily with essentially NO modification to the existing wiring harness, just a reconfiguration of the old low beam relay and a new SPDT relay, as follows:

If it isn't obvious to you, the way this works is as follows:

 -  Fresh 12V supply is taken straight from the battery to provide the power for both low and high beams.

 - The existing wiring from the main light switch (yellow/white for low beams) and stalk (white/blue for high beams) are used as the inputs to control the relay coils.

 - The relay outputs then hook up to the rest of the existing wiring to the fuse panel and then to the lights themselves.

 - The low beam relay coil is grounded THROUGH the high beam filaments, so that the low beams turn OFF when the high beams come ON. (This is the right way to do things, for both legal and practical reasons).

Depending on your exact year, you may need to run a new large (say 10) gauge wire (preferably RED!) from the battery for the 12V supply, and if you do NOT have a later model with the factory high beam relay, then you will need to find the connector #89 where the high beam white/blue wire meets the white wire, disconnect these, and run new leads for both from that junction up to your new high beam relay:

If you're planning to upgrade to H4 or similar higher output headlights, most would advocate replacing the existing wiring to the lights themselves with new, larger gauge wire.  While this is generally a wise idea, if your existing wiring is in reasonably good shape my above circuit should still also work without overtaxing anything.  This is because the overall run of the wiring (from the battery to the lights to ground) is now much shorter than the original path that went all the way up through the switches and back again.  I like this because it helps to keep the wiring cleaner and easier to sort out again in future, but if you're at all nervous, by all means error on the side of caution and run big new wires in place of the original ones!

Additional fog and/or driving lights

Now that we have the main lights all sorted out, it's time to move on to adding those beautiful auxiliary lights on to our 2002s!  Here there are basically two options: fog lights and driving lights. But which ones do you want?  Fog lights, as the name implies, are intended for driving in foggy or other poor-visibility conditions and provide diffuse, low down light that is not reflected off the fog/rain/snow etc. and back up into your eyes.  Driving lights, on the other hand, are more like high beams and are aimed higher and are meant to provide additional illumination down-road in the dark when weather conditions are more favorable and there is not any oncoming traffic to worry about blinding.  Unless you live in a crappy climate in Europe where fog lights are truly warranted, I think that most 2002 drivers are better suited with driving lights.

An additional bonus that I like to leverage with auxiliary lights is that, if done correctly, you can also use them all the time at lower intensity as daytime running lights (DRLs)!  I accomplished this by using a DPDT relay to power my driving lights in SERIES (which makes them about 30-40% of full brightness) whenever the car is on, and then switching them to PARALLEL (100% full brightness) whenever the high beams come on.  This also means I don't even need to run a separate switch for them, as they just use the existing high beam switch.

Whatever the case, we are fortunate enough to have a connector provided by BMW up in the nose specifically for the addition of such driving lights.  It's the #9 connector on the white/purple high beam wire most wiring diagrams:

If you don't care about DRL functionality, then you can simply add in a relay, again run a new red wire straight from the battery to supply the power, and use this connector to trigger the relay coil, and you're done!  But if you're like me and feel that higher visibility = greater safety (shout out to all Colorado, Inca, Golf, Mint,Verona, and Tiaga cars!), then here's the circuit to wire them up to double as DRLs:

I didn't draw it because I already had one, but for this you CANNOT take your +12V power straight from the battery, otherwise the DRLs would stay on even when you turn the car off, and quickly kill you battery.  So you need to find/make a source for switched +12V power instead.  For this I recommend adding another relay that is powered straight from the battery but that is triggered (on squaries) using the green wire that originally powered the headlight relays (which is now unused and available after upgrading those relays), or (on roundies) you can run a new spur of green ignition wire from one of the unused terminals on the back of side of fuses (3, 4, or 12 on 12-fuse versions):

Now, you will have a relay that provides power only when the ignition is on that you can use for all sorts of things, such as the power source for these DRLs!

You should also add a new fuse (I use 20A) somewhere in your fog/driving light circuit in order to keep everything properly protected!

When it's all done, here's how it works:  With the ignition on but the high beams off, the current flow is through the lights in series as follows:

But when the high beams are switched on, the relay switches the path and the lights are instead powered in parallel, thus:

Well, I hope that some of you find all of this helpful, and are able to use this knowledge to make your cars brighter and more enjoyable!  Feel free to comment or PM me if you find any errors or have further questions!

-Carl

P.S. A quick note of thanks to @Ireland Engineering, @Nijn, and @323IJOE all of whom have created wonderful colored versions of the 2002 wiring diagrams! I'm not sure whose specifically I used shots of here in this write-up, but you all deserve thanks for your efforts!