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Headlamp Conversion Revision

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Headlamp Conversion Revision

Post by ScottAndrews » Sat Dec 25, 2004 5:27 pm

After taking some good natured pokes from some of you about my messing up the terminology and Hi/Low beam definitions, I went out to my car and realized my error. Hella makes H1 and H4 bulbs, as well as H1 and H4 lens/bucket assemblies. It makes a big difference in talking about wiring which exact filament you are discussing. I was also a little confused since, while the Hella H4 has 60 and 55 watt filaments, the H1 (high beam) is only 55 watts...So I was calling the lower wattage filaments "LOW" beam.. I am still not quite sure which filament is which in the H4 lamp (ie is the low beam filament actually the higher wattage, but positioned differently? I suspect so).

So, to set the record straight, I revised my article and diagram. See below. This is also posted inthe BCG tech library for future reference.


Converting from a Four Filament Headlamp System
to a Six Filament System

Scott Andrews, 12/2004 (Revised)

Like many such cars, my 1980 Euro spec BMW 635 had been somewhat butchered during the Federalization process. One of the Federalization steps was to toss out the high quality European spec headlamps and replace them with cheap sealed beam units. This not only resulted in poorer lighting, but it also ruined the look of the headlamps, and included a lot of dubious wiring.

Daniel Stern ( has compiled some useful information on overall lighting systems. Also Joseph Peterson published a very good article on rebuilding the front wire harness and replacing the crappy lamps with high quality units. ( ... 4913276280)

The best lighting systems typically use two types of lamp. What is commonly called the H1 unit is a single filament high beam lamp. The position of the filament in the reflector, and the intensity of the lamp make for a beam that lights the road in a long far reaching beam. The other lamp is commonly known as an H4 (these are Hella names, so others may use slightly different conventions). The H4 is a dual filament lamp that has the same high beam filament, and a second low beam filament. The low beam filament is positioned differently, so it lights up the road in the relatively nearby front of the car. I also think the H4 headlamp bucket and lens tend to aim the light for the high beam down a bit, so when both high beams are on the car seems to light up the entire road, from close in front all the way out to Zion.

This solution works quite well, except it doesn?t fit well in the BMW headlamp wiring. The issue is that the BMW is made for a four filament system (A single hi/low lamp on the outer lamp position, and a high beam in the inner lamp position). Most of the articles I have read have incorrectly wire the four to six filament conversion.

When using six filaments you are seeking the following lamp behavior:

Low Beams: Outer LOW beam filaments ON, Outer HIGH beam filaments OFF, Inner HIGH beam filaments OFF

High Beams: Outer LOW beam filaments OFF, Outer HIGH beam filaments ON, Inner HIGH beam filaments ON

The idea here is to use the outer high beams to light the road directly in front of the car, and the inner high beams to light the whole world way out ahead. There are two other important things to consider when going to this setup.

1) You do not want to have the high and low beam filaments in the H4 bucket on at the same time. This generates too much heat and will cause the filaments to fail early (and in some cases might also crack the lens)

2) You do not want to run the inner and outer high beam filaments off the same wires, since this will draw a lot more current through the wire and will cause both heating and voltage drop (if it doesn?t also blow a fuse).

In many of the articles I have read the wiring system fails to meet one of these two goals. In many diagrams (Stern?s for example) they use single pole single throw relays to control the lights. These do a fine job of controlling the lights, but they either leave the outer low beams on with the high beams, or they double up the current on the high beam wiring when the hghi beams are on.

To avoid this, I developed the circuit shown below.

In this circuit, I use Bosch 0 332 209 150 relays. These cost about $18 each and I had to order them since they are not your typical cheap headlamp relay. The really useful thing about them is that they are single pole double throw relays, so we can do some cool switching using them.

The operation of the circuit is as follows: In low beam mode, the left and right low beam wires are powered from the standard BMW lighting relays (in the fuse box). This current passes through the two new relays (in terminal 30 and out terminal 87a) and on to the outer low beam filaments. The relays are normally in this position (that is when the coil is not activated by passing current between terminals 86 and 85).

When the high beam switch is closed, the BMW high beam relays activate the high beam wiring. So, for a four wire system, we would have both the high beam and the low beam wires hot. The high beam wiring is connected directly to the high beam filaments in the outer H4 buckets, and also to the two new relays. So, when the high beams are on the relays switch over so that the low beam signal goes to Terminal 87b. This is routed to the inner high beam filaments. Using this setup, the high beam current is basically used to switch the low beam current from the outer low beam lamps to the inner high beam lamps. Only one filament is run by any one of the four lighting wires.

In installing this system, I placed the relays on the left inner fender right next to the battery. I also rewired the headlamp harness using heavier wires, and that allowed me to add the extra wires for the fifth and sixth filaments. So, I have the four original lighting wires coming from the fuse box down through the harness to the battery area where they emerge from the harness, and drop down to the relays. The six new wires go back up from the relays to the harness, where they head out to the front of the car, as originally designed.

Bill Wolf (from the Big Coupe Groupe) suggested the alternative wiring shown in dashed lines. This basically crosses over the right and left inner high beams so that if you blow a headlamp fuse you can still get both sides of the car lit.

In use, this system works extremely well. The effect of the low beam switching makes the lighting changeover rather dramatic, and unlike the four filament setup, the high beams in the outer position really add to the overall lighting effect.

Going to good euro lamps was a huge improvement. Wiring them properly just makes the whole thing that much better and more reliable.

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