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Sources for relays are any autoparts store. HOWEVER, www.EasternBeaver.com sells complete plug and play kits for your motorcycle, and they even have a version like that WITH a modulator if you wanted that too.

What follows, below, is an extension of article 24A; and, is an edited version of my reply to him on 01-13-2008. It is a fairly complete detailed explanation of the headlight switching circuitry, the stock headlight relay, the diode(s) involved, etc.

It is important to realize right off the bat that the MAIN purpose of the stock headlight relay is to turn off the headlight, leaving the dash lights and rear running lamp ON.......all during the CRANKING time. The stock headlight relay is NOT a relay that simply switches the high or low beams on or off. In fact, it does not do that EXCEPT during cranking. It may not even defeat the High Beam momentary flash button.

Here is how the headlamp relay and its associated circuitry works on the 1981-1984 R100RT, and similarly on others, in most ways. There are some minor details that are a bit different on the ST, G/S, R65 and R45; and also if one has the Euro switchgear, but the basics are VERY similar.

Firstly, it is IMPORTANT to understand that the NORMAL mode of the headlight relay, with the engine running (or, ignition on and starter NOT energized for cranking) is for the relay coil TO BE energized. Since relay coil IS energized during running of the engine, I will assume that energized coil condition of the relay for the following explanations, but I will describe the on/off of the relay and switching, etc...as I proceed. This will save some verbiage.

1. The battery feeds TWO "arms' of the switching contacts inside the headlight relay.   The connection for the battery to the relay is a RED wire, and the associated relay terminal is #30. One of these two arms contacts is touching the contact 87a that has a gray wire. That gray wire goes to the dash and rear running lamps.   A diode inside the relay keeps the dash and rear running lamps ON, during starting/cranking (more on this a bit later here), as the diode is fed by a green/white wire from the ignition switch (terminal 86 at this relay), when the ignition switch is ON. The diode is connected with its anode side to that terminal 86, and the cathode side is to that mentioned 87a.

2. The other "arm" that internally is from terminal 30, is touching the contact that connects to the yellow-white wire, which is terminal 87.   This yellow-white wire is the one that supplies power to one of the two power connections to the 3 position headlight switch.

3. The RELAY wire I had not mentioned yet is the black wire from terminal 85 of the relay. This one is SLIGHTLY TRICKY!   This wire connects to the STARTER motor.   When the starter motor is engaged and is cranking the engine, the starter has +12 volts on its solenoid terminal. If this same +12 volts is applied to BOTH sides of the headlight relay coil, then the headlight relay coil has no voltage drop across it, meaning it DE-energizes.   The internal diode keeps the dash and running lamps ON during cranking, but the HEADLAMP is turned OFF.   There is a minor complication, that I should mention, since some sharp-eyed person like Joe 'Cuda, might.   Tthere may be a second diode in this black wire circuit. It could be inside the relay (very late models), or elsewhere's. It's function is not important here.

4. Now...to the lights switch:
Feeding the lights switch with power is the yellow-white wire from terminal 87; as previously noted. That wire is NOT energized when the relay is NOT energized. Thus, no power to yellow-white wire through the relay if the key switch is OFF. BUT, there is another path to the bars switch for power. More later.

There are TWO VERSIONS of the high/low bars switch wiring to the headlight bucket and ignition switch. NOTE that your BMW owners manual schematic may well show the wrong one! It is not easy to find the schematics for BOTH versions, to see side by side. One version allows high/low flashing withOUT the ignition key being ON, the other version turns off all lighting function possibilities (including the momentary flashing switch) if the key is not ON. BOTH types have been shipped to the USA and elsewhere's!    It is a matter of where a GREEN wire from the headlights switch is connected.   USUALLY, in the USA, that green wire from the headlight switch assembly is connected to the OUTPUT side of the ignition switch.   Thus, the headlight switch has power.

SIDE NOTE #1: Some folks have made small wiring changes, moving a single wire (push-on type), in the headlight bucket. This can enable the ignition to be ON in the PARK function....which some can then use to energize a 10 watt quartz lamp to the headlight shell (side of the headlight itself) on the RT/RS, ....etc.   This leaves the headlamp bulb itself UNpowered on the PARK position.   Thus, they can drive with a fairly bright headlight (10 watt quartz), and save 45 watts, for such as accessories or better alternator functioning, or call it better battery charging. I am not getting into that here, other than to mention it, as it may throw a slight wrench into those trying to follow my wiring and explanations, if they don't know about someone modifying their bike before they purchased it. It was moderately popular to do this.

5. There are TWO sections to the headlight switch on the bars, and they are mechanically interconnected.   One section simply selects the high beam momentary, or so-called FLASHING mode. This is the spring loaded switch. UP is HIGH beam, MIDDLE is LOW beam, and pushed DOWNward (spring loaded to go back up to mid-position) is the flashing position as in flash to pass.   That section gets power from BOTH the relay terminal 87 (yellow-white wire) AND the mentioned green wire going to the ignition switch. THUS, in STARTING function, the flashing mode is always available. The 'arm' of this side of the switch simply selects high beam momentary flashing mode, or not....but the electrical power to the HEADLIGHT LAMP ITSELF, whether high beam or low beam, MUST GO THROUGH THIS SWITCH SECTION.   So, this is a section that does NOT appreciate high power headlights. The contacts on this bars switch are NOT massive.

6. The other section of the bars switch ALSO has the "I don't like high powered lamps" problem.....because....this section receives all the power from the first section, via an arm connection in this bars switch....and it is this other section that selects HIGH beam, or LOW beam. NOTE that the FLASHING function is separate, and the HEADLIGHT RELAY is NOT in the actual function of the flashing portion, due to the two paths for power to the bars switch.

7. Thus, both sections of the bars switch must pass ALL of the headlight current. THAT is why the switch generally, not always, tends to fail from higher wattage headlamps.

SIDE NOTE #2: VERY rarely this happens, but when it does, it will bug you:    If the diode in the headlight relay shorts, then the bike engine, once started and running will stay running if you then turn the key off....unless you either turn off the gas and wait awhile, or you stall the engine, or unhook the battery....and, of course, you will likely move the right bars KILL switch during this fun and games. Then the situation repeats after the next startup.