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Headlight switches & relays operations, in disgusting detail.
Copyright, 2012, R. Fleischer


This article was written because someone asked about potential problems with the left-hand switch of BMW Airhead motorcycles if using higher powered headlight bulbs.   He did not understand why the existing stock headlight relay did not automatically take care of the wear on the switchgear.   The reason is that the stock headlight relay does not do what many think it does.  Variances of this question have come up many times, often on the Airheads LIST, but also on various lists & forums for other BMW models. 

It is important to know that the MAIN purpose of the stock headlight relay FROM 1978 is to turn off the headlight, leaving the dash lights and rear running lamp ON, during the time the Starter Motor is actually CRANKING the engine.  The stock headlight relay DOES NOT simply switch the high or low beams on or off.  The relay switches the headlight off during cranking.

The headlight relay does not defeat, nor is it directly involved, with the High Beam momentary flash button, depending on year, model, country shipped-to.

Using additional relays to control higher powered than stock headlamps is a must; although some have gotten away with it for years.   Note that the left bars switch assembly is NOT CHEAP.  It is NOT designed to handle higher-powered headlamps.  Using relays will reduce wear on the left switchgear EVEN WITH the STOCK headlight; and usually will increase light output.

Sources for adding relays are any auto-parts store.  HOWEVER, sells complete plug and play kits for your motorcycle, and they may still have a version WITH a modulator if you wanted that.

Relays: Versatile relays that can work fine for most functions in your motorcycle, such as switching lamps, running horns, etc., is the Bosch (now Tyco) 330-073, rated at 30/40 amperes and 12 volts, SPDT, 5 pin, with tab for screw (tab area can be removed); or the Blazer DF005 or DF005W which also has a tab/screw mounting. 

Some headlight relays have a diode inside. The BMW relays are pricey.  You can substitute, and add a diode.

What follows, below, is an extension of article 24A; and, is an edited version of a reply I did 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.

If you are the inquisitive type, you will want to read all the rest of this article; and, perhaps, as I suggest... you will follow along on the schematic diagram for your bike.

The Headlight relay function was totally different before 1975; and from 1975 through most of 1978.  I will describe the switch and the relay usages below, but the rest of the article assumes a 1979+ motorcycle (perhaps some of 1978).

The /5 bikes (to 1973) had a plunger-pin type key switch mounted on top of the headlight bucket.  This 'peg-key' operated the internal switch that controlled lights and ignition, directly, without a relay.  

In 1974, BMW retained that plunger type key switch, but it had 5 positions:  OFF; Parking lights; Ignition only; Ignition and parking lights; Ignition and headlight.  

In 1975 BMW used a separate lights switch on the handlebar. BMW also added a headlight relay.  Ever since, all UNITED STATES models have had a THREE position ignition switch, with a conventional KEY:  OFF; Parking lights; Ignition. 

From 1975 to 1978 the headlight relay actually did directly control the headlight; the function of the relay was simply to reduce current through the bars switch.   Does that mean it is possible to use an increased wattage headlamp bulb on the 1975-1978, if the headlight relay is actually checked and found to be connected for direct headlight control?  Yes, within reason.  I'd not go over 80 watts.

From 1978 BMW used the headlight relay for different functions, and THAT, since it is a bit complicated, is what this article is primarily all about.

Section 1: Here is how the headlamp relay and its associated circuitry works on the 1981-1984 R100RT, and similarly on others from 1979 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.

1.  In the NORMAL mode of the headlight relay, with the engine running; or, ignition on and starter NOT energized for cranking; the relay coil IS ENERGIZED. 

2.  The battery feeds TWO "arms' of the switch 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 contact is touching the contact 87b 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), 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 87b.

3.  The other switch "arm" also connects to terminal 30, but 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.

4.  The RELAY wire I had not mentioned yet is the black wire from terminal 85 of the relay. This 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 wires 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 by the relay.   There is a minor complication, that I should mention, since some sharp-eyed person might.   There 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.  The important thing is: When the starter is NOT in cranking mode, there is no +12 volts at the starter, and the starter looks like a very low resistance load to ground, as far as the relay coil is concerned. Thus, the relay is energized, and the headlight is on.  WHEN the starter is energized, the relay contacts OPEN, and the DIODE provides power to the dash and running lamps.

5.  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. 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. I have seen one other version, and am not sure it is stock.  That Airhead was wired such that in the PARK position, all flashing functions were still available.  This Airhead could have its ignition key removed in the Park position.

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 slightly confuse 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.

6.  There are TWO sections to the headlight switch on the handlebars, 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.

7.  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.

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

9.  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 even if you 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 try moving the right bars KILL switch during this fun and games.  Then the situation repeats after the next startup.

Section 2: This section was added in August 2011, as there was an inquiry, prompted by something I had posted on the Airheads LIST, and I took the opportunity to make a very complete answer: The query on the LIST:  

"""Let me start by saying that both my bikes are Euro spec so the headlights can be switched off at any time. I gather that the US imports were headlight on with ignition switch on. Is that right?  I have read about this wiring setup before, and each time my mind just shies away from the idea of ground (earth) and hot wires at the same terminal.  Let me see if I understand this correctly just for my own edification. If we take a simple SPST 4 pin relay as a headlight relay feeding the Hi-beam, the connections to the four terminals would be as follows:
85 = switched power to relay
30 = Battery+ fused power into relay (maybe directly from the battery)
87 = relay-fed battery+ power out of relay to headlight hi-beam
86 = ground BUT CONNECTED TO THE BATTERY TERMINAL AT THE STARTER INSTEAD OF TO THE FRAME, so that when power is fed to the starter large terminal by the starter solenoid, it cancels the power coming into the relay via the 85 terminal, and kills the headlight.

1) Is this basically correct?
2) If so, what colour would the wire be to the starter terminal? Red, brown or something else?

Here is my rather complete (& then some!) reply (slightly edited/abbreviated here):

You are basically correct.  Just to make this absolutely clear, since lots of Airheads read the Airlist:

Terminals 85 and 86 are the relay's solenoid COIL connections.  On most relays, those are completely isolated and independent (NOT so on later Airhead headlight relays...see below; and the starter relay is also special).

For common relays (NOT STOCK later headlight nor starter relay in Airheads): Either 85 or 86 can be used interchangeably for coil power + and coil power -.  The stock headlight relay on the later models had an internal diode, and the way that diode was connected mandates that 86 is the + from the output of the ignition switch; and 85 went to the starter motor.

There is a page on my website that describes DIN standards for wire color codes, if you are curious as to what colors mean.  More on that page too about wiring.

In the Airheads that DO ground the relay to the starter hot terminal (well, really the solenoid hot trigger terminal); the wire to the starter that grounds the headlight relay coil is BLACK, from terminal 85 of the headlight relay.

That black wire, in the wiring loom, typically ends at the starter relay, NOT at the actual starter or starter solenoid.  Electrically, it is the same place.

Power goes from battery + to and through the starter relay (internal jumper, red wires) to the ignition switch... all the time.  When the key is turned ON, the headlight relay coil gets power to terminal 86.  Terminal 85 is grounded (early models with headlight relay), or, goes to the starter motor via the starter relay connection point (later models with headlight relay).  The starter relay connection point I am talking about is the the output of that starter relay.  That starter relay output terminal is +12 ONLY when the starting push button switch is pressed.  When you press the starter button, that +12 goes, via a black wire, to the small terminal on the Starter Solenoid.  That energizes the solenoid, and the starter hopefully starts rotating.  Since the headlight relay has +12 at terminal 85 AND +12 at terminal 86, there is no voltage drop between them, and the COIL of the headlight relay is NOT energized (during cranking).

If you have followed all this, you will also understand that when NOT cranking, but ignition is ON, the headlight relay coil is energized, and you know which contacts are closed!

Early Airheads that DID have the headlight relay, had the relay GROUNDED via a brown wire.   If I remember correctly, the transition year for wiring that headlight relay to the starter motor began with the 1978 model year, and a year later, in 1979, the diode was installed in the relay.  There were some anomalies.  If I remember correctly, the 1979-1980 R65 model did not use the diode and the relay went to GROUND, not the starter.

There are some strange anomalies, besides the R65.  The most complicated one I know about is for the RT models when they has the swing-out 'fog lamps' in the fairing....several extra relays and some 'funny' connections and another diode; and a warning buzzer on some, made it way complicated to understand.  I have an article on the website that describes all this, if one needs the information.  It may buzz your brain though.

Please refer to item #6 in the following article:

Section 3, replacing headlight switch gear:
This section has been entirely moved to:

01/14/2008:  clarity; and same, again on 06/08/2011.
08/03/2011:  Add section 2.
05/16/2012:  Correct typo in #1, as terminal is 87b, not 87a.
10/03/2012:  Add QR code, language button, update Google Ad-Sense code; add information on 1975 & prior controls & 1975-1978 relay function.
2013:  remove troublesome language button javascript code
09/15/2014:  very slight updating, strictly for clarity
03/05/2016:  metacodes and layout
07/16/2016:  Update metacodes, H.L., scripts, slight changes to fonts and layout.  Clarify details (minor).

Copyright, 2012, R. Fleischer

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Last check/edit: Tuesday, July 26, 2016