This article has TWO sections. The second section is a different way of talking about some of the same things, but with extra details...AND...a simpler! way of explaining how it all works!!
How the headlight switch and relay works, in disgusting
detail ...
on BMW Airhead Motorcycles (and
similar on Oilheads and Classic K-bikes)
© Copyright, 2012, R. Fleischer
headlightrelay.htm-24B
This article has TWO sections. The second section is a different way of talking about some of the same things, but with extra
details...AND...a simpler! way of explaining how it all works!!
This article originally came about because of someone asking 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 and tear on the switchgear. The reason is that the stock headlight relay does not do what many think it does. This question has come up many times. But, there is a LOT MORE to just that question and answer.....:
FIRST....using additional relays to control higher powered headlamps is a must; although some have gotten away with it for years, but note that the left switch assembly is NOT CHEAP. It is NOT designed to handle higher powered headlamps. Using relays will reduce the wear and tear on the left switchgear EVEN WITH the STOCK headlight; and may increase light output some too!
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.
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 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 know that the MAIN purpose of the stock headlight relay 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. It switches the headlight off during cranking. Typically, the headlight relay does not even defeat the High Beam momentary flash button, depending on year, model, country shipped-to.
If you are the inquisitive type, you will want to read the rest of this article....and, perhaps (and I do so suggest) you will follow along on the schematic diagram for your bike.
***NOTE! The Headlight relay function was totally different before 1975. I will describe the switch and the
relay here, but the rest of the article assumes a 1979+ motorcycle.
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 added a separate lights switch to the handlebar. BMW also added the headlight relay. Ever since, all UNITED STATES models have had a THREE position ignition switch, with a real KEY: OFF; Parking lights; Ignition.
From 1975 to 1978 the headlight relay actually controlled the headlight; the function of the relay was simply to reduce current through the bars switch. From 1978 BMW eliminated the bars light switch in its prior setup.....and BMW used the 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.
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 that relay's coil TO BE
ENERGIZED. Since the relay coil is energized during running of
the engine, I will assume that particular 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 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 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 87b.
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/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 like Joe 'Cuda, 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. 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. So, the relay is energized, and the headlight
comes on.
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.
I have seen one other version, and am not sure it is stock.
This 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.
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 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 (and then some!) reply (slightly
edited and abbreviated here):
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.
CLICK
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 this, you will understand also that when NOT
cranking, but ignition is ON, the headlight relay is energized!
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 with 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.
rev:
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 the 1975
and prior controls and the 1975-1978 relay function
© Copyright, 2012, R. Fleischer
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