Hi folks,

So I’m on the very last step of rewiring the BMW K100. Everything works (as far as I can tell) however I’m having trouble figuring out how to wire the Axel Joost neutral switch to the M.unit and an aftermarket speedo. I attached the sheet they provided me below, it has a diagram and German instructions.


Separately, I’m also interested how people wired in a clutch switch to the m.unit.

If you couldn’t guess I’m a newbie to electrical wiring and whilst I can follow a very basic diagram, any level above that requires a lot of research. Anyway I’d appreciate any help.

I had to read that a few times before I figured out what they are doing with the gear indicator switch. There should be a cable that exits from the top of the gearbox casting and terminates in a black four-pin connector. The wire colours are brown, yellow/blue, yellow/black and yellow/white. Connect the yellow/x wires to the three yellow (gelb) wires on the Axel Joost unit (I see the English comment but the German says the same thing - any gear indicator wire to any yellow wire) and the brown wire to earth.

The orange (Orange) wire goes to the negative side of the neutral LED.

The black (Schwartz) wire goes to battery negative.

The green (Grun) wire goes to pin 85 (brown/red wire) on the starter relay. It looks like someone has already picked the brown/red wire, decided it was wrong and should be the black/yellow wire, then gone back to their original idea. I'm basing that on the length of the words that have been crossed out.

The red (Rot) wire is supposed to pick up a twelve volt feed from the ignition switch ('hinter dem Zundschalter'). You'd think that would be the red wire on the ignition switch but no, you need the green one because that only goes live when the ignition switch is turned on. So that one goes to the positive on the LED as well as the 12 volt input to the unit. It also advises putting a 1 amp fuse in line between the ignition switch and the unit.

Can't help with the clutch


Anyone want to make a more accurate assessment?

English translation for all Joost product instructions is available at the Joost website. The instructions for this product are found at this page.

So I can still read German (sort of), even if I can no longer speak it! Now there's a surprise.

Dai wrote:So I can still read German (sort of), even if I can no longer speak it! Now there's a surprise.

You left off the part about having fun with the product.

Dai wrote:
The green (Grun) wire goes to pin 85 (brown/red wire) on the starter relay. It looks like someone has already picked the brown/red wire, decided it was wrong and should be the black/yellow wire, then gone back to their original idea. I'm basing that on the length of the words that have been crossed out.

The red (Rot) wire is supposed to pick up a twelve volt feed from the ignition switch ('hinter dem Zundschalter'). You'd think that would be the red wire on the ignition switch but no, you need the green one because that only goes live when the ignition switch is turned on. So that one goes to the positive on the LED as well as the 12 volt input to the unit. It also advises putting a 1 amp fuse in line between the ignition switch and the unit.

Can't help with the clutch


Anyone want to make a more accurate assessment?

So, mostly correct Dai.

- the LED shown in the diagram is the Neutral LED indicator inside the aftermarket speedo.  Connect the output labelled "LED" on the Joost product to the Neutral input wire on your aftermarket instrument. Most aftermarket instruments use active-LOW inputs for their indicators so there are no other connections to be made between the Joost product and your speedo

Can't say what you've removed/modified as a result of installing the M.unit and that will determine how the clutch switch gets wired.  That clutch switch is meant to supply 12V over a Black/Green wire terminating on the start button on the right hand switch cluster

- The Relay output of the Joost product should be connected to the other Black/Green wire going to the start button on right hand switch assembly.  You'll be connecting the Relay output to the end of the wire that used to be connected to Pin 5 of the OEM gauge cluster.

The rest of what Dai wrote is correct.
- any Yellow/XXX wire from the Transmission Gear Position switch can go to any Yellow wire on the Joost product
- The brown wire from the Transmission Gear Position switch must go to ground
- Power for the Joost product goes to 12V input and can be picked up from most any Green/Black wire inside the K-bike relay box, most conveniently on the Connector for Special Equipment
- Ground for the Joost product can also be found in the K-bike relay box, most conveniently on the Connector for Special Equipment .

Dai wrote:I had to read that a few times before I figured out what they are doing with the gear indicator switch. There should be a cable that exits from the top of the gearbox casting and terminates in a black four-pin connector. The wire colours are brown, yellow/blue, yellow/black and yellow/white. Connect the yellow/x wires to the three yellow (gelb) wires on the Axel Joost unit (I see the English comment but the German says the same thing - any gear indicator wire to any yellow wire) and the brown wire to earth.

The orange (Orange) wire goes to the negative side of the neutral LED.

The black (Schwartz) wire goes to battery negative.

The green (Grun) wire goes to pin 85 (brown/red wire) on the starter relay. It looks like someone has already picked the brown/red wire, decided it was wrong and should be the black/yellow wire, then gone back to their original idea. I'm basing that on the length of the words that have been crossed out.

The red (Rot) wire is supposed to pick up a twelve volt feed from the ignition switch ('hinter dem Zundschalter'). You'd think that would be the red wire on the ignition switch but no, you need the green one because that only goes live when the ignition switch is turned on. So that one goes to the positive on the LED as well as the 12 volt input to the unit. It also advises putting a 1 amp fuse in line between the ignition switch and the unit.

Can't help with the clutch


Anyone want to make a more accurate assessment?

Thanks for the help, what does pin 85 do on the starter relay? I know it leads to the ignition control unit but that’s about it.

Brown wires on the K-bike are grounds.  The Brown/Red wire to pin 85 of the start relay is also a ground but it is a very special ground wire.  The starter relay would not be able to engage with the press of the start button without a path to ground.  The Brown/Red wire goes to the ICU and inside the ICU, is a switching transistor in series with that wire.  When power is applied to the bike, the ICU switches on that transistor and now there is connectivity to ground for the starter relay current.  The start button gets pressed, the starter relay engages, the starter motor spins the engine, the engine fires and RPMs increase past 700.  The ICU can sense the RPMs of the engine through the Hall effect Sensors.  When the ICU senses the RPMs are above 700, it turns off this switching transistor and now there is no longer a path to ground for the current. This means the starter relay will not engage when the start button is pressed while the engine is running.  This is a BMW safety feature.  If you want, you can wire the starter relay pin 85 directly to ground but then be aware that you are disabling a key safety interlock on the bike.

Ha wrote:
Thanks for the help, what does pin 85 do on the starter relay? I know it leads to the ignition control unit but that’s about it.

This is the way I think about it. When the ICU gets the signal that the rider has pressed the start button (and other start conditions are met) the ICU sends earth to pin 86 (edit, pin 85) of the starter relay. Pressing the Start button also provides power to pin 86 of the starter relay. With power and earth to the starter relay coil, a magnetic field is created, causing the contacts between pins 30 and 87 to close. This allows power from the battery to reach the the starter motor, so the starter motor spins.

Thanks Rob! I was reasonably sure that I hadn't got it all correct because no M.unit to look at, hence the request for a checkover by someone else.

I found two M-unit schematics on my hard disk that I had squirrelled away some time ago (I'm a hoarder).  One schematic is for a K75 and the other is for a K100.  In both schematics, the authors have done away with the clutch switch.  They have both modified the start button on the right hand controls so that when it is pressed, it supplied an active-LOW signal out a wire, and not an active-HIGH signal as in the OEM harness.  That wire is connected to the M.unit "Start" input terminal.  The M.unit's "Start" output terminal is connected to pin 86 of the K-bike starter relay.  When the M.unit sees an active-LOW signal on the "Start" input it will output an active-HIGH signal on the "Start" output.

With this configuration, you cannot make the clutch switch work.  In its normal configuration, the clutch switch will output an active-HIGH signal not an active-LOW signal.  Furthermore the intent of the clutch switch on the original K-bike harness is to provide another alternate source of power for the starter relay (instead of the TGP switch).  Thus the rider can either start in neutral or start in gear.  With the m.unit wired the way it is in these two schematics, you don't have that option.  Heaven help you if you forget to pull in the clutch and try to start the bike in gear.

Thinking about this even further, you cannot use the "Relay" output on the Joost product to power your starter relay.  That product assumed you haven't modified the start circuit on your K-bike.  Unfortunately with the M.unit, you have modified that circuit.  The only purpose to which you can put the Joost product is to run your neutral light on your gauge.

robmack wrote:I found two M-unit schematics on my hard disk that I had squirrelled away some time ago (I'm a hoarder).  One schematic is for a K75 and the other is for a K100.  In both schematics, the authors have done away with the clutch switch.  They have both modified the start button on the right hand controls so that when it is pressed, it supplied an active-LOW signal out a wire, and not an active-HIGH signal as in the OEM harness.  That wire is connected to the M.unit "Start" input terminal.  The M.unit's "Start" output terminal is connected to pin 86 of the K-bike starter relay.  When the M.unit sees an active-LOW signal on the "Start" input it will output an active-HIGH signal on the "Start" output.

With this configuration, you cannot make the clutch switch work.  In its normal configuration, the clutch switch will output an active-HIGH signal not an active-LOW signal.  Furthermore the intent of the clutch switch on the original K-bike harness is to provide another alternate source of power for the starter relay (instead of the TGP switch).  Thus the rider can either start in neutral or start in gear.  With the m.unit wired the way it is in these two schematics, you don't have that option.  Heaven help you if you forget to pull in the clutch and try to start the bike in gear.

Thinking about this even further, you cannot use the "Relay" output on the Joost product to power your starter relay.  That product assumed you haven't modified the start circuit on your K-bike.  Unfortunately with the M.unit, you have modified that circuit.  The only purpose to which you can put the Joost product is to run your neutral light on your gauge.

If what you are stating is true, then can I assume the bike should start even if it is in gear? Meaning that something hasn’t been wired properly.

Separately I also contacted the manufacturer, they said the product is compatible. Their explanation is that the green wire is also connected into the start button on the input side with a small relay. I’ll attach a picture below.

Harry.Dip wrote:

robmack wrote:I found two M-unit schematics on my hard disk that I had squirrelled away some time ago (I'm a hoarder).  One schematic is for a K75 and the other is for a K100.  In both schematics, the authors have done away with the clutch switch.  They have both modified the start button on the right hand controls so that when it is pressed, it supplied an active-LOW signal out a wire, and not an active-HIGH signal as in the OEM harness.  That wire is connected to the M.unit "Start" input terminal.  The M.unit's "Start" output terminal is connected to pin 86 of the K-bike starter relay.  When the M.unit sees an active-LOW signal on the "Start" input it will output an active-HIGH signal on the "Start" output.

With this configuration, you cannot make the clutch switch work.  In its normal configuration, the clutch switch will output an active-HIGH signal not an active-LOW signal.  Furthermore the intent of the clutch switch on the original K-bike harness is to provide another alternate source of power for the starter relay (instead of the TGP switch).  Thus the rider can either start in neutral or start in gear.  With the m.unit wired the way it is in these two schematics, you don't have that option.  Heaven help you if you forget to pull in the clutch and try to start the bike in gear.

Thinking about this even further, you cannot use the "Relay" output on the Joost product to power your starter relay.  That product assumed you haven't modified the start circuit on your K-bike.  Unfortunately with the M.unit, you have modified that circuit.  The only purpose to which you can put the Joost product is to run your neutral light on your gauge.

If what you are stating is true, then can I assume the bike should start even if it is in gear? Meaning that something hasn’t been wired properly.

Separately I also contacted the manufacturer, they said the product is compatible. Their explanation is that the green wire is also connected into the start button on the input side with a small relay. I’ll attach a picture below.

Can confirm the wiring was incomplete, bike can start without a neutral switch or clutch in. Solves that problem

Thanks for the diagram.  I see what Joost is proposing.  Good to hear that you K-bike  starts.  Hope that you ended up solving your problems.

Not undermining this specific thread at all, but is there any real need to interlock the sidestand/clutch switch/ignition module etc. with the operation of the starter? It's a problem that often crops up on this forum. Many heads have been scratched, for what?
For me, the black and green tentacled monster deserves to be euthanased, cut out and thrown on the workshop floor, along with the starter relay switched ground from the ignition module.
No-one seems to have any emotional attachment to the parking light circuit, which alone is responsible for about sixteen miles of wiring on a K. Cripes, take both circuits out and you have enough spare wire to build another bike.
I'm having a radical morning. Merry Christmas to everyone. Even the sun-bathing Aussies 

  Yeah - I dumped that load of worms too. I did it the easy way though by just looping a bit of wire across a terminal block and plugging them into the loom. No more lockouts because they're permanently locked in.

As for the starter circuit; IMO that was being smart just for the sake of it. I actually like the ICU killing the starter motor but - ! Grounding the load shed through the starter motor? Surely someone with any mechanical sense could have seen the trouble that would cause down the line? The only problem with 'fixing' that is it means adding another relay to drop the load shed out when the start button is hit. Either that or a major redesign of the loom not to use another relay. Having surveyed the route, I'm not going there.

And while I'm ranting about the load shed relay (morning Chris!), that, IMO, is also a questionable design decision. Currently it is permanently pulled in when the bike is running. Okay, so relays are pretty reliable but... we do get load shed relay failures and when it fails, bike won't start. Change it for a standard changeover relay, connect both green/red wires to pin 87a and rewire the coil so that it pulls in only when the starter circuit is active. Why? Two reasons. 

a) there is less stress on the relay because it is relaxed when passing power to the circuit
b) if it does fail, the bloody bike will still start.

Suzi Q wrote:I'm having a radical morning. Merry Christmas to everyone. Even the sun-bathing Aussies 

I'm having a good day so I'd also like to say merry Christmas to everyone, even those who don't acknowledge Christmas, and yes, even to Englanders.

Deleted irrelevant sentences.

A couple of thought bubbles. Bikes have small batteries. Not a big deal in OZ, but in cold countries you want to make it as easy as possible to start during winter morning's, so it has a load shed relay. When I first saw the brick unloader I said whaaaat, but I'm used to it now. There's still some weirdness about it. Dai I like your way that it only works when the starter circuit is going, but that's what I like about the bricks load shed relay too, it only sheds while the starter turns, cause it relies on power to the starter to deny power to the lights and that. And I like your idea that it should not prevent the bike from starting if it doesn't work, but the bricks load shed relay doesn't prevent a start either, if you remove it, the bike will still start, but it's just that the lights won't cut out during a start.

The side stand switch. Yep if it's not working, can be annoying to diagnose, but that's not as annoying as spending three months in hospital, which happened to a mate who went around a corner not knowing the side stand was down. Most bikes have an over centre side stand, so if it goes back it also has to go up, you don't want the back tyre lifted up while leaning around a corner. Sounds rare but hospitals are full of victims of rare events. Maybe they can be converted to the earlier brick system, mine works great, I can't think of a reason to disable it, though some do.

The park light circuit. Yeah not much need for it on first world roads. I've only used lights on without key once so far. There was a tree fallen on the road around a bend, I had the hazards on for the side with not enough warning while I was removing the tree. Only needed it once so far, but it's times like these that makes you appreciate this feature, as well as it's battery saving, and time saving, benefits (While riding with the park lights on, you also want the instrument cluster lights on. In parking only mode, and if away from the bike, you don't want the keys left in the ignition, and the cluster lights remain off, as a battery saving feature. It also prevents someone from say turning on your headlight as a lark, to flatten your battery)

The ICU control of the starter relay. Probably not what you would first think ( to prevent the starter from turning while the engine is running) cause the sprag clutch makes that irrelevant. Since there could be a number of reasons, any one reason is probably wrong. A quick way to test the air/ fuel ratio is to press the start button while the engine is running, you don't want the starter running for this.

I look into a crystal ball and I see a meeting of BMW blokes. It's in the early eighties, they're talking about a higher than usual number of start button failures recently. The return spring is weak, or it's broken, or it's missing, or the spring retainer is missing (One of mine had a broken button and a missing spring) The starter doesn't have a cooling system since it only runs for about one second. Spose you have permanent earth to the starter relay and the starting switch is faulty, you're on a two hour ride, the starter might have been running for an hour or two (if say it's night time and the starter is running, you haven't got headlights, not good) So one of them says "Hey let's deny earth to the starter relay from the ICU when the engine is at or above idle speed, or something like that" And they say "Yeah"