85 K100RS blows #1 fuse shortly after starting (w/strange symptoms)

[boostd4]

Looking for any electrical guru insight.


I'm continuing my diagnosis started in this thread:


http://www.k100-forum.com/t12767-sudden-no-start-with-starter-button-where-to-check-next


But symptoms have changed a bit and was looking to maybe get some new eyes on the issue.


As it sits right now, my 85 K100RS will blow the #1 fuse every time I start it after it's sat after running.  Sometimes it will do it when I put the key on, then turn the choke on (fuse pops before engine starts).  Sometimes it will do it a few seconds after I put the key on without touching anything.  And sometimes it will let me start the engine, it will run for sometimes a few seconds, sometimes it will go about a block of riding before popping the #1 fuse.  Engine still runs and all, just functions tied to that fuse stop working (most importantly brake lights that keeps me from riding it anyway).


A month ago, it was doing this completely randomly (not just shortly after startup).  I pulled the tank and hooked a headlight bulb in line with the #1 fuse and jiggled wires until I found what I think was the clutch switch connector on the left tray under the tank would pop the fuse whenever I jiggled it.  I cut it out and wired it straight and it stopped blowing the fuse for a few days until the above current scenario.


Link to connector I eliminated that would pop the fuse when I tapped it:


So why don't I just continue using the headlight bulb and find the short?  Here's the weird part - After it blows the fuse, I can replace that fuse and it will start and run normally - FOR AS LONG AS I RIDE IT... whether that be 5 minutes or an hour.  I've tried working every component while riding and nothing blows the fuse.  If I shut the bike off and let it sit for at least 10 minutes, it will blow the fuse on restart (in the above ways).  Then it works just fine after replacing the fuse.


The same goes for the Headlight bulb trick.  When I pull the tank and hook the headlight bulb up to the #1 fuse, it glows bright (indicating there is a short) for about 1 second, then it goes out (no short present), and stays out until I actually RIDE THE BIKE for a while, then let it sit again.


So strange!


Any thoughts?

[rbm]

On early K100's fuse 1 protects the following circuits:
- BMU
- Connector for Special Equipment
- front brake switch
- rear brake switch
- clutch switch
- Pin 6 on the instrument cluster that powers
  * BMU light
  * 4 & 7 low fuel light
  * temp light
  * choke light
  * oil light
  * gear indicator
  * charge light
  * tach
  * speedo
- Heated grips
-  Additional Instruments connector

What you are grabbing in the picture is the clutch switch connector.  I don't believe the clutch switch was the problem in the first place.

I have a feeling that there is an abrasion somewhere that is touching part of the frame randomly.  The symptoms you describe don't point to any single failing part in the list above.  I've worked on K-bikes where weird things happen and part of my diagnosis is to follow the wiring completely looking for either breaks in insulation or breaks in wires.  I'd suggest you do the same.  Try following the cables along to the parts on the motorcycle that are listed above.  Possibly there is a wear point in the cabling going to the instrument cluster where it exits the frame at the steering head.  Possibly there is an abrasion on a cable deep under the tank that you can't see unless you really dig the cable out and look.

Try disconnecting different circuits from fuse 1 and see if the problem alters with the removal.  For example, pull the plugs for front brake and test.  Then if the problem persists, pull the rear brake switch.

Eaton / Bussman manufactures ATC sized circuit breakers (Cooper Bussmann CB223-7-1/2).  They are sold through online electronics supply houses like Mouser and Digi-key.  That might save you pulling out your hair since they are self-resetting once the short gets removed.  It will help to speed up the diagnosis.

[boostd4]

Thanks for the advice.  I know the clutch switch may not have been the underlying problem, but was undeniable when I found it.  With the clutch switch connector's male and female ends plugged in, I could pull it up out of the bundle, hold it with one hand and tap it with the other and the tester light would blink every time I tapped it.  Plus, before I found that short, the fuse would blow every time I rode the bike within the first 5 minutes.

It's so strange that now it only does it once and then it's fine for even a 2 hour ride.

[rbm]

The heated grips connector is in the same region as the clutch switch.  Do you have heated grips (OEM or after-market)?  Did you examine that particular connector during your earlier diagnosis? 

[boostd4]

This being a cafe project, I don't need a lot of the stuff on Fuse 1 anyway.  I'm going to remove the BMU and hard-wire the lights (plenty of documentation about that on this site) to eliminate that.  I don't need the clutch switch (I don't believe - I'm smart enough not to start it in gear).  I don't have heated grips.

Where is the connector for special equipment and the additional instruments connector?  I don't believe I need those either.

[rbm]

The Connector for special equipment is inside the relay box.  It's a 5 position circular connector.

The connector for special instruments is under the tank on the right side of the frame, near the middle.  It is a connector that looks like one of those old AMP disk drive connectors.

[boostd4]

If this is the heated grip connector:




..then I've seen it under there.  I tugged good and hard on it but it didn't affect the test light.  Of course it is open ended (no heated grips installed).  I'd have have no problem cutting it out of the circuit but I'd have to open the whole harness back to the fuse box to trace the wire.   I suppose I could just cut the connector end off and tape up the ends to eliminate the connector itself as shorting.

[boostd4]

I have a feeling I may end up just pulling the harness out, unwrapping the whole thing, and looking at wires.  Not fun.  I'll try unplugging what I can first.

The hard part is there are branches and legs off the harness everywhere, I have a feeling I'm going to create more problems than I solve! ;)

[rbm]

If this is the heated grip connector:

Yes it is.

..then I've seen it under there.  I tugged good and hard on it but it didn't affect the test light.  Of course it is open ended (no heated grips installed).

So, what's that big black wire hanging off the connector?  It certainly looks like it's in use.  Maybe not for heated grips, but for something. A stock K100 without heated grips will have nothing connected; it will just be that connector end with the pigtails.

[boostd4]

^^ I stole that pic from google.  Not my bike.  Sorry, I should have mentioned that.

[rbm]

Ok, well don't snip off that connector willy-nilly.  If it looks intact and undamaged, just leave it alone as it is doing its job of keeping power on Green/Black away from ground on Brown.