Here's one for you learned people....it's been bugging me for a while (OK - most of the year).

I have ABS on my '91 K100LT (8V) and it all does exactly what it is supposed to do right up to the point where I get my revs somewhere over 5000 rpm and then it starts flashing at me as if to tell me there is a fault and my ABS is disabled. Turning off the ignition resets the ABS unit and it all behaves as it is supposed to until I get above 5000 rpm again.

In normal riding:

• ignition on - ABS flashes, Rear Light Monitor (RLM - Triangle indication on instruments) steady red.
  
• operate front & rear brakes - ABS and RLM both flash in unison.
  
• move off - ABS and RLM lights go out
  

This is exactly what is supposed to happen. 

However, regular as clockwork, if I get on an open stretch of road and crank open the throttle, somewhere between 5000 and 5500 rpm the ABS and RLM warning lights start to flash at me. At times, I can even be sitting at steady revs above 5000 rpm but sooner or later, the ABS/RLM will start to flash. Using the ABS switch on the dash, I am able to stop the flashing and get the steady red warning lights for a period of time after which they start flashing again and need another push of the switch. 

My initial thought was it was some problem with the ABS sensor gap - but that doesn't explain it working perfectly for 99% of my riding (1000 - 5000 rpm). My second thought was that it was a vibration issue (a combination of engine vibration and vibration induced by the road surface) - at higher revs, could this affect the ABS brain, electrical connections or the sensors? As the ABS brain resets as soon as the ignition turns off, I haven't determined any fault codes....is there a test set up I could fix up to identify the fault code while riding (a lash up that I could safely use on a clear straight section of road)? My third thought was ignore it - accept that it will happen and accept that, when it does, the ABS will be inoperative, although it is distracting having red lights glaring at you - especially at night.

On longer distance rides, I have got in the habit of turning the ignition off to clear the fault whenever I pull up at traffic lights etc. I have even tried turning the ignition off and on again while riding at a good road speed and effectively bump starting the bike again - it works....but it's not the safest operation and probably not too good for the bike either.

I have only experienced this problem in 5th gear....mainly because that's the gear I am in when doing the sort of speeds that 5000+ rpm give you in 5th. Is it worth cranking up to 5000+ rpm in a lower gear to see if the same fault occurs. I reckon I get the usual amount of vibration from the engine at higher revs for a K100, certainly nothing extraordinary or uncomfortable. Is the ABS brain (located in the duck tail) particularly susceptible to vibration - should I add some rubber padding to the mount to help isolate it more?

Has anyone else experienced this fault? Any suggestions what the problem is and what to do about it?
Sorry for the 1001 questions...but as I said, this fault is bugging me!

Usually turning it off and back on it means the supply voltage is down a bit. I presume you ride with your lights on so turn them off and see if it happens, which would confirm that supply voltage is the problem.
Possible cause could be that the brushes in the alternator are well worn and they can tend to float above the slip rings and cause the power output to drop off.. It's not the only cause but it's the first thing that comes to mind. Could be bad joints (change dealers) but that would likely be more erratic.

RicK G wrote:Usually turning it off and back on it means the supply voltage is down a bit. I presume you ride with your lights on so turn them off and see if it happens, which would confirm that supply voltage is the problem.
Possible cause could be that the brushes in the alternator are well worn and they can tend to float above the slip rings and cause the power output to drop off.. It's not the only cause but it's the first thing that comes to mind. Could be bad joints (change dealers) but that would likely be more erratic.

Thanks Rick,

That's a useful line of enquiry and an easy check to make. I have not touched my alternator (rule 1 - if it ain't broke, don't touch it) so it could quite likely have worn brushes and that will eventually lead to more problems. I reckon I'll pull the alternator off my donor bike and get that spruced up ready to transplant if/when required.

I do ride with my lights on (always) but this problem only occurs >5000 rpm. I thought that at higher revs, the alternator could be chucking out more volts, not less. There are no other (obvious) electrical issues but I will hook up a voltmeter to see what is going on in that department when riding...hmmm wonder where I could get my hands on a little data logger?

Bad joints indeed!!!! Reminds me of a dodgy character who confronted me many years back when I was a young pup in a less than savoury place in the caribbean. To my "no thanks mate, I don't what any of that sh*t" his eyes lit up and he opened his coat to reveal a whole pharmacy of what he obviously considered better sh*t for the more discerning junkie!

Touch of mispent youth there. Now wondering where its legal to do over 5.000 rpm in 5th!!

My car did the same thing. Battery was the cause. Since you have the spare altenator I would pop it in.

If it fails over here you could be weeks in a pub waiting for another and only Guinness to live on.

Remember a similar problem in a other forum, I think it ended out that it was caused
by a wrong sensor gap (which was just around the limit).

Try a fault code memory readout, it might narrow it down a bit.

Touch of mispent youth there. Now wondering where its legal to do over 5.000 rpm in 5th!! 

 erm, anyway...5000 rpm isn't that fast and my speedo does read high when compared with the GPS

If it fails over here you could be weeks in a pub waiting for another and only Guinness to live on.

What a terrible thought....  ....maybe I'll leave the alternator alone, book a ferry and live in hope

Born Again Eccentric wrote:Touch of mispent youth there. Now wondering where its legal to do over 5.000 rpm in 5th!!

Where I live it is legal if there are no cops and you don't get caught to do 7500 revs in 5th :-)) cos there's no one to tell you its not(around mach 2)
If there's cops then it is clearly NOT legal above about 4000.
I don't think they like seeing us go over 100kph (60mph) and I think they must really hate it when we are over 180k's
I assume they must really hate my guts 95% of the time that I am on the road . . .

Inge K. wrote:Remember a similar problem in a other forum, I think it ended out that it was caused
by a wrong sensor gap (which was just around the limit).

Try a fault code memory readout, it might narrow it down a bit.

Thanks Inge,

I'm not able to get a fault code memory read out as I don't have a suitable LED to hand at the moment. However, having found an ancient set of imperial feeler gauges, I have just checked the sensor gaps and get 0.014" (0.356mm) front and 0.050" (1.27mm) rear. 

According to my Haynes manual the gap should be 0.0138" to 0.0256" (0.35 - 0.65mm) so clearly the rear sensor gap is well out of spec and this could be root of my problem. I only measured the gap in one position, again Haynes says to take 6 readings at ~60 degree intervals, so I will repeat the measurements later. Sounds like I need to remove shims / insert thinner shims on the rear sensor. 

It is odd how the ABS works fine at all speeds/revs below 5000 rpm - and, on the odd occasion when I have had to brake suddenly and been a little over zealous on the rear brake, the ABS has definitely functioned correctly. 

As I understand it, the sensor (mounted on the brake caliber body) constantly detects the movement of the teeth of the impulse gears as the wheel turns. I am guessing that this is a pretty small signal and over my relatively large gap it must be even weaker still - I presume, therefore, as my road speed increases, that the ABS brain eventually loses this signal and goes into fault mode. This will be easy to prove by riding at high revs (5000+ rpm) in 4th gear - if it doesn't throw up the fault, it will almost certainly mean that the problem is associated with wheel speed (rather than engine speed) and the size of the sensor gap. If this is the case - dead easy to fix. Updates to follow.

For completeness, an update on my flashing ABS issue (above 5000 rpm) ....

Since my last post, I had ridden the bike at >5000rpm in 4th gear and can confirm that the ABS issue was related to bike speed and not rpm, as suspected, as I could not generate the flashing fault.

This morning, I removed the rear wheel and re-checked the clearance between the rear ABS sensor and the impulse gears (confirming that the gap was 0.050" (1.27mm), as previously measured) i.e. well out of spec. (0.0138 to 0.0256" or 0.35 to 0.65mm).

On removal of rear ABS sensor (held in place on rear brake calliper with 2 torx machine screws and does not require removal of calliper) I found 3 shims fitted to it. Removed 2 of them - total thickness removed = 0.030" (0.762mm) and reassembled. 
Gap clearance now set at 0.020" or 0.508mm all round (well in spec.)

Took bike for a spin and glad to report that this has fixed the issue and I no longer get the annoying ABS warning coming on above 5000rpm in 5th. 
Result!!

Christmas Eve, blue sky, bright sunshine and 8.5 deg C....guess we're not going to get a white Christmas around these 'ere parts.