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ABS fault-finding, and anyone know what this part is?

3K views 15 replies 5 participants last post by  djlacey 
#1 ·
Hi All!

New to the site and looking to improve the way I use my F650 GS Dakar (it mostly loses out in competition with my R1200GSA :-( )

I'm experiencing a problem with the ABS on the front wheel - kicking in every time I brake. I suspect it's a sensor issue. It arose in connection with some maintenance on the forks I had carried out at the BMW dealer.

I'm not looking to make any repairs myself, but would like to see if I can determine the fault before taking it along to the mechanic.

Can anyone point me to a fault-finding procedure for ABS on this great site? I've made a determined attempt to search for myself, but with no success. Your help would be much appreciated.

Also, does anyone know what the cylindrical part highlighted in the attached jpeg does? I've seen it depicted on very few images of the F650, and the manuals I have access to (Haynes for example) don't reference it. Again - help much appreciated!

Thanks!

//Derek

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#3 ·
Hi Derek,
welcome to the forum, you not got an easy choice with the R1200GSA in your stable too!

suggest you check ABS sensor and rota for muck etc, also check wiring for chaffing / connections , usually the cause, if back not playing up isolated to front, unless you are giving a good hand full when braking, I found it not easy to get front abs to kick in, easier on back tho
 
#6 ·
Hi KuyaJonathan!

Thanks for the suggestion - I'll take a look later today, and give things another good clean just in case. Good point that isolation to front wheel would tend to indicate the fault lies there...

Regarding the question of how hard I'm braking - the ABS effect (pumping in the brake lever, poor braking effect) occurs regardless of how hard I brake. In fact I even get it a slow/crawling speeds as I'm testing things in the car park where braking is hardly necessary because of the speed. Workaround for the moment is to turn off ABS...

//Derek
 
#8 · (Edited)
Hi
Are you sure it the ABS kicking in and not warped disc or caliper issue ? causing pulsing brake lever.
But you did say it is ok with ABS off so possibly not the above.

Would suggest sticking it on a GS911 or BMW diag to check front and rear sensor speeds. it might not be just front issue.
 
#9 ·
Hi Dereck,

Could you describe your problem in a bit more detail please, because I might be able to help?

The reason I ask is that my F650GS had an ABS problem when I bought it; the front brake lever would pulsate under normal braking but braking would then stop under heavy use. It turned out that there were two dings in the reluctor ring which resulted in brake lever pulsation due to the continual on-off signal from the sensor and then the complete loss of braking at low speed because the slower rotation meant the computer interpreted the break in the signal as wheel lockup. This was diagnosed by Steve Scriminger (well known and respected in BMW circles) and pressing the reluctor flat solved the problem.

Regarding comparing the F650GS with another bike, even another BMW, is a little unfair: I have an R1200ST and it's a completely different beast to my GS - if I were going a long way on good roads I would choose the ST every time, but for local journeys and even long journeys on poor roads then I would pick the GS - horses for courses and both bikes do what they are good at very well. :thumb:
 
#10 ·
Hi Andy, All that have helped,

I've solved the problem and as you've suggested it was related to the sensor/reluctor. How the problem arose is still a mystery however.

I live in Norway and the roads get icy in winter, and that's when I use the 650.

I have 2 sets of wheels, one with studded tyres mounted, and I swap them around as the seasons dictate. The thing is that being a cheapskate I swap the disc and reluctor between front wheels at the same time as swapping he wheels over, so I know how they're mounted.

You can see from the two "Reluctor" jpegs attached that there's wear where there's been contact with the sensor. When I took a close look the intermittent contact and poor alignment was clear to me. I removed the disc and found a bunch of shims had been installed behind the disc - it MUST have been done by the BMW workshop doing the work on my forks (certainly not by me)... The shims pushed the disc out of alignment!!! That was a surprise. I removed all the shims, re-installed the disc, checked the runout (perfect) and I've just come back from successfully testing perfectly functioning ABS brakes!

Thanks to you all for taking the time to help!


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//Derek
 
#11 ·
Hi Anthony - your spidey senses were right, though the disc wasn't warped... it was mounted on the hub with a bunch of shims behind it (asymmetrically I must say - not all the screws were shimmed, one had 2 shims!) so that it was misaligned! See my latest post. Cheers! //Derek
 
#12 · (Edited)
Hi Dereck,

Great news that you've found the problem, congratulations. :thumb:

However, there's something else that worries me: what was whoever fitted those shims trying to achieve and for what reason? I wouldn't it was to adjust the sensor-reluctor gap because it's pretty obvious that the sensor is adjustable by sliding in and out to get the correct position before clamping it. The only thing I can think of is that they were trying to centre the disk in the caliper, but why? :think:

Edit: I just saw your subsequent post to Anthony and from this all I can conclude is that whoever did this was a complete wazzock!
 
#13 ·
Hi Andy, I can only agree... I have no idea what the shims were supposed to do or why they were installed. The work done was to correct an oil leak from the bottom of the forks arising AFTER the same workshop had done some maintenance work on the forks - guarantee work. Brakes fine when I rode the bike to the workshop for oil leak correction (turned out they were overfilled), not fine when I rode home again...

Just by removing the sensor / wheel / disc / caliper, removing the shims and re-installing, everything was restored to full and proper alignment. :dunno::wall: Shims discarded...

It's a while ago since the problem arose. I picked the bike up late in the day so didn't turn back to address it immediately... drove home and then considered it unlikely they'd accept responsibility for the brake problem... started to doubt and thought "heck, now I have a new problem with the ABS...". The worst case is (and I prefer not to think this way) that there's an outside chance there's a plan behind this... maybe I was expected to bring the bike back for more repairs?

//Derek
 
#15 ·
It seems pretty obvious to me that the uneven distribution of the shims, now removed, were the cause of the problem, the disk was in fact okay, as demonstrated by the fact that Dereck removed them and the problem went away - or have I misunderstood what you mean? :)

Further, if the disk now squeals and he finds this unacceptable he can always put them back, distributed correctly!
 
#16 ·
Hi again - yeah, I think Andy has it right. The "shims" (anti-squeak washers) were distributed unevenly - 2 on one screw, 1 on three screws, 0 on two screws... 5 washers altogether on six screws. That was enough to misalign things. As I now understand things, the wheel was true in the forks, but the disc was misaligned on the hub causing the pulsing. You can see how about 1/3 of the reluctor has been polished by contact with the sensor in one of the earlier images I posted.

Both wheel and disc are true now, I've measured them with a runout gauge.

I'm still mystified because as I mentioned I've regularly swapped the disc between summer and winter wheels. In fact I bought and installed a brand new disc early on because the original was worn. I have never seen these washers before discovering them this time. I don't recall receiving any with the new disc.

Anyway I'm happy now. I've ridden only the 650 since I got this sorted. The 1200 is getting a bit grumpy I can tell... I genuinely thought the problem must have been something new and expensive revealed accidentally by the visit to the workshop. It's been nagging me for a while.

No squealing either, but if I do get that I now also know what to do about it!

Cheers!

//Derek
 
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