ABS modulator failed!!!!!! - K-Bikes.com - Excellence in Motion
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post #1 of 14 (permalink) Old Jul 10th, 2009, 10:15 pm Thread Starter
 
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ABS modulator failed!!!!!!

My rear power brake servo has not worked all year and the ABS red dash lights have been on. The rear brake still works but there is no ABS and no power brakes. Power brakes still work up front.

Finally got it into my stealer today and was told the ABS modulator has failed. Cost to replace with labor is $2900!!!!!!!!! Was also told that the K12RS has had a high failure rate for this unit. I had not heard of this prior - have any of you?

The parts guy told me to watch Beemerboneyard.com as once in a while one will come up and usually sell around $500 but there's no guarantee how long IT will last.

Here's the interesting part. I asked the service manager if the front ABS would work even if the rear wasn't. He did not know. So on my way home I tested it in a gravel parking lot and the front ABS was definately working. There is only one ABS modulator unit and it seems if one end didn't work the other wouldn't either. No the case. I tested it 3 different times and it is most definately working up front. Are there two seperate sending units coming out of the modulator?

Also, if the rear portion has failed is it just a matter of time before the front does as well?

Right now, the bike is very driveable as front brakes are working fine and rear brake still works - you just have to press harder. If the front fails as well I don't know if there will be adequate braking. Any thoughts?

Thanks for the help.
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post #2 of 14 (permalink) Old Jul 11th, 2009, 7:26 am
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What year is your bike?

I got the "You went a little long on the brake flush treatment" at one dealer. He said "No more than two years between flushes." I went nearly 3 years and he said "It came out a little yucky and brown." Hope mine holds out (mine's an 2006).

Good luck on your search. Personally, I'd be a little worried since it might experience dry-rot on the internal rubber/seals if it has been in salvage too long.


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post #3 of 14 (permalink) Old Jul 11th, 2009, 9:56 am
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I only know the older one's which have two separate circuits and share a common pump. If yours is like mine you will see 2 bleed nipples on the top, one for the front circuit and one for the rear. As far as I know they just activate mechanical clutches for each circuit.

The modulator hydraulics sit around a long time not doing a lot. Their position on the bike means they couldn't have designed a better device to trap water and not drain it out. In fact, if it was pretending to be a water trap it just needs a couple of bleed of points at the bottom!

There isn't much reported about failures and it seems that regular and thorough brake flushes keep them happy along with occasional activation. There have been attempts at poking around inside, but you need the time, patience and inclination. There are 'no serviceable parts' available as spares. Most problems have either been the electrical relay or seizure on a piston. A check is done by the ABS computer at startup and it is quite sensitive at detecting a stuck piston - which then puts on the warning. The BMW diagnostics machine can interrogate the fault code which more or less says the ABS unit is faulty and the Stealer starts rubbing his hands for the $$'s.



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Last edited by voxmagna; Jul 11th, 2009 at 10:06 am.
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post #4 of 14 (permalink) Old Jul 12th, 2009, 9:13 am
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Quote:
Originally Posted by voxmagna

... it seems that regular and thorough brake flushes keep them happy along with occasional activation...
This may sound like a dumb question, but: How does a conservative rider occasionally activate the ABS? Without fear of dropping the bike, that is.

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post #5 of 14 (permalink) Old Jul 12th, 2009, 9:30 am
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10mph on gravel or dirt track. The rears are no probs. The front takes some guts. I don't have linked brakes or servos.

The rear is easy on a wet road.

One day I'll build a box so I can do it on the center stand.



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post #6 of 14 (permalink) Old Jul 12th, 2009, 10:49 am Thread Starter
 
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Quote:
Originally Posted by six_rabbits
This may sound like a dumb question, but: How does a conservative rider occasionally activate the ABS? Without fear of dropping the bike, that is.

Actually, I forced myself to test the front brakes to see iff the ABS was still working and it wasn't bad at all. Gravel, dirt parking lot, 10 -15 mph, going dead straight and hit the brakes hard. I did it 3 separate times and all was well. ABS definately working ok still on the front.

BTW, my K12 is a 2002 and I think it has the ABSII system (I know brakes are linked so if you use the front brake it activates the back as well, but not the reverse - back brake lever just controlls the back brake). I am still dumbfounded as to why the front works but not the back...........

Does anyone have any idea as to what kind of brake power I would have on the front if the modulator unit went completely? Does the servo pump activate more pistons and thus provide additional braking power? Is the servo pump and the modulator unit one and the same or two separate components - or is the servo housed inside the modulator unit?

Is there any chance the battery could be just weak enough it won't properly run the entire modulator unit? It seems ok with good cranking power but as far as I know it's the original battery. I have owned the bike two years and have not changed it - not sure if the original owner did but I doubt it as there were only 10K miles on it.
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post #7 of 14 (permalink) Old Jul 12th, 2009, 1:28 pm
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Testing out your ABS probably isn't something for 'conservative' riders.

I can only say about my earlier ABS11 modulator, but I don't think BMW philosophy would have changed.

The ABS computer has to do voltage checks all the time and if it gets the faintest sniff of a low battery the ABS will lockout faster than you can blink. It then puts up the front warnings.

On my ABS11 the brake fluid passes through the piston chambers. The purpose of the modulators is to remove braking effort not add to it. If you think about it, you have a wheel lockup ready for a skid and the ABS has to remove the effort you have on from white knuckles and/or a heavy boot first, then re-apply it. It wouldn't put more on than what you started with - that's like saying if I take hands and feet off the brakes the bike will stop on its own.

That's the scary thing if you've been doing some testing as it can remove braking effort when your brain thinks you might need it. During one slow speed test I definitely thought I would have hit something sooner than if I'd had no ABS. Their algorithm is quite strange for slow speeds. I think when you brake hard from say 70mph the number of brake 'pulloffs' you get for that stopping distance will probably save your life but previous posters have had ABS come in at speeds below 30mph and found the ABS pulloff time so relatively long over a short stopping distance, they get the brown stuff in their pants. That's the problem the GS guys used to have off road and why I think they fitted an ABS disable switch.

Your slow speed testing is informative and I wonder how you felt if you were braking for real whilst the ABS was pulsing 'off'? The ABS algorithms may change depending on what firmware the ABS computer has in it which could be different year to year.



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post #8 of 14 (permalink) Old Jul 14th, 2009, 10:06 am
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My friend is also an independent BMW wrench. I was over at his shop and recently a bike came in with the same symptoms and dealer resolution. He found that it was not as the dealer had said but did need flush and resetting some things and I think a sensor. It ran about 600. This is not a quote about the problem ,work or anything. Just a remembering that sometimes there are other things that can be done to resolve the situation. Perhaps get a good second opinion.

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post #9 of 14 (permalink) Old Jul 15th, 2009, 11:37 am
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ABS pump issues

Moosedog,

Interesting problem, that the rear does not have ABS function. I had the flashing light come on on about 1 out of every 90 brake applications. I finally picked up a used pump, but over time, as in 6 months, my brake issue went away.

So I have a used pump here if you're interested.

But I would get a brake flush done first, and have a few other checks done.

I would be looking for $450. for the pump. I tcame off a 2004 K1200GT with 12K on it.

My bike is also an '02 RS. Pump should be compatible with your bike.
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post #10 of 14 (permalink) Old Jul 15th, 2009, 10:50 pm Thread Starter
 
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Thanks for the reply's and the offer of the used pump. Right now I have it working properly again. I kept thinking about the fix cited by a couple of others in other threads, that the brake lever was not returning all the way up. As I started it up to go out for a ride the other day, I bent down and pulled up the brake lever by hand and the lights went off and the servo worked - for about 30 seconds - but enough to make me think "hmmmmmmmmmm". So when I got home that night I worked the brake lever and assembly by hand and sprayed it up good with silicone and let it sit overnight. Next day it worked fine all day long.

Trouble with computers they can only tell you what it knows - it can't reason out a problem. I am a bit (understatement) disappointed with my stealer as they would offer no other ideas of what it could be - just "oh the computer says it's the abs modulator - $3000 please". Thanks to you good people on this site who gave me other possibilites to at least try.

Now, I'm not sure how long this fix will last and I'm honestly trying (agonizing) to decide whether I should trust the bike and keep it or get rid of it while all is working. Is it normal for bikes to have a $3000 part on it that has a fairly high failure rate or just this particular bike?
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