How does ABS fail? - a question for an expert

[Peter Room]

There have been recalls of R and K series bikes that have the generation II ABS (non-servo) in various countries, for example

 

UK

http://www.dft.gov.uk/vosa/apps/recalls/searches/expand.asp?uniqueID=7802F713BF36E6898025713B003ECCD2&freeText=Blank&tx=

 

Australia http://www.recalls.gov.au/content/index.phtml/itemId/953937

 

The problem is described: "In certain exceptional circumstances (constant use of the ABS) it is possible for the ABS to stop functioning. This is as a result of excessive pressure building up in the ABS system." There is no suggestion this causes a brake fluid leak.

 

... and the solution is "to fit a pressure restrictor into the hand brake control hydraulics."

 

Is there an ABS expert out there who can tell me what causes the ABS to fail in this situation?

 

Does excess pressure trip a software or hardware fault, or stop/burnout the ABS pump, or something else?

[Satguy]

The problem is described: "In certain exceptional circumstances (constant use of the ABS) it is possible for the ABS to stop functioning. This is as a result of excessive pressure building up in the ABS system." There is no suggestion this causes a brake fluid leak.

 

... and the solution is "to fit a pressure restrictor into the hand brake control hydraulics."

 

 

This recall you quote above was actually only performed on the bikes with the servo assisted ABS brakes. While I worked in a BMW M/C dealership, I installed hundredes of the banjo bolts with built in restrictor on the front brake master cyl.

 

I don't know exactly why this was done, but I know that if the bike was used in practice stopping manouvers (like during rider training) the ABS could fail due to low voltage. This failure had something to do with low engine RPM's and lack of battery charging, along with pressure build up in the control circuit of the ABS controller/modulator.

 

I'm sure others will add aditional info.

[KDeline]

We had some clown, Opra Hazard I believe, wanted and tried to get a civil suit started. What a couple of threads that was.

[Rodney]

My 07 non-servo ABS failed at 30,000 miles . Diagnostic readout indicated "low voltage". The tech said ABS pump failures are not uncommon. I asked him if there's anything I did to cause it or anything I can do to prevent it and he said there wasn't. I only engaged the ABS a few times in all 30,000 miles.

[dirtrider]

Morning Rodney

 

That pump on your 07 I-ABS gen 2 system runs every time you use the front brake lever with the rear wheel turning.

 

To see--- Put the bike on the center stand, turn the key on, spin the rear wheel with your foot, then pull the front brake lever.

 

 

[Peter Room]

Thanks for the feedback Bruce, Rodney & DR.

 

DR is right that the ABS pump runs every time the front brake is applied but I still don't understand the recall.

 

I don't see how excess pressure in the front brake line could be linked to low voltage which causes permanent ABS failure.

 

Could excess pressure keep the ABS pump switched on full time and burn it out while draining the battery? But surely this would mean the rear brake would be on full time until the pump burned out and the rider would notice the heat and smell from the rear and reduced performance?

 

We need an expert to explain!

 

Peter

[TestPilot]

The recall in question is 06V164000. Documents pertaining to the recall are available on the NHTSA ODI web site.

 

None of the documents I viewed say anything about low voltage. They simply describe an over-pressure condition. They also state the condition is temporary and "ABS function will return under normal riding conditions."

[NewBlue]

Seems to me Satguy's explanation was pretty good. The braking pressure is controlled by the pressure input from the master cylinder, servo system. Adding the orifice to the brake master will reduce the flow to the ABS controller (thinks you're not pulling the lever as hard) Reducing the flow will reduce the pressure on the controlled flow side of the orifice, for a little while. Depending on how long you hold the lever the pressue will equalize on both sides of the orifice. One of the down sides to using a restrictor, not so much in this application, is heat.

 

They are adding a delay but will give full braking if you hold it. Long term use at idle with the servo system can cause greater heat due to higher amp draw caused by lowered voltage at idle.

 

I'm still looking for an expert but it's .02 worth

[dirtrider]

Afternoon Peter

 

I think you are mixing your ABS systems—

 

The Older 05 & early 06 I-ABS (wizzy) brake systems were the ones effected by the recall you have a link to. There is no brake fluid leak associated with the malfunction or this recall. There were also certain models under this recall period that got thicker front brake disks as part of the brake campaign.

 

The later (non servo) I-ABS gen/2 braking systems were NOT effected by that recall but had a recall of their own & that one DID involve front brake line replacement due to a possible fracture at the ABS controller end fittings causing leaks and possible brake failures.

 

I have a couple of the old recall bulletins & campaign notices out in the shop somewhere but I am no where near them now.

 

The campaign on the 2005/early 2006 I-ABS (not the I-ABS gen/2)

 

Read something like this GS campaign----

 

“””Summary: On certain motorcycles, under rapid, repeated, and unusually harsh braking, loss of ABS capability could temporarily occur.

 

Consequence: Temporary loss of ABS braking could increase the risk of a crash.

 

Remedy: Dealers will add a physical restriction, in the form of a banjo bolt with a flow restrictor to the brake line, and on certain models dealers will install new front brake discs of a slightly larger thickness free of charge. The recall is expected to begin during May 2006. Owners may contact BMW at 1-800-831-xxxx.”””

 

Note the “TEMPORARY LOSS of BRAKING” (not permanent failure).

This seldom if ever happened in on-road motorcycle usage. It was more for motorcycle training like police training or motorcycle safety range work where the brakes were pushed into ABS repeatedly therefore overheating the pump & or depleting the battery to the point of low battery power to the ABS controller.

 

I have never inspected the restrictor that was installed under the campaign against an original but seem to recall reading in a BMW tec bulletin or advisory that it improved braking performance in residual braking once the ABS pump shut down. (I have no actual data showing this though) so take this for what it is (a vague remembrance)