St1100 ABS(l) - it is a bug or feature?

[Alberto]

Riding on a bumpy dirt road, I noticed that on bumps rear modulator engages by itself, even without applying rear or front brakes. After 15 min of it drops off error code 4 (rear abs hydraulic failure).
Going to replace abs electronic unit, but the question is- is that normal behavior or not? Have anyone encountered that rear modulator swirls on bumps without doing anything?
Also wanted to add, that except this misbehave abs works as it should, if I apply brakes and wheel locks abs engages normally

 

[jfheath]

Have you checked the condition of the downward facing surface of the rear sensor, checked for debris (the surface is magenetic and may attract eg wire bristles), and checked the air gap between the pulsar ring and the surface of the sensor.

Another place to look is the pairs of connectors that connect the sensor and the power to the wiring harness.

Check your rear brake fluid reservoir isn't over-full. Don't ask - its easy enough to check.


There is no way that the ABS should be whirring into life unless the brake has been applied. I'm thinking that it may be having difficulty getting into the correct position, or reading what position it should be in.
Agitate your brake pedal - see if the lights flicker. The switch may be set to be too sensitive.

 

[Alberto]

As I remember right - fluid level was normal, just around upper level mark on reservoir. I didn't think that was something wrong with abs sensor - because abs works when I apply brakes. I'll check rear brake switch though - looks like good idea, thanks)

 

[jfheath]

Assuming the wiring is all OK, then two things have to be happening. The brakes have to be on, AND the wheels have to be going at different speeds relative to each other (as indicated by the pulse streams going to the ECU).
The latter is almost guaranteed - going over bumpy ground, slight wheel slip on loose ground - and I imagine (but may be wrong) that the suspension bouncing at different rates has an effect on smooth circular motion of the wheels - all of which will be picked up by the ECU.

So all it needs is to think that the brake is being applied.

 

[Larry Fine]

. . . abs works when I apply brakes.

What makes you say that?

 

[Alberto]

What makes you say that?

I mean, when I try to lock one of the wheels on a slippery asphalt or dirt road, abs kicks off as it should.

 

[Larry Fine]

I mean, when I try to lock one of the wheels on a slippery asphalt or dirt road, abs kicks off as it should.

 

[Alberto]

Just checked rear brake switch. Not sure that something wrong with it - need to firmly press brake pad for 1.5-2 cm to activate brake lights. Anyway, need to dug into and check of it is possible that switch can be activated under strong vibrations / impacts.
Found road with speed limiting bumps and discovered that rear modulator activates not only when rear wheel crosses the bump but front too.
After 2-4 braking with rear ABS activation it brings error code 4 and dies. I'm puzzled for now).

 

[Alberto]

Finally found source of the problem with abs: that pressure sensor (under white plastic cover on the modulator) was bent a little bit and loose contact on every decent impact or vibration ( or slight pressure by finger), and this caused a modulator starting to pump. I just bent it back to place sensor closer to piston end and problem was solved.
I highlighted above mentioned sensor on the pic

 

[wjbertrand]

How can you tell the ABS is kicking on? I know the TCS will kick on when riding on a bumpy loose surface, and the TCS light will flash. If the ABS is kicking on maybe there is a weak or missing brake pedal return spring?

 

[Erdoc48]

Glad you found it…on my 94, I’m having issues with the front sensor (3 flashes) and I’ve cleaned it - I can’t tell what’s up with it- brakes fines, but now the lights are flashing rapidly even when just starting out, so I assume I have no ABS at this time (can’t even pull the code now as it doesn’t do the normal startup sequence). I like the idea behind these systems but sometimes I think they’re a royal PIA.

 

[Alberto]

flashing rapidly even when just starting out

It looks like abs brain doesn't see speed sensor at all and refuse to perform self test. I would start with checking wires.

 

[Alberto]

How can you tell the ABS is kicking on

Actually, is was Abs motor, not abs solenoids started to buzz. But it sounded very similar and I didn't manage to recognize what kicked off exactly.