ABS operating when unrequired.

[Yaristotle]

Yaris 2004           1.3 petrol (manual)         30,000miles

Has anyone experienced a problem with this model where the abs can cut in even when only modest braking pressures are applied? The result of this anomaly is substantially firmer braking than required. The trouble is that it is neither a regular, nor frequent, experience but occurring at irregular times.    

Is this issue likely to be a consequence of a defective abs sensor? Both front tyres are new and the rears are in good condition.  If anybody is able to suggest any areas of investigation it would be much appreciated. Thanks for any advice.

[bathtub tom]

Do you mean the EBA (Electronic Brake Assist) is activating unnecessarily? If the ABS is operating at modest braking pressures then all that would do is intermittently release a brake.

I believe the EBA is triggered by either pressure applied to the brake pedal or the speed it is pressed. Could be a difficult fault to diagnose.

[Yaristotle]

 

Yes, Tom, good point, it is the EBA we are referring to here.

What I should add is that the car belongs to my dad who is relaying the symptoms down the phone. In addition I should have mentioned that this seems to only have happened when he was carrying two passengers in the rear with the front passenger seat unoccupied (don’t ask ). So this phenomena seems to arise when weight is distributed rather irregularly, perhaps unwisely so.     

[Madasafish]

First thing to do is check the tyre pressures.

Then the tracking (4 wheel tracking ). Often problems caused by kerbing/mounting pavements .

 

Then check the wheel bearings and braking systems work normally.

Then- and ONLY then - if all are correct, there is a possible fault.

 

 

 

 

[Bomber209]

Hi, EBA kicks in with a speed signal, or a weight  signal. It helps to proportion more fluid to the rear brakes as required. If your dad has weighted the rear of the car (two pax) then it will enhance braking effect. He has probably never experienced it before. If it doesn't happen without weight in rear then the system is working OK.

[bathtub tom]

1 hour ago, Bomber209 said:

Hi, EBA kicks in with a speed signal, or a weight  signal. It helps to proportion more fluid to the rear brakes as required. If your dad has weighted the rear of the car (two pax) then it will enhance braking effect. He has probably never experienced it before. If it doesn't happen without weight in rear then the system is working OK.

Sorry, got to disagree with your weight assertion. From Wiki: " By interpreting the speed and force with which the brake pedal is pushed, the system detects if the driver is trying to execute an emergency stop, and if the brake pedal is not fully applied, the system overrides and fully applies the brakes until the Anti-lock Braking System (ABS) takes over to stop the wheels locking up."

I suspect there's a proportioning valve that limits pressure to the rear brakes and by putting a couple of passengers in the back the valve's fully open allowing more braking effort to the rear wheels than the driver's used to.

[Peter S]

On 21/08/2016 at 2:33 PM, bathtub tom said:

Sorry, got to disagree with your weight assertion. From Wiki: " By interpreting the speed and force with which the brake pedal is pushed, the system detects if the driver is trying to execute an emergency stop, and if the brake pedal is not fully applied, the system overrides and fully applies the brakes until the Anti-lock Braking System (ABS) takes over to stop the wheels locking up."

I suspect there's a proportioning valve that limits pressure to the rear brakes and by putting a couple of passengers in the back the valve's fully open allowing more braking effort to the rear wheels than the driver's used to.

This also my understanding of how EBA works, applying normal braking pressure, no EBA, "stamping on the breaks" as in an emergency EBA takes over and applies full breaking pressure, ABS then prevents the wheels from locking.

[Bomber209]

Hi, OK your Dad's problem is not EBA but EBD. EBA as you suggest is for emergency situations, and EBD is weight distribution affected. More weight in the back means back brakes have more applied pressure.