Brakes: Occasional brake pedal 'hammer'

[Moose Test]

Hello all,

I'd welcome any thoughts on the symptom my vehicle has started to exhibit this past month.

It falls into the intermittent category: happens probably every other third short trip, and therein possibly just once in 5-10 miles of driving. No pattern to it, as far as I discern.

Coming up to a road junction around town when I gently apply the brakes (so typically when travelling between 25-30 MPH), as the car starts to come to a halt (say 5-10 MPH) the brake pedal 'gives' once or twice in quick succession. I'd call it a hammering of sorts, as the pedal travels a short distance downwards and rapidly returns. When this happens there is an accompanying sound which seems like a fluid kind of noise (whooshing, or squirting).

I'd suspect ABS or DSTC activation but the Stability/DSTC warning light on the dash RHS doesn't pulse (it does light during ignition-on self-test, so it's not a false negative).

I get no ABS warning light, no central amber/red warning light and no messages on the DIM. VIDA is free of fault codes (excepting the customary, always present AQS sensor fault).

Fluid level in the reservoir is fine.

Any thoughts? Air in the fluid? Servo leak?

TIA.

[cheshired5]

Have you inspected the ABS/speed sensor rings?

[Moose Test]

Originally Posted by cheshired5

Have you inspected the ABS/speed sensor rings?

Tbh I haven't but whenever I've had issues of that type (corrosion, cracks/splits) in previous vehicles (V70, XC70 and this specific XC90) it's always been readily apparent from the presence of ABS and anti-skid warnings and fault codes. The built in diagnostics on these cars are pretty good at detecting those faults.

I'll take a look.

[TruckbusUK]

Defo, check the reluctor rings, I had similar, and it was a cracked/split ring spinning/moving on the hub.
It eventually fully split and the world lit up like a Christmas tree with ABS/Anti-Skid and stability control messages, and the ABS tell-tale warning lights on the dash.

Hope it helps

[Moose Test]

Thanks both. It's due in at my preferred local garage next week for its annual service and MOT so I'll have them check the rings over whilst it's there, as they can ramp it.

I'll post any relevant updates.

[Tannaton]

Does it feel like a normal ABS intervention? There is no warning light for that...

Given that it is only happening when you are braking there must be a clue there.... wheel bearing? Have you checked tyre pressures??


Might mean an imminent trip to Norfolk.............

[Moose Test]

@Tannaton

Yes, it feels exactly as I would expect of a normal ABS activation.

The reason I've initially discounted that possibility is that I have always believed that the DSTC Stability light illuminates during such. I may be deep into false memory territory but I'm 'sure' that in both the XC70 and XC90 I've seen that lamp light during known ABS excursions (albeit at greater speed).

Tyre pressures should be spot on, I checked them only a week ago and wear appears even on all wheels.

It's a shame that there doesn't appear to be a way to graph the sensor signals in VIDA. The BCM is only offering up pressures, pedal travel and temperature.

[SwissXC90]

Originally Posted by Moose Test

It's a shame that there doesn't appear to be a way to graph the sensor signals in VIDA. The BCM is only offering up pressures, pedal travel and temperature.

There is
You can graph the wheel sensor pulses
I've seen it

Look further: CEM, ECM, BCM or TCM. One of them has it.

[Tannaton]

Originally Posted by Moose Test

@Tannaton

Yes, it feels exactly as I would expect of a normal ABS activation.

The reason I've initially discounted that possibility is that I have always believed that the DSTC Stability light illuminates during such. I may be deep into false memory territory but I'm 'sure' that in both the XC70 and XC90 I've seen that lamp light during known ABS excursions (albeit at greater speed).

Tyre pressures should be spot on, I checked them only a week ago and wear appears even on all wheels.

It's a shame that there doesn't appear to be a way to graph the sensor signals in VIDA. The BCM is only offering up pressures, pedal travel and temperature.

There is no visual indication of an activation of ABS caused by brake action alone - I.e. intervention by the car to reduce brake effort on one or more wheels.

You do get an orange light on the DIM if the car applies the brakes during an intervention, I.e. to prevent a skid (stability control - any of three or more systems) or wheel spin (spin control).

[Moose Test]

If you said ABS reluctor ring...you were correct!

I asked my local Volvo garage to make a visual inspection during its annual service today and they identified a crack in the front nearside ring.

Many thanks to all who made suggestions.