Wierd ABS Issue

[keithy1959]

My 1999 V70 petrol has just passed its MOT, despite having an intermittent ABS issue not showing up on the roller test When I drive and brake, the ABS kicks in when slowing normally, even as low as 30mph, and there is a ticking noise ( imagine a bit of sticky tape stuck on a tyre ) whch speeds up as I do. If I remove the fuse it all goes away.
There are no warning lights, and no fault logs on the computer
My mechanic is flummoxed, and not sure where to start with this, and I don't want to spend too much this close to Christmas. Has anbody experienced anything similar ?

Thanks in advance

[myfirstv70]

There are some tone wheels or phonic rings just inboard of the discs and on the drive shafts. These rings (they look like a gear wheel) are shrunk on the drive shaft and tend to corrode and split giving erroneous signals to the ABS. When I went round a corner the ABS and brakes would intervene and cut power and all sorts of crazy stuff going on - it was these 'rings' , one had split and started creating havoc. Easy fix, a few quid if you can do it or maybe £100 tops if you get a garage to do it.
HTH?
Nobby

[Georgeandkira]

Just FYI: Lots of "lower end" car diagnostic tools fail to retrieve certain codes.
The lower the tool the more it'll fail to retrieve.

See if your center bolts have loosened.
These are the ones immediately behind the front wheels' center cap. If you can coax the center caps out with double faced tape, you don't even need to remove the wheels to check.
Be aware these are TTY (Torque to Yield) bolts which are designed for single use. In short, they've been stretched and are prone to breaking if reused.

edit: Visually check the pulse rings for cleanliness; you want clean rust-free teeth. Also check for cracks. They do break.

[VOLVOBOY]

OP.
Where is the "ticking noise" coming from ?
Is it there all the time , even when not braking ?
Is it a mechanical tick (like a cam lobe for example) or an electrical tick (like a relay) ?

[Laird Scooby]

Most likely cause is either a dirty sensor tip, broken, distorted or otherwise damaged and/or dirty reluctor rings or maybe the sensor is getting past its best and causing erroneous triggering of the ABS system.

The "clicking" noise would be more like a muffled light "thunk" noise, fairly rapidly at a guess? If so, this would tie in with the above.

Most ABS systems won't pick this up as a fault until the sensor fails to respond, regardless of how good or bad your fault code reader is for the simple reason there isn't a fault to pick up - not within the system anyway.

The fault is somewhere external to the system as outlined above. Clean the sensor tips and reluctor rings, checking for cracks, chipped teeth etc on the reluctor rings and clean the rings too.
Reset the ABS system fault codes regardless of whether there are any or not and see how it goes.

[keithy1959]

Originally Posted by VOLVOBOY

OP.
Where is the "ticking noise" coming from ?
Is it there all the time , even when not braking ?
Is it a mechanical tick (like a cam lobe for example) or an electrical tick (like a relay) ?

Coming from the front, Left hand side of the bonnet, as much as I can tell when driving.
It's there all the time.
Its a mechanical sound- not dissimilar to a piece of card in a bicycle wheel

[keithy1959]

Thanks for all your advice on this one - much appreciated !

I'll print these off and give them to my mechanic. I'm using a professional garage / MOT station and he works with Volvos a fair bit, so I imagine he would have a reasonably comprehensive diagnostic setup but I will check with him.

Thanks again,

Richard

[VOLVOBOY]

Hi.
I`m wondering ...
A mechanical problem with the ABS Modulator and/or the pump motor on the top of it.
An electrical issue causing the above. The fuse/relay box is in the same area. Are you sure it`s not a relay constantly triggering = more of a rapid ticking ?
Another thing...
How are the brakes ; good , bad , spongey?
I`ve heard that if air gets into the Modulator block it can be a real sod to get out again. Some systems have to be pressure bled with the engine running.
Have you ever changed the brake fluid ?

[Laird Scooby]

Originally Posted by VOLVOBOY

Hi.
I`m wondering ...
A mechanical problem with the ABS Modulator and/or the pump motor on the top of it.
An electrical issue causing the above. The fuse/relay box is in the same area. Are you sure it`s not a relay constantly triggering = more of a rapid ticking ?
Another thing...
How are the brakes ; good , bad , spongey?
I`ve heard that if air gets into the Modulator block it can be a real sod to get out again. Some systems have to be pressure bled with the engine running.
Have you ever changed the brake fluid ?

Live bleeding can also destroy the modulator block and/or ECU on some systems so check carefully before live bleeding!

Having just seen the responses above since my post, i'd also tend towards renewing the brake fluid as a first course of action followed by checking the sensors/reluctor rings and resetting the ABS ECU - sometimes an unintentional wheelspin can trigger the ABS ECU into thinking a fault exists or even turning while accelerating.

As the system tries to resolve the fault so it can either ignore or flash up the ABS light, it often repeats the self-test function causing the chattering sound from the modulator/pump area - also worth checking the earth for the modulator/pump as well as that can cause similar.

[VOLVOBOY]

Live bleeding can also destroy the modulator block and/or ECU on some systems so check carefully before live bleeding!

Any idea which it is on P2`s ; whether you can or not ? I`m not sure.