Subaru ABS Speed Sensor Circuit Distrotion

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Subaru ABS Speed Sensor Circuit Distrotion

Postby Steve Smith » Thu Aug 25, 2016 3:38 pm

A shout out for help as I have been pulling my hair out on this one for some time.

To cut a long story short I have been looking at an ABS issue with a Subaru Legacy for a colleague here at Pico, details below:

Make – Subaru
Model – Legacy SE
Year – 2005
Engine Size – 2457cc
Engine Code – 056542
Model Code – BP9
Chassis Number – JF1BP9KK44G007376
4WD – Yes
Mileage –99000

My tests have been carried out occasionally over lunch times and any additional time I can find during early mornings or late afternoons.

The issue surrounds ABS pump operation when the rear of the vehicle rides over speed humps, the ABS will operate the pump for up to 3 seconds and then revert to normal operation.

A fault code scan initially revealed Code C0115 only which refers to “Wheel Speed Sensor”.
The code seldom appears and the ABS light does not illuminate to indicate a fault. (ABS warning light is normal at engine start)

As the rear of the vehicle rides over speed humps, noise/distortion is present throughout the ABS speed sensor circuits (voltage) front and rear.
The noise is not so apparent in the ABS speed sensor current
This event may or may not be accompanied with ABS pump operation.

The image below highlights the brief period of ABS signal distortion prior to pump operation

ABS SPEED SIGNAL DISTORTION.jpg
ABS Speed Signal Distrotion


TEST 1 ROADTEST ALT DISCON.psdata
ABS Speed signal distortion psdata file
(1.44 MiB) Downloaded 5 times


I believe there is sufficient reduction in the rear speed signal frequency to initiate ABS solenoid and then pump operation, but I cannot prove why!

ABS NOISE.jpg
ABS Speed Signal Noise


NSR WHEEL SPEED SIGNAL.psdata
NSR Speed signal voltage and current
(1.9 MiB) Downloaded 8 times


Road wheels and tyres are correct, rear brakes do not drag/bind, ABS sensor wiring secure and routed correctly.
G sensor signal wire valid between 2.34 V to 2.8 V depending on posture of vehicle on road test.
Parking brake correctly adjusted and rear wheel bearing “play” is zero

My thought now move to the 4WD system and the distribution of drive to the rear wheels.

Any help, advice, assistance or feedback would be most welcome and allow me to sleep once more


Take care………Steve
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Re: Subaru ABS Speed Sensor Circuit Distrotion

Postby steevegt » Thu Aug 25, 2016 4:00 pm

Could't it be from worn shocks or bad springs?

Going over a speed bump, even if the car stays in the same linear speed, the up and down motion of the wheels will change the relative wheel speed momentarily. If the shocks are worn, and the up and down amplitude is higher, could't it fool the ABS module that the wheels are binding or dragging?

Just a guess...
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Re: Subaru ABS Speed Sensor Circuit Distrotion

Postby STC » Thu Aug 25, 2016 7:44 pm

Hi Steve

I'm on a tablet at the moment so cannot open the PS Data Files. I do have a clear view of the snapshots in the thread.

I have seen a couple of ABS Sensor Issues that were labelled as unsolvable in the past. I have one that is on-going now. None are Suburu's

A Ford Galaxy that would throw an OSF WSS fault, mostly when turning left. This car had a new ABS Controller, Countless Wheel Sensors, Wheel Bearing with Phase Wheel & the Powershift Gearbox rebuilt.

I was on that car for hours at a time over 3 days, Load testing, driving, scoping ......

I pulled the loom to bits, even the part in the wheel arch that is insulated in Silicon rubber very carefully with a scalpel.

I then cut the original wires at both ends and ran new wires in parallel (twisted them first with a drill) to the loom and gave up - well nearly !

Then, I'm not sure why, I then routed my new twisted pair across the front under the slam panel (rather than in the engine loom) and then back to the ABS controller. Fixed - I live to tell the tale, Just have no clue how !!! Interference ??

The Other was a Reluctor / Phase Wheel with a hairline crack. I read the post through again and saw no mention of that being checked.


We know that the Engine ECU will ignore "Bumpy" signals from CKP when It is sure form ABS that the car is being driven on rough ground. - Maybe not the answer but could be put into the equation ?
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Re: Subaru ABS Speed Sensor Circuit Distrotion

Postby Steve Smith » Tue Aug 30, 2016 10:42 am

Thank you so much for the feedback.

I know the feeling well (3 days on a particular fault)

There is just something inside that will not allow you to "let it go". Its like admitting defeat.

I believe the ABS is responding as expected to a reduction in wheel speed and so I have been chasing a symptom rather than the fault!

The image below indicates 4 x wheel speeds as the vehicle rides the speed humps (Freq Maths channels applied to all four inputs)

DROP IN FREQUENCY.jpg
4 x Wheel Speed signals over speed hump


The front wheels see minimal reduction in wheel speed, where the rears see a drop of 15 Hz in 30 ms.
Could this be the criteria in which the ABS determines imminent wheel lock? (At this low road speed)

Once again the scope reveals in depth information about the opertaion of components that there is simply no data to compare with. (No manufacturer specification)

The suspension ride height I had not considered and I must confess the rear of the vehicle is looking "tired"
It has been a towing vehicle and its posture does "sit down" towards the rear when unladen.

Could this be an issue with the adhesion of the wheel to the speed hump during such momentary suspension deflection? (Poor damping)

I am going to experiment with Tyre pressures and then see how I could increase ride height during testing.

Thank you again so much and rest assured I will feedback

Take care.......Steve
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Re: Subaru ABS Speed Sensor Circuit Distrotion

Postby STC » Tue Aug 30, 2016 12:11 pm

The front wheels see minimal reduction in wheel speed, where the rears see a drop of 15 Hz in 30 ms.
Could this be the criteria in which the ABS determines imminent wheel lock? (At this low road speed)


Indeed there is no need for ABS to do anthing at low such low road speeds, but that does not mean it wont. It does have a mind of its own (Well that of the Bloke / Chick that wrote the software) You will know as we all do, that overlooking an unreasonable theory has the potential to come back and bite you - Hard !!!
Let's face it, both rear VSS's are showing a similar variation in frequency (one dips a tad more). Unlikely they are telling the same lies.

Could this be an issue with the adhesion of the wheel to the speed hump during such momentary suspension deflection? (Poor damping)
I am going to experiment with Tyre pressures and then see how I could increase ride height during testing.


Not Sure how Increasing Ride Height will help, unless you put new springs and shocks on it ?

If it is an adhesion issue, tyre pressures ? Lowering them would help and/or putting some weight in the back ?? Easier and quicker to achieve too.

The front ones do have the unfair advantage of an engine holding them down.
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Re: Subaru ABS Speed Sensor Circuit Distrotion

Postby Dcunning35 » Thu Sep 01, 2016 4:12 pm

Hi please forgive me if I've missed something out of the thread but I'm viewing this on my phone , I would go with the theory of this being mechanical as you suspect differing roadspeeds between axles and in relation too steering angles etc.
If you can check the geometry to ensure there is sufficient tyre footprint on the road I've seen this a lot with the focus st and rs due to bush deterioration.
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Re: Subaru ABS Speed Sensor Circuit Distrotion

Postby STC » Thu Sep 01, 2016 8:49 pm

Danny
check the geometry to ensure there is sufficient tyre footprint on the road


You should be a carpenter !!! :D :D :D
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Re: Subaru ABS Speed Sensor Circuit Distrotion

Postby Mark Dalton » Mon Sep 26, 2016 1:32 pm

Hi Steve.
Is the driver the original owner reporting an issue that was never there before or is (s)he reporting an issue they noticed after they took ownership of the car? I've seen something similar on a different make and model where the ABS would activate at fairly low speed when the front tyres crossed gutters into driveways. It needed to be just the right conditions for it to occur, but but the ABS activation was unexpected at that speed over those bumps, but the ABS obviously saw it as an issue. Does the issue occur over any and every speed hump at any speed. Can you drive it so only one wheel goes over the speed bump.
Although a thought just occurs to me, is this with the brake pedal pressed? Are we talking an ABS issue or Traction Control issue.
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