ABS light on: system overhaul, pics request

[rmcdonnell]

Hi Folks,

My ABS pump started getting a little noisier about a year ago, with a brrrhh! noise at 3mph rather than a gentle whirr. I noticed a month or two later that the actual ABS function, which was working well before then, was starting to limit a bit early and pulse before the wheels would have locked up anyway, but I think that was probably air in the system. Recently I've fitted new front pads, and got the main system bled OK, so I now have a high and firm pedal, with good brakes.

Today the Antilock light has started coming on, after the pump test, after the car is moving and gone over the 3mph point. It stays on.

So this is probably a faulty wheel sensor, or poor sensor connection, I will check these.
Are there some typical oscilloscope pictures of normal waveforms, somewhere?
The electrical diagram shows all the sensors converging on a connector before the ABS control unit, is this at the control unit?
Or maybe the pump is not working correctly? Air in it?
Or the relays maybe corroded or malfunctioning: I think there are some behind the pump with a loom connection maybe? This is behind the wheel arch liner, and I haven't had a look for a while.
Are there any more relays elsewhere, apart from the main ECU relay area?

Where is the ABS control unit (the brain?), and what does it look like? I think it may be behind the dashboard (my car is RHD) on the right of the car. I think there is a multipole connector for ABS somewhere there too, is this close to the right hand front wheel arch, or nearer the center somewhere?

I have stuck my head under the right hand footwell, (this hurts my back on the handbrake, RHD remember!) but I'm not sure what I'm looking at, or for..!

And to check the system, if air has got in the ABS pump, does anyone know if I can I operate the ABS valves, and bleed the system completely?

It would be good to get all this info together, so any photos of ABS bits would be useful, rotors, sensors, pump, relays, connectors, ABS brain etc

Any info appreciated.
Best regards

Russell

[924RACR]

Not air or pump, unless the pump itself died. If it was making much more noise, that might be a bit concerning.

The 3mph test is to run the pump and cycle the valves. So step one is check the relays and fuses - sadly I'm not so familiar with the old systems in the 944, so can't speak to location of those parts. But IIRC you should have a pump motor relay and maybe one other...

The early control would make me suspect wheel speed sensor(s)... but not sure if this is in addition to relay/pump/valve problems...

[RC924]

Originally Posted by rmcdonnell

So this is probably a faulty wheel sensor, or poor sensor connection, I will check these.
Are there some typical oscilloscope pictures of normal waveforms, somewhere?

Yes, would check the wheel sensors first. Scoping the output signal is best however a rudimentary check can be made on a DMM with frequency. Jacked up, spinning each wheel. No signal = no reading, spinning faster will increase frequency.

Since these are variable reluctance sensors the output signal is a distorted sine wave. Amplitude and frequency will increase with RPM. The trace should look similar to this pic although maybe transposed +/- depending on direction of rotation. The control unit registers at the zero crossing line, processing the frequency as RPM value and not the signal amplitude.

[rmcdonnell]

Brilliant. Might be able to check this out on Sunday, depending on the weather. The sensors or a connection are the most likely culprits. I've had front pads in and out recently, so maybe I've hooked them somewhere they shouldn't be, and now one is complaining. I can fix my brake pad light while I'm in there.

cheers
R

[bazar01]

You'll be lucky if it was just a bad or dirty wheel sensors. I fought this issue for a long time checking wheel sensor signals while spinning the wheels with a scope meter at the ABS computer connector. I ended up disabling the ABS system and unplugging the T40 connector to get rid of the ABS fault.
Good luck!

[Aufgeladen944]

ABS is overrated, anyways.

[924RACR]

Originally Posted by Aufgeladen944

ABS is overrated, anyways.

(says the Siena grad)
(groan)

As those sensors are passive, not active, they aren't direction-sensitive - they'll read the same, both on an o-scope and to the ECU, regardless of direction. Not that it matters much for your diagnosis...

[RC924]

Yes, Vaughan is technically correct, the direction of rotation will not alter the phase. The sensor polarity will. For this diagnosis though it is irrelevant if the output goes positive on approaching teeth then negative or visa versa. Either way results in an AC waveform. The diagram I posted above was simply a quick link from a well known site and is more ignition specific, since the teeth on dizzy pickups or toothed wheels generally have a smaller tooth and larger gap. Commonly ABS sensor "wheels" have an even 50% coverage (the tooth is as wide as the gap) which results in a much closer to perfect sine wave. The actual trace should look somewhere between a pure sine wave and the one pictured above.

The timebase setting on the scope will expand or compress the X asis. As each tooth passes the sensor it will generate a positive, (or negative) peak then go negative (opposite) and repeat for each and every tooth. What you need to look for is a consistent output signal at a consistent RPM.

These links to the Picoscope site will explain more. Some aspects and settings may depend on your particular scope. The timebase setting of 10mS looks like a good start but as usual recommend starting at the highest ACV setting and dropping down. Should be < 1V though if only spinning by hand.

Real-Life Diagnostics - Diagnosing ABS

ABS speed sensor

Good luck, hopefully is only a dodgy connector or a sensor at worst.