Just when it was all working funny noise from brakes

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sandyt

Well-Known Member
Posts
1,615
Location
Wraysbury Nr Heathrow
Help needed I have searched and read a bit but cant get to the problem. I have a p38 and driving along the ABS failure comes on plus traction failure seems to be at odd times also does it first thing in the morning where to start? any ideas welcome
 
ABS and traction control is signalled from the wheel sensors. Check that the wiring and connectors are in good condition, also check that the sensors are in contact with the reluctor rings on the CV joints.
 
Will do thanks for that thinking it may be a rear wheel bearing as there is a vibration just started through the body at about the magic 53 mph will let you know
 
Hi Sandyt
I have just been through exactly the same as you, ABS Fault and Traction Failure!
My warnings had a habit of not coming on straightaway on start up but after a couple of miles or even longer, 50 miles!
I took the excellent advice of the forum members and started with the cheapest option, the brake switch! (£10) After getting halfway to Newcastle from Cardiff without the dreaded ABS Fault etc I had hoped I had cured it! Sadly not!
Next off was to do a check with a Blackbox Solutions Faultmate which showed up the front off side ABS sensor fault, this was confusing as previously no fault had shown itself on the Faultmate!
By chance I had a spare ABS sensor from an old front axle bought for the wheel bearings!
An honest hours work later I had the 'new' ABS sensor fitted and so far (1000+ miles) no ABS fault has shown.
The 'bad' ABS sensor looks as though someone has hit the end of it with a hammer and squashed it a bit! How it got like that I haven't a clue! No damage to the ring etc.
So my advice get your car Autologiced or Faultmated to determine where the fault is, it could save you a lot of time and money as ABS sensors aren't cheap!
When you know which ABS sensor is giving the fault then check that wheel for bearing play.
Incidentally the old one isn't open circuit on the meter but obviously doesn't work on the car!
Hope this helps.
Ian
 
Thanks yep I think I will take it to my trusty inie and get him to do a bit of diagnostics for me and if Chelsea win he might do it for sod all so long as i say they deservedto (thats the hard bit) will let you know
 
Irishrover said:
ABS and traction control is signalled from the wheel sensors. Check that the wiring and connectors are in good condition, also check that the sensors are in contact with the reluctor rings on the CV joints.
Click to expand...

Surely you mean NOT in contact with the reluctor ring. It's the sensor touching the reluctor ring that causes the problems. There has to be a slight air gap between the two. Any wobble in the reluctor ring caused by a failing wheel bearing causes the sensor to strike the ring and be pushed out. Giving too large and air gap for the sensor to work properly.
 
To elaborate....when the sensor is touching the ring there is a uniform wave pulse when checked on an oscilloscope. If there is a gap, the wave can be erratic.
If there is a wobble and the sensor is displaced, which it does, the thing is to eliminate the wobble and ensure that the ring runs true again.
If the sensor is inserted until it contacts the ring, any very slight run out will push the sensor up the bush, it will not "Grind" the sensor away.
Some cars allow for height setting of the sensors by fitting shims to give clearance which can be measured. On the P38 there is no way of getting in to measure any clearance, that is why they are pushed in to contact. The sensor itself is away from the metal casing that encloses it.
 
Irishrover said:
To elaborate....when the sensor is touching the ring there is a uniform wave pulse when checked on an oscilloscope. If there is a gap, the wave can be erratic.
If there is a wobble and the sensor is displaced, which it does, the thing is to eliminate the wobble and ensure that the ring runs true again.
If the sensor is inserted until it contacts the ring, any very slight run out will push the sensor up the bush, it will not "Grind" the sensor away.
Some cars allow for height setting of the sensors by fitting shims to give clearance which can be measured. On the P38 there is no way of getting in to measure any clearance, that is why they are pushed in to contact. The sensor itself is away from the metal casing that encloses it.
Click to expand...

The metal casing should NOT touch the ring in use. For the sensor to work there has to be a uniform air gap between the sensor and the reluctor ring. Try positioning your crank position sensor until it touches the flywheel. I see where your coming from in saying push until contact but i always position sensor just back a little. Both methods will work on a good hub. The sensors work when the teeth on the reluctor ring passing it cause a change of flux in the magnetic field of the sensor, these pulses are counted by the ECU and compared for each wheel. We are saying the same thing basically BUT if this guy has a hub that is running out and he pushes the sensor in until it touches, ( all well and good on a good hub) then as soon as the hub turns it will push the sensor away, if he has set it on the low spot, or there will be too large a gap if it contacted the high spot. Giving a big air gap on that wheel, the ECU will detect a weak signal and show a fault.
 
Agreed up to a point, the gap must be at a minimum...almost zero. The point is if the ring is not running true then that must be corrected first unless a mechanism is fitted to the sensor to raise and lower it in unison with the ring run out to ensure a constant clearance is maintained. In other words fix the cause first...not adapt everything else to compensate for it.
Hall effect sensors are a pain at the best of times but rely on consistency of excitation pulses.
 
Irishrover said:
Agreed up to a point, the gap must be at a minimum...almost zero. The point is if the ring is not running true then that must be corrected first unless a mechanism is fitted to the sensor to raise and lower it in unison with the ring run out to ensure a constant clearance is maintained. In other words fix the cause first...not adapt everything else to compensate for it.
Hall effect sensors are a pain at the best of times but rely on consistency of excitation pulses.
Click to expand...

Hall effect? Thats interesting, all the sensors I've destroyed have just been a magnet with a coil around it, the rotating ring just affecting the magnetic field to produce a voltage/current.
The little cylindrical magnets are very powerful and I find them useful for other things.
As RAVE ony shows 2 wires to the sensors I presumed that they were the normal magnetic jobbies. Hall effect usually requires 3 wires, supply, ground and signal return.

Happy to be corrected if I'm wrong:confused:
 
If you read page 836 on RAVe regarding sensor refit, it clearly states "Push the sensor into the bush until it makes contact with the reluctor ring. Automatic adjustment will take place when the ring is rotated" or words to that effect.
 
Irishrover said:
Agreed up to a point, the gap must be at a minimum...almost zero. The point is if the ring is not running true then that must be corrected first unless a mechanism is fitted to the sensor to raise and lower it in unison with the ring run out to ensure a constant clearance is maintained. In other words fix the cause first...not adapt everything else to compensate for it.
Hall effect sensors are a pain at the best of times but rely on consistency of excitation pulses.
Click to expand...

Yes i agree with that. Most sensors will have witness marks on them were the ring has kicked them away. Even rust buildup in the ring gaps can cause malfunctions. But if this fellows hub is wobbling about like a whippets dick no matter how many times he pushes the sensor in the fault will come back. It really does sound like his hub bearing is on it's way out i agree he needs to sort that out first. If there is no play in the bearing he is fighting a different battle. :)
 
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