Hi guys, I’m a HGV Mechanic so have some knowledge but just struggling with this one and wondering if anyone else has had similar problems. I have a mk 6 fiesta with an ABS fault that’s dragging on. A few months ago at the start we plugged it in showing the right (o/s) rear abs sensor invalid signal, carrying out live data tests which fluctuated a lot (driving and idle) compared to the other sensors. So I ordered a rear Abs sensor (on these they come together n/s & o/s as one for the rears) whilst waiting to fit the sensors the dash went on the car rev counter, centre digital screen, ABS light, handbrake light and engine management. We fitted the ABS sensors for the rear but still no joy. I then found a ‘good’ 2nd hand ABS pump on eBay with same serial no. Which I was told would fit straight on and need no programming. So I fitted the pump and bled the brakes. All lights went out. Yay! .. but then after a short road test the dreaded ABS light come back on but yet everything else was working again. After plugging it in again the fault now was - wheel speed sensor right rear tone ring tooth wheel missing fault, so I checked the sensor position- all okay and clean then ordered another tooth/ magnetic wheel. After fitting that the fault still remains and whilst monitoring live data it reads all okay but every now and again will drop and show an invalid reading. So now I’m out of ideas... any help much appreciated. 

Must be the gap between the sensor and magnetic ring. Is there too much play in the wheel bearing, does the wheel move from side to side. Is the bearing pressed into the hub fully. 

Hi, the face where the sensor is bolted is clean and fully tightened. The bearing has no obvious play (I will double check)... if the air gap was to be the fault surely the different reading would be more frequent and not a small blip every so often?.. 

If you have a multimeter connect it to the wheel sensor. You will need to disconnect the sensor plug and gently push the meter probs to the pins. With the wheel jacked up, spin it at about 1 turn every 2 seconds and you should read 0.2vac or more.

If the reading is lower check the magnetic wheel has the correct number of teeth for the tyre size. Also check it against the other side to check what voltage you get.

 

Good morning, I have carried out the following checks...

Battery voltage is at sensor plug from front.

480 ish ohms taken from each side sensor (with the loom unplugged) 

when carrying out voltage tests I get nothing from either sides testing from the end of sensor loom with wheel stationary or spinning.. 

little confused, help much appreciated.

The sensor on the 2006 must use hall effect so it wont give ac voltage. The ohm test is the same for both so I guess that both sensors are ok. 

Could check the wiring for a dirty connection. You could spray wd40 into the connectors and see if the signal picks up.

My 2004 says measure 1.5vac to 2vac when spinning the wheel. It might be different than what the manual says though.

Also, measure the pins on the other side of the connector that the sensors plug into. With the power off check the resistance on the pins for both connectors that go to the rear sensors. The resistance should be the same across for both pins.

Thanks for you help mate. In the mean time I swapped the drums/bearings from one side to the other and making a small adjustment on the magnetic wheel and so far so good. 
 

Thankyou very much, will keep you

updated if any problems. 

I've just read another post that had the same problem. The sensor reads the signal at the inner side and not on its end. It might be possible to push it inwards towards the wheels centre to get it closer to the magnetic ring.

Swapping the hubs was a good move. Looks like you have sorted the problem.