splin

Mystery solved - the new sensor is faulty. It has the same part number as the old one - 7M3 927 807 N (front sensor with pad wear connector) but the old sensor contains a magnet, whilst the replacement doesn't, or it's too weak. I also discovered that the old sensor, which was only replaced three years ago, wasn't fitted properly and the suspension has chafed through two of the wires. That also explains why the pad wear warning light was coming on intermittently. Having repaired and refitted it my ABS warning light has gone out at last. The old sensor does react to the non-magnetic reluctor ring moving past it which clearly modulates the strength of the field from the magnet in the sensor. For anyone who needs to test their active sensors, its quite easy - connect an ammeter set to 20mA or 200mA range in series with the sensor. Looking at the 4 pin socket with the clip at the bottom, rounded side to the left, the sensor sockets are top right and bottom right. You can push thin wires into the connector to make the connections. With the ignition turned on, rotate the wheel and you should see the current change between approximately 14mA and 7mA. If not, check the voltage across the sensor which should be approximately 12V; the bottom right should be +12V and top right 0V. If you don't have 12V at the sensor then its probably an ABS control or wiring problem. If the current is significantly different to 7 or 14mA then probably the sensor is faulty. If you have 7 or 14mA but it doesn't change then it could be the sensor or the reluctor ring, or the gap between them is too big. You should be able to check the sensor by bringing a strongish magnet as close to the sensor tip as you can get - also reverse the magnet to see if the current changes. It isn't particularly easy to connect to the male pins of the mateing plugs on the car because of the shroud - I used some small croc clip leads to the rear offside socket which is much easier to see than the front plugs. I connected a long cable so I could test all the other sensors from that plug. Even better is to connect a 6 to 12V power supply, with a current limit set to 50mA or less, directly to the sensor. Its not essential to have a current limit but you must get the polarity right as the sensor appears to contain a diode which will conduct if you accidently reverse the connection, clamping the voltage to < 1V. Without a current limit, excessive current may well destroy the sensor. My sensor works down to around 4V with the currents reducing to around 4mA and 8mA so you could use a series resistor of approx 500 ohms for protection which will limit the current to less than 24mA worst case. Note though that with 12mA flowing the 12V battery voltage would be reduced to 6V at the sensor but when it switches and reduces the current, the voltage would rise meaning the difference between the on and currents will be less than when operated at constant voltage. The sensitivity of the sensor reduces at lower voltages so probably best to ensure the sensor voltage is at least 5V, at which level the currents were 5.4mA and 10.8mA. Another option is to use a partially depleted 9V battery - short it out with the ammeter and if the current is at least 10mA but less than, say 100mA, the sensor shouldn't be damaged if you inadvertently connect it with the wrong polarity.

I have a 2003 1.9TDI which has active sensors. I have a replacement sensor and I bought a 'reluctor ring' just in case as they are pretty cheap. But my understanding is that active sensors require magnetic sensor rings - so have I got the wrong one? All the adverts seem to suggest that the reluctor ring is the same for all 1995 to 2006 models, whereas the sensors changed from passive to active at the end of 2002. Applying 12V to the new sensor, a current of approx 14mA flows changing to 7mA when a magnet is brought to the tip. Moving the reluctor ring or any steel object past the sensor doesn't cause any change. I assume these sensors use hall effect switches which sense magnetic fields. The sensor doesn't appear to be magnetic and neither is the new reluctor ring so how is it supposed to work? When I get round to examining the car I can get more information but until then does anyone know what I'm missing? Incidentally, virtually all posts that I have found here or elsewhere talk about passive sensors which can be tested with an ohmmeter. And many Ebay and other sellors of ABS passive sensors don't point out that they are not suitable for post Dec 2002 1.9tdi models so beware - I nearly bought the wrong part.