08-04-2018, 09:42 AM
A few little factoids you may or may not be unaware of;
ABS wheel sensors are divided into two types, Passive and Active....Or should i say Magnetic Inductive and Magneto Resistive- Hall Effect!
As far as i can see the only real-world advantage of the latter is it can distinguish between forward and reverse motion, so can be used to give feedback to yet another of those inane driving aids- Hill Start Assist. With a 'scope the Passive can be identified by it's sine wave generation, although i identified my own Peugeot system as Passive by detecting a double pulse of DC which the car sends to the wheel sensors to establish they are intact when the ignition is switched on. On a multimeter the Passive sensor will return a lowish resistance typically under 2,500 ohms, but the Active is much higher at several Meg. The basic construction of the latter, as i understand it, is two parallel resistors leading to a magnetic substrate which is of low conductivity.
I had thought that testing a wheel sensor would be as straightforward as testing a (coil and magnet) crankshaft sensor for continuity as around 500 ohms.. but no (or rather- not necessarily!) Worrying to note in the case of my crankshaft sensor that when it fails (and it did) everything else stops working- no ignition and no fuel injection. There is no double redundancy. I now carry a spare and the splined bit required to change it.
By contrast the 1987 Cavalier is inherently more trustworthy- the humour of the MOT tester came out when he parallel parked at the end of the test- 'Your parking sensors and your power steering aren't working; other than that it's passed'
Dave
ABS wheel sensors are divided into two types, Passive and Active....Or should i say Magnetic Inductive and Magneto Resistive- Hall Effect!
As far as i can see the only real-world advantage of the latter is it can distinguish between forward and reverse motion, so can be used to give feedback to yet another of those inane driving aids- Hill Start Assist. With a 'scope the Passive can be identified by it's sine wave generation, although i identified my own Peugeot system as Passive by detecting a double pulse of DC which the car sends to the wheel sensors to establish they are intact when the ignition is switched on. On a multimeter the Passive sensor will return a lowish resistance typically under 2,500 ohms, but the Active is much higher at several Meg. The basic construction of the latter, as i understand it, is two parallel resistors leading to a magnetic substrate which is of low conductivity.
I had thought that testing a wheel sensor would be as straightforward as testing a (coil and magnet) crankshaft sensor for continuity as around 500 ohms.. but no (or rather- not necessarily!) Worrying to note in the case of my crankshaft sensor that when it fails (and it did) everything else stops working- no ignition and no fuel injection. There is no double redundancy. I now carry a spare and the splined bit required to change it.
By contrast the 1987 Cavalier is inherently more trustworthy- the humour of the MOT tester came out when he parallel parked at the end of the test- 'Your parking sensors and your power steering aren't working; other than that it's passed'
Dave
