I've only just got a laptop sorted for my new scope so havent had a chance to use it yet so forgive me for any ignorance...

At work today i got a vauxhall insignia 09 with the ABS light on.The DTC would clear but the brake pedal would pulse under braking (light or hard) and the light would come back on. C0050 rear right wheel speed sensor signal erratic.

There are known water ingress issues with the block connectors in the rear arches so i investigated there first. Everthing looked ok visually. I removed the sensor from the hub and that too looked ok.
I checked the wiring with the sensor plugged in back probing with my DMM. 12V down one wire with no short to earth then carried out a 'dynamic' test on the signal wire. 7-14Ma spinning the wheel.
I then repeated the test up at the ABS control unit with the same result, also tested the NSR speed sensor which was also the same.
I then reported back to VX technical who wanted me to swap the signal wires in the ABS plug, i decided against this as room is very tight so for ease and speed, swapped the rear sensors over.
Cleared the DTC and re-roadtested, same DTC came back so i suspect its the actual bearing as the magnetic pick-up is built into it. I will fit one in the morning.

my question is, if i had my scope obviously i could check the signal from the sensor. How would i workout if the sensor was faulty or the magnetic-pick up? would it be obvious if i looked at the waveform?

thanks in advance
Russ

GRUSS wrote on Jan 25^th, 2012 at 11:52am:

7-14Ma spinning the wheel.

so these sensors are magneto resistive?  the kind that don't really output a voltage trace, but an amperage trace?  if so, take a look at this thread:

http://www.autonerdz.com/cgi/yabb2/YaBB.pl?num=1276215303

So am I right in thinking that this is a 'active' Type and to be properly tested requires the use of a micro amps clamp?
Vx checking procedure advised to check the sensor with it plugged in, so I persume it needs a 12 volt feed and a earth.?

The other day I checked a rear speed sensor on a chevy aveo, I unplugged the sensor and probed the 2 pins and spun the wheel. Is this classed as a passive type and if so, creates its own voltage? I had a nice waveform on the scope but was unable to save due to the now known 'bug' with the low sample rate the Preset gives.... I really need to stop using the presets!

Where would I get a micro amps clamp from? Do pico make one?

GRUSS wrote on Mar 3^rd, 2012 at 2:33pm:

So am I right in thinking that this is a 'active' Type and to be properly tested requires the use of a micro amps clamp?

or place your trust in a good graphing scantool.


GRUSS wrote on Mar 3^rd, 2012 at 2:33pm:

I unplugged the sensor and probed the 2 pins and spun the wheel.

GRUSS wrote on Mar 3^rd, 2012 at 2:33pm:

I had a nice waveform on the scope

if it gives a waveform unplugged then it is an analog sensor. 


GRUSS wrote on Mar 3^rd, 2012 at 2:33pm:

Where would I get a micro amps clamp from?

tom has some.  here's a link:  http://www.autonerdz.com/microprobe.html

Yes it is better to use a micro amps clamp but you can still test the output on a voltage scale.

The voltage change on the signal is usually only about 0.5 volts and this signal is 'piggybacked' onto a voltage supply wire to the sensor.
This voltage supply can be set to whatever the manufacture requests so again good samples are always handy to have, ie tested one yesterday on a VW which had the signal voltage set at about 12v and I have saved captures from a Land Rover which shows the voltage signal at under 1v!

You can set the scope to AC which will zoom in on the image and allow you to use a smaller scale.

Interestingly the latest gen active sensors will generate a signal pulse even when stationary, can measure the air gap and differentiate between forward and backward movement (for hill start)


Hth
Sean