Crankshaft Sensor

[Bixxer Bob]

While I had the tank off today, I thought I'd have a look at the output from my crankshaft sensor.  I'd been reading about how to do this with an oscilloscope recently and was keen to have a go.

Firstly (after taking the tank off) I realised the sensor plug was difficult to get at so traced the wires to the next point on the main loom, which happened to be a plug to the right of the fuel rail, just next to the oil breather pipe where it comes out of the airbox.  I identified the senor pair as being a white with blue stripe and blue with grey band.  The plugs have watertight seals on them so I very carefully slipped a pin down the side of the wires as far as the connectors.  Just to be sure I had touched metal, I put a multimeter across the pins and got about 6.7 Ohms so good enough.  I next clipped the 'scope sensors to the two pins like this:





Then set up the 'scope inputs to AC, 0.2 milisecs and with a bit of fine tuning got a picture:



Being new to this, I'm not very sure what I'm looking at; but here goes;

1.    Pulse to pulse (almost the width of the pic) is one revolution.
2.    The gap with the spike is the reference gap on the alternator marks.
3.    I wouldn't be surprised to see the alternator has 22 marks to match the 22 peaks in the wave pattern.

I also noticed that the second pulse is a lot cleaner than the first which has a couple of shadows on it.  I checked this several times and kept getting the odd pulse that wasn't clean.  I'm not sure yet whether this was down to my lash-up or a faulty crank sensor - I've had the odd time where it's just died on me when coming to a halt, but no classic crank sensor breakdowns - so I'll just keep an eye on it for now.  At £50 I'm not just replacing it for fun.

[iansoady]

Sounds like you have time on your hands.

How much does an oscilloscope cost? I'm always ready for new toys.

[haroldo_psf]

John,

  The odd timing which causes it to stall is the very symptom I had which was cured by replacing it. I too have looked at the oscilloscope signal for my crankshaft sensor, and also my exhaust cam sensor (other motorcycle) and the thing is, without specs from the manufacturer, it is hard to diagnose the sensor. In my case, I had broken teeth on my crank shaft sproket (long story), so it was easy to see in the scope what was wrong.

This is my horror story and complete ordeal, but I think two thirds into it you will find the screen captures of my scope measurements, and a godo dialog with Brian, who knows a lot about these sensors, but still hard to diagnose without manufacturer specs:

http://forum.concours.org/index.php?topic=64175.0 (http://forum.concours.org/index.php?topic=64175.0)

Ian,

 Oscilloscopes can cost anywhere from $500 to $10000, depending on what you want. For a simple two channel digital storage oscilloscope, 1GHz sampling rate, you can get a decent one with removable storage and color screen for about $800 bucks.

You can probably get analog phosphorus (sp?) screen scopes like the one John is using for dirt cheap off ebay, or sometimes even for free from companies that are disposing of obsolete equipment.

[Bixxer Bob]

Thanks H, I'll have a read.  

Ian,  I got mine off Ebay for £45.  It's old but works perfectly nd came with the original manual.  You can pay a bit more than that for a digital hand held one from China -again from Ebay.

[Bixxer Bob]

Well,  I guess I know you a whole lot better now Haroldo; you're one of those guys that stuff happens to eh?  :lol:

I wanted to get a single pulse pic but need to work out a trigger.  If these things had a normal coil and HT lead, I have a clamp sensor that could trigger the scope from the HT pulse.  Hang on, I could probably trigger from the LT plug on the coil stick.  Worth a go anyway.

At this morning's attempt the wave pattern was too erratic to be sure that the shadow was coming from the sensor and not from the setup.  Lots to learn yet.... :)

[haroldo_psf]

Well,  I guess I know you a whole lot better now Haroldo; you're one of those guys that stuff happens to eh?  :lol:

I wanted to get a single pulse pic but need to work out a trigger.  If these things had a normal coil and HT lead, I have a clamp sensor that could trigger the scope from the HT pulse.  Hang on, I could probably trigger from the LT plug on the coil stick.  Worth a go anyway.

At this morning's attempt the wave pattern was too erratic to be sure that the shadow was coming from the sensor and not from the setup.  Lots to learn yet.... :)

Yes, stuff just happens to me! :(

Keep us posted!
Also, it could be a more straight simple way to document a quick was to diagnose it if, in case you decide to just replace the sensor, to capture the signal just like you did and compare old vs new  sensor. Even if we don't understand exactly what is going on with the signal, the difference in plots could be very valuable info and  good guideline for anyone who would want to measure it in the future.