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Old 11-03-2016, 08:27 PM   #15
bmwman91's Avatar
Join Date: Oct 2004
Location: South Bay Area
Posts: 2,158
Originally Posted by pklym View Post
Forgive these O-scope newbie questions, we have access to an o-scope to help with troubleshooting that is documented here:

We are trying to scope both the crank and cam position sensors using a fluke 125.

Should we probe both pos & neg wires, or just positive and use a chassis ground?
What should we set the scales too for volts and time?
Is it DC or AC volts?

We are getting some readings but they don't look anything like the pictures up top.

Crap, sorry I didn't see this! It's a bit late, but it looks like you figured it out with the right sensor.

1) Where should you probe?
Obviously for the crank & cam you want the positive probe tip on the black & yellow wires' terminal ends, respectively. For the ground, the bolt where the ECU grounds might be acceptable, but I used the sensor's second wire (the corresponding yellow & black wires). This helps with noise reduction.

2) Time and voltage division settings?
Both depend on RPM. At idle the 5V/div and 10ms/div should work. At 3000RPM you probably want 10V/div and 2ms or 5ms/div.

3) DC or AC coupling?
I had the inputs set to DC coupling.

What did your readings look like? I am guessing that they were super noisy or something. I did some pretty heavy post-processing on the scope captures because there was a lot of noise to filter out to get the proper waveform in a clean format. The ECU does its own filtering and stuff, but if you are probing from the outside you'll probably need to do some of your own.
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