Connecting scope stalls engine

[totalminibmw]

Good afternoon,
I am troubleshooting a 2008 BMW Z4M with an S54 engine. I have VANOs faults stored for the intel camshaft sensor tooth. I have done some VANOS adjustments prior to this and all looks perfect with respect to how it can hydraulically move the camshafts and positions reached. On my test drives is when It sets the faults. I am trying to hook up my scope to both Cam sensors and crank sensor along with the signal of the #1 ignition coil so that I can watch the events in the bay with the car stationary and also driving down the road. When I connect the sensor lead(im using the yellow with a AC 20V scale) the engine runs rough and stalls 80% of the time. I hooked up a DMV separately and am getting 4v steady with a .1v drop when connecting the scope lead(again making it stall). I swapped to the red lead and channel with the same result. I swapped to a BNC(non auto identify lead off of an old OTC kit) with the same result...Why? Any help appreciated-Chad

[Rfmotors1]

Hi, I have never heard anything like that.
The only explanation would be if t
you have connected the ground side to sensor but I am sure you didn't.
Assuming you have all the negative leads on battery, scope set to DC coupling and connect the signal sensor wire to positive lead.

If the sensor is two wires passive inductive type, then best is to connect both wires, each to positive separate channels.
Also in some inductive sensors I had to use either high speed x10 probe or x10 or x20 attenuator, this causes smaller voltage drop on very weak signal sensors (Ferrari gearbox for example I had to use this attenuation).
Don't know if it helps but it's something to think about.
And please update us here once you get some more info.
Regards,
Roman

[totalminibmw]

Hi, I have never heard anything like that.
The only explanation would be if t
you have connected the ground side to sensor but I am sure you didn't.
Assuming you have all the negative leads on battery, scope set to DC coupling and connect the signal sensor wire to positive lead.

If the sensor is two wires passive inductive type, then best is to connect both wires, each to positive separate channels.
Also in some inductive sensors I had to use either high speed x10 probe or x10 or x20 attenuator, this causes smaller voltage drop on very weak signal sensors (Ferrari gearbox for example I had to use this attenuation).
Don't know if it helps but it's something to think about.
And please update us here once you get some more info.
Regards,
Roman

Roman,

It is an inductive Bosch Crank sensor with 3 wires, looks like ground, shield and AC signal. Yes, I am set to AC settings for that channel. Using the schematic and verifying wire colors etc...I should be in the right pin and the signal when I can see it on the scope verifies that and I can see the TDC in the signal....What is this different probe you are talking about. Link? I can see this throwing a wrench into anyone's diag in the future so I would like to get it figured out. Thanks, Chad

[Rfmotors1]

I think the best way is to follow the Guided functions, Sensors, Crankshaft sensors and i believe it is safer to use the floating running sensor to start with.

I am usually setting the scope to DC but AC is not wrong, actually mostly recomended. See the connection picture, it is taken from the fuided function.

(The shield wire is not in the picture)
The x10 or x20 probe I mean either passive high speed probe set to x10 but better would be to use attenuator x10 ot x20 on each channel, it means two of them as you need to monitor both wires. (if the sensor is floating, the scope may corrupt the signal if connected only to one wire.
Here is the link to attenuator: https://www.picoauto.com/products/acces ... attenuator
As I wrote above, in the past I had sensors which especially at low RPM have weak signal and the scope just lower the amplitude so the CM cannot read it. This I have solved with using the x20 attenuators or high speed probes set to x10.
I hope it helps,
Roman

[totalminibmw]

I did the guided test and got the same results with signal pattern as what I had tuned knowing kind of what I was looking for. My sensor is a 3 wire though(maybe that diagram is not accounting for the shield). Just again it stalls the car. I also tried an amp clamp but could not see the signal clearly and certainly could not see TDC. I have the attenuator ordered with a few other clamps similar. If it wasn't a $200 sensor from dealer only, I would have put one in there. Why are they using 2 channels to look at sensor ground and AC signal separately, how would it corrupt the signal?

[Rfmotors1]

Let's say if the channel A probe positive goes to one of the two wires on the crank sensor and the same probe Ch A negative side on the second sensor wire. The scope should measure the voltage across this two wires. However, there is not positive or negative side as it is basically AC signal.
Then on the same scope, channel B is connected to another sensor, maybe COP coil driver signal for sync and negative side to battery ground.
If the scope is 4423 with non floating ground where the ground of each channel is shared inside the scope, then such connection will basically shorten the inductive sensor to ground via the scope common ground.

Then as the sensor has basically two "signal" wires, two channels are used, one for each side.
This will put the same condition on each sensor signal wire and keep it in ballance.

Below is the picture where only one channel was used and caused signal corruption, see the spikes on the top and especially on the missing teeth gap.

Regards,
Roman

[Grant_Baker]

See Steve Smith's reply in the below linked thread on shared and common grounds:
viewtopic.php?p=102838#p102838

Note all channel grounds should all be at battery negative and your channel inputs should be on the positive signal wires for all AC generating sensors.

[totalminibmw]

Thankyou for the explanation in detail, makes more sense now. I took your recommendation and ordered a few attenuated type products. Lead TA506 was the simplest of them and hence the one I tried first and worked perfectly. I can now examine the signals without stalling.

[Rfmotors1]

Thank you for the update.
If you would have time, please try to upload the waveform here.
It will help many other Picoscope users as we all need to learn.
And this forum here is the best place to share our stories from real diagnostic situations.
Best Regards,
Roman