Help for the newbie

If you are not a PicoScope user and want advice with a diagnostic problem then post here. Forum members may be able to help.
Automod
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Help for the newbie

Post by Automod » Sat Sep 07, 2019 6:17 am

Good morning

I have been passed a car to look at as its been causing loads of problems, I was given this car origianlly to replace the engine loom as the wires where just falling apart, after replacing the loom the car still had the same problems of injectors dropping out, its an old merc c220.
Now as im new to pico I thought I would start testing sensors and having a play to see if I could get to grips with my new tool.
As the sensor on this car has a sheilded wire all the way to ecu I had to to back pin there.
So attached is a recording of the sensor.
Sensor ohms should be around 860 when unplugged im getting about 900 which increases when I it warm up, so to me this is already out of spec, but after looking at scope its all over the place and at first thought it was just my cheap Hantek leads( Because im new to pico only have base model 2205a) the car starts fine and runs on all 4 for a short time but then drops out, if there is somebody out there that can understand the signal abit more and confirm my thoughts I might be able to sleep at night and help this garage finally give customer car back. I have also attached a cam and crank scope of the same car.

Thank you in advance to any help sent my way.
ps Please be gentle with me im new to all this :D
Attachments
Merc CAM CRANK.psdata
(139.55 KiB) Downloaded 16 times
merc crank.psdata
(119.36 KiB) Downloaded 10 times

victor2k
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Re: Help for the newbie

Post by victor2k » Sat Sep 07, 2019 8:05 pm

Hello,
Please add some car/engine/year info,a scan with stored faults can help.
Also make a new capture but selecting DC coupling for both channels(in your capture one was off) and a maximum resolution(8 bits for your scope).
Here is an example of good signals
Best regards
Attachments
c202.JPG
20 ms/div,1 v/div

Automod
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Re: Help for the newbie

Post by Automod » Sun Sep 08, 2019 9:27 am

Morning
Thank you for reply, I'm abit confused why I need to set to DC as these are Inductive crank sensor 2 wires in a shielded sheath, so I thought these produced AC output.
Now looking at your picture, I'm guessing that top crank signal you can see a gap where I'm presuming TDC is, on mine there is no defined gap for TDC which I'm guessing at the moment that's why injector 1 and 3 keep dropping out. I've ordered a new crank sensor just so I now know that both cam and crank sensors are new.
Before I got it they replace cam sensor, injectors, ecu, and had some other wiring loom repairs.
The only codes that come up are misfiring on 1.

victor2k
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Re: Help for the newbie

Post by victor2k » Sun Sep 08, 2019 11:53 am

Automod wrote:
Sun Sep 08, 2019 9:27 am
Morning
Thank you for reply, I'm abit confused why I need to set to DC as these are Inductive crank sensor 2 wires in a shielded sheath, so I thought these produced AC output.
Hello,
Because of noise present when engine is off(1,5 Vpp),this is a requested value at starting :)
Also try to remove any laptop charger,monitor supply or the external charger for car battery.
Some data to check in WIS(VIN,year of production,type of engine)?
Attachments
Capture.JPG

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Autonerdz
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Re: Help for the newbie

Post by Autonerdz » Sun Sep 08, 2019 8:07 pm

Automod wrote:
Sun Sep 08, 2019 9:27 am

I'm abit confused why I need to set to DC as these are Inductive crank sensor 2 wires in a shielded sheath, so I thought these produced AC output.
It is a common misconception that AC signals need AC coupling. This comes from a misunderstanding of what AC/DC coupling does. Many of the automotive menu selections also show AC coupling for AC signals. This is incorrect setup.

AC coupling filters out the DC component of the signal. DC coupling allows all the signal to be displayed. By choosing AC coupling you may filter out important information you need. There is no reason to select AC for AC signals. In fact, there is almost never any reason to choose AC with a PicoScope. Even when you need to see a small AC ripple riding on a DC signal, this can be done with Pico's powerful zoom and filter capabilities.

Automod
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Re: Help for the newbie

Post by Automod » Wed Sep 11, 2019 7:16 am

Thank you,
So in my image of the crank it wasn't showing the gap for TDC, if I had that set on DC would that of picked it up? Sorry for stupid question.
I've ordered a new sensor anyway because it was a little bit out of spec, and when it comes I will scope and compare.
This car is a pain in side and hoping I'm the last one to try and fix problem.

Automod
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Re: Help for the newbie

Post by Automod » Wed Sep 11, 2019 7:05 pm

So I got bored today and thought I would bore scope where crank sensor goes and saw something I've never seen before..... there was no normal castle flywheel to see, but there was 2 plates one had a recessed ball these where 180° apart, when the one with ball was in line with crankshaft sensor piston 3 was at the top. This has now confused me even more because there doesn't seem to be a ref for TDC can anyone help me lol.

Automod
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Re: Help for the newbie

Post by Automod » Fri Sep 13, 2019 8:49 pm

Thank you all for help car is now fixed.

Technician
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Re: Help for the newbie

Post by Technician » Sun Sep 15, 2019 4:14 pm

and the fix was...

Iver
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Re: Help for the newbie

Post by Iver » Mon Sep 16, 2019 12:13 am

This has now confused me even more because there doesn't seem to be a ref for TDC can anyone help me lol.
The Design engineer and ECU can take the TDC reference from not only the Crank Sensor and the Missing tooth !!

Go back to some BMW Diesels with the Bosch VP44 pump. For sure there are better and more efficient ways to collect "COMPRESSION STROKE TDC" data from.

The Crank sensor is retained for other reasons. Misfire detection, Smooth Running ... ..... .....

In answer to your question, there will always be a reference to TDC, Has to be !!

We dont need to know what the fix was.

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