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VW Passat No Start Case Study, PT.2

posted Jul 03, 2016 14:08:33 by Noah
The link to the part in the video is here
Around the 20 minute mark, paul uses his multimeter to check the 3 pins on the cam sensor (hall effect).
The signal wire showed 4.99v
The 5v ref was low at just over 3.8v
The ground voltage was high, at 4.22v
All measurements with the sensor disconnected. I get that a bad ground is going to have high voltage. What I am having trouble understanding is how can the ground have higher voltage than the feed to the circuit? I would expect it to be the same 3.8v measured on the ref circuit or lower.
Furthermore, assuming the computer has one internally shared 5v regulator, why would you have a good ref voltage on the signal wire disconnected and not on the feed?
Of course, if all the questions are answered in upcoming parts of the series, feel free to not spill the beans.


NOT related, I enjoyed the format of this video, going back and forth between the projector screen to show the hookups on the car, and the lab scope.
I think I saw one video in the not so distant past where one of the overlays was semi-transparent. I think that would work well when you have the scope traces on the screen, but still want to show what you're doing under the hood.
[Last edited Jul 03, 2016 22:56:39]
Massachusetts, USA
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5 replies
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Tyler said Jul 03, 2016 16:24:17
Agreed on the format, loving it. Very easy to follow along with the testing and theory shown. Also, I kind of like being able to hear the students questions/comments/jokes? Kind of nice to hear the gears turn inside the students heads during the diagnosis.

I was thinking about the voltage levels, too. I think you could explain the elevated ground by saying that there's probably other 12V components that share that same computer ground? Thus, the voltage level on that (bad) ground could exceed 5V.

Then there's the signal wire voltage. SUPER glad you brought this up, 'cause we've got the other VW thread going in the Repair section. The one about the 0-12V pull down CMP sensor? I'm wondering if this is the same design, and it's actually supposed to have 12V on the signal wire unplugged, not 5V. That might explain the differences in voltage level between the signal and the reference.
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Noah said Jul 03, 2016 16:54:53
Then there's the signal wire voltage. SUPER glad you brought this up, 'cause we've got the other VW thread going in the Repair section. The one about the 0-12V pull down CMP sensor? I'm wondering if this is the same design, and it's actually supposed to have 12V on the signal wire unplugged, not 5V.

I was really hoping this case study would shed some light thread as well.
I was kind of thinking that the 5 volts on that other car may have been some kind of bias voltage with the connector unplugged, and 12v plugged in. I think it's unlikely that's the case however.

Having other 12v components share that ground would make sense as to the elevated voltage levels. I wonder if a voltage check on pin 4 of the dlc would have yielded any useful information.
Massachusetts, USA
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Tyler said Jul 03, 2016 18:35:00
Having other 12v components share that ground would make sense as to the elevated voltage levels. I wonder if a voltage check on pin 4 of the dlc would have yielded any useful information.


O_O Wow, good idea about pin 4, man. That's a really interesting thought, about how you could have followed the no communication issue and ended up in the same place. Not that Paul was wrong to follow the spark/timing path, as I probably would have done the same.
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Tyler said Jul 08, 2016 01:01:13
Just finished part three, pretty happy with where it ended up. I think most customers in a retail environment would have pulled the plug after hearing about the PCM and the possibility of bent valves.

Just thinking here. Repairing the PCM board itself would be difficult to the size of the pins, understood. I was wondering if there's a reason the external grounding could not be made permanent? As in, this is my car and it's my problem, not for a customer.
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Noah said Jul 08, 2016 11:42:04
I'd try it were it my car.
I feel kind of bad for whomever bought that car.
They've gotta have all kinds of money tied up into it between parts and labor of the initial timing belt job, plus who knows whatever parts put into it to correct the no start, hours of diag before it made its way to Rosedale tech, tow fees etc. Now it needs a pcm,(and I bet it's not just plug play on an 06 VW) and the timing belt has to be re done, and it still might need a head job!
Some cars a just bic lighters... you don't mind filling a Zippo when the fluid runs out, but a bic goes in the trash when it's done.
Massachusetts, USA
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