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AC Coupling question RE: 2013 Lincoln MKZ Para Drain / RGTM Part 2

  • Smeter12
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25 Mar 2020 15:08 #38625 by Smeter12
Hello there,

In ScannerDanner's Youtube video termed "2013 Lincoln MKZ Parasitic Drain/Rear Gate Trunk Module (RGTM) No COM (Part 2) at time marker 21:33, ScannerDanner turns on AC Coupling to test the lift gate's control switch input to the module. How does AC coupling help in this situation. I guess a better question is, what is AC Coupling?

Thoughts appreciated and thanks in advance.

Robert

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25 Mar 2020 18:13 - 25 Mar 2020 18:15 #38629 by Andy.MacFadyen
I haven't watched part 2 that of particular video yet so I don't know Paul's reason for using it on that module but I will explain what AC coupling is.

AC coupling is very simple. A signal can be an AC signal like mains voltage or DC like any battery but it can be an AC voltage superimposed on top of a DC voltage.
Selecting AC on an oscilloscope blocks any constant or slowly changing part of the signal and leaves just the AC part. The circuit in the oscilloscope that does this is very simple just a capacitor which sits in series between the oscilloscope's analogue to digital convertor and the + terminnal of the test lead.
Capaictors cannot pass DC current or voltage but pass AC so the DC part of the signal is removed leaving the AC.

" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)



Last edit: 25 Mar 2020 18:15 by Andy.MacFadyen.

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26 Mar 2020 04:27 #38638 by Andy.MacFadyen
I watched part two of the video --- Paul was using AC coupling to allow smaller changes in voltage when the buton was pressed to show on screen.
Much more everyday use for AC coumpling is when doing a cranking compression test or fuel pump current or cheching alternator AC voltage ripple.

For example on cranking voltage compression test switching to AC coupling will allow a 5v (or lower) scale on the the oscilloscope instead of a 20v. The lower voltage scale will make a lower compression cylinder easier to spot.

" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)



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26 Mar 2020 09:37 #38658 by IanMc
Very good reply Andy.
Anyone want to talk about sensors in airbag control units lol.

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26 Mar 2020 13:45 - 26 Mar 2020 13:56 #38661 by Andy.MacFadyen
I don't anything about the internals of the actual modules except not to turnover or jar the modules when they are powered up.

On all the euro cars I have worked the airbag module puts a 12v pull up voltage to the instrument pack cluster to extingish the dash light

Side and front impact sensor are prone to corrosion damage, I have no idea about the internals of these units (I really should cut one open) except they are smart sensor and the airbag module looks for a very distinct self-test response from the sensor Paul did a video on a corrosion damaged front impact sensor on a GM pick-up.

This is a known good I caputered from a 2003 MG ZT/Rover 75 it has the same SRS MRS system as the BMW e46





www.scannerdanner.com/media/kunena/attac...450/sideimpactlh.jpg

GM Taho Front Impact Sensor

" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)



Last edit: 26 Mar 2020 13:56 by Andy.MacFadyen.

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