Weekly Waveform 4 - AC coupling starting and charging

This weeks assignment is to use AC coupling on our starter system and charging system. We use AC coupling to remove the DC voltage from our waveform (normally 12 volts) so we can focus on what's happening on the surface of the waveform. We can change our scale from 20v down to 1volt or even less. This enhances our accuracy and the size of waveform drastically but removed our ability to measure the voltage on the trace compared to battery negative.

[@ThePracticalMechanic]

My information was inaccurate on the Alternator ripple. We should be using a faster time setting on the snapon scope so we can measure the actual ripple. In the video I showed measurements that include the rpm fluctuations as well. This will indicate a higher ripple than what the vehicle actually has. I also forgot to apply loads to the system. It's best to test ripple with some steady loads enabled(headlights, defroster grid, seat heaters). Snap-on settings will be 20MS with peak detect on, AC coupled, .5v to 1v. I will post an update to this video once filmed.

[@8power0]

AND THIS IS EXACTLY WHAT I MEAN, YOU ARE VERY IN-DEPTH WITH YOUR OBSERVATIONS I HAVE THE GREATEST CONFIDENCE IN YOUR MOST DETAILED GUIDANCE IN THIS CHANGING FIELD. WOULD LOVE TO HAVE SOME BASIC SPEC READINGS I WOULD SUPPOSE TO BE GETTING WHILE I'M TESTING . THANKS SO MUCH AND LOOKING FORWARD TO ANY AND ALL YOU HAVE TO OFFER .

[@8power0]

I AM GOING TO COME RIGHT OUT AND SAY IT, I AM NOT A BIG TIME SCOPE OPERATOR BUT WITH THESE VIDEO CLASSES I CAN BE AT A VEHICLE AND WANT TO DO A TEST BUT NOT FAMILIAR WITH CARRYING OUT THE TEST AND ALL I HAVE TO DO IS WATCH YOUR VIDEO AND FOLLOW ALONG FOR THAT TEST. SO IT DOESN'T LOOK WEIRD TO THE CUSTOMER THAT THIS GUY IS NOT SORE WHAT HE'S DOING. PLUS THEY ARE SHORT AND TO THE POINT WITH A LOT OF DATA GATHERING. JUST NEED A COUPLE MORE USUAL TESTS. THANKS FOR HELPING US ALL OUT.

[@bartscave]

The ocean example for AC coupling helps to understand it. Thanks for this series of exercises!

[@billyyoder8171]

Thank you Chad and Val. Good job again. Have a blessed and safe week to you and your family.

[@nickayivor8432]

SHREWD The Practical Mechanic
👍
From Nick Ayivor from London England UK 🇬🇧 ⏰️ 07:36am Good Morning 🙏 🌄

[@georgebonney90]

Thank you sir👍

[@spelunkerd]

I was looking for a frequency estimate for ripple that comes from the alternator, and on looking at your trace the frequency at idle is ~ 25 cycles per second. Of course frequency will depend on engine rpm, load, and the relative ratio of pulley size. Other components like ignition etc are also dependent on rpm, so recognizing artefact from other components is not so easy. It is so much better to evaluate this on a scope, to help identify artefacts that may throw off multimeter estimates of ripple. I'll just have to get in the habit of checking this often to get a feel for normal values.

[@rwrobs777]

I was doing a cranking incylinder at the intake manifold with my WPS and the vacuum pull looked pretty jagged so I increased the resolution enhancement from 12 to 16 bits and that cleaned it up...i was told that using active filtering MAY mask a problem...your thoughts and you've got a great channel !!

[@DependableAutoTruck]

should my vantage legend have ac coupling

[@grabovg]

Yes definitely. When you’re in your scaling area there should be one by the trigger/filter selection.

[@jonathanhart54]

Are you able to use an amp probe with ac coupling to alternator diode checks? I know voltage measurements are good but a lot of times the B+ terminal on the alternator is difficult to access

[@Valamilk]

Fiiiiiiirrrrrrrsssssssssttttttt 😉

[@Valamilk]

Ask me if I watched it tho 😈🤣

[@ThePracticalMechanic]

@@Valamilk I made you watch it live

[@Valamilk]

TWICE. yes, I remember. 😬

[@ThePracticalMechanic]

@@Valamilk I didn’t like the first version.

[@diymisfit_Mechanic_UDX404]

Cheater!!!!

[@DependableAutoTruck]

i know i am very uneducated what is the main purpose of checking the alternator i understand the starting

[@ThePracticalMechanic]

The diodes in the alternator are supposed to smooth the voltage exiting the alternator in a nice consistent DC voltage to charge the battery and run our accessories. If the diodes are weak or fail the pattern will have very large peaks and valleys. If we measure from the peak to valley we will get our alternator ripple. Some ripple is normal and unavoidable. Anything over 300mV I consider excessive because it can cause interference with other sensors and communications on modern cars. Older cars were less sensitive.

[@DependableAutoTruck]

@@ThePracticalMechanic good to know will hook up some tomorrow and see what i see hopefully will make since thanks. my uscope master kit is supposed to be here tomorrow

[@ThePracticalMechanic]

I will probably do a separate video on the uScope. The uScope doesn’t have the setting to change from dc to ac couple but there is an attachment that goes on the scope lead for AC coupling.

[@DependableAutoTruck]

@@ThePracticalMechanic ok will use the snapon for the information you want i need the practice where ya'll can tell me what i am doing wrong or right . i have several some older like early 90's and then couple newer. i have a 94 dodge that has high power ignition system with 10mm wires and special coil wil be interesting to see if that shows up

[@steveo782812]

So you do turn on AC coupling when doing a relative compression check? I always assumed it wasn't on.

[@ThePracticalMechanic]

It depends on the scope. Since we cannot zoom in vertically on the snapon scope after the capture has been made it make it easier to check the variance between cylinders if we turn on AC coupling and turn our scale down to 1 or 2Volts.