Come along with Eric O. as he has a look at this 2005 Chevy 2500HD 6.0. The Check Charging System indicator (battery light) is on and we suspect the alternator has failed. The question is, how do you know if it is the alternator, the belt, the battery, corrosion, a broken wire or the ecm? Simple, you test it! --Enjoy!
Pomona 6405/POM Insulation Piercing Clip Set (Pack of 2) :
Interested in stuff seen in the videos or the tools and toys Eric O. likes and uses? Check out the S.M.A. "A-Store" and help support the channel! :
If an SMA Video has helped you out please consider giving using "Patreon" to help support us. The videos take real time to create and pull us away from real work that pays our bills. CLICK HERE:
--Eric O.
Simply send and E-mail to SMAstickers including: your shipping address and e-mail address in the text, I in turn will send you a paypal invoice for $5 each USA or $6 each international. Let me know how many you want and as soon as the paypal invoice is paid I will ship them out ASAP! Thanks for your support of our channel!!
--Eric & Vanessa O.
Feel like sending some swag to SMA because you love the videos but don't know where to send it?
Just ship it here:
South Main Auto Repair
47 S. Main St
PO Box 471
Avoca, NY 14809
OTC/Actron Provided By Bosch
Due to factors beyond the control of South Main Auto Repair, it cannot guarantee against unauthorized modifications of this information, or improper use of this information. South Main Auto Repair assumes no liability for property damage or injury incurred as a result of any of the information contained in this video. South Main Auto Repair recommends safe practices when working with power tools, automotive lifts, lifting tools, jack stands, electrical equipment, blunt instruments, chemicals, lubricants, or any other tools or equipment seen or implied in this video. Due to factors beyond the control of South Main Auto Repair, no information contained in this video shall create any express or implied warranty or guarantee of any particular result. Any injury, damage or loss that may result from improper use of these tools, equipment, or the information contained in this video is the sole responsibility of the user and not South Main Auto Repair.

50 thoughts on “Alternator testing – gm”
  1. Avataaar/Circle Created with python_avatars Anton R says:

    Interesting, GM's RVC system states 5v 128hz control from ECU. Any idea why GMC got 8v 400hz ?

  2. Avataaar/Circle Created with python_avatars Norman Martin Jr. says:

    This topic reminds me of a charging system I looked at and repaired like 22 years ago. Drove someone to pick up a used mid 1980's ish GM sedan (cavalier/delta 88 I think) car they just purchased, as soon as they started it battery light was on (previous owner claimed to not know about it). Got the vehicle back to their residence (after several jumper cable charge and start sessions), pulled out my digital multimeter and ran some voltage checks. Key off had no voltage on heavy and medium red wires (alternator had the medium and small wires in a white connector on the side), with key on had voltage at the small wire. Knowing the large and medium wires were supposed to be always on, and medium wires usually routed to battery post on the alternator, i pulled out my jumper cables and made a direct connection between the battery positive and battery postin the alternator. Started the car and it started charging. After all that background I traced the large red cable to a set of fusible links at the battery post of the starter (of course the links for the alternator wires were blown). Repaired the links with suitable replacements and made the people happy that they could drive the car they just purchased.

  3. Avataaar/Circle Created with python_avatars Clifford Buckman says:


  4. Avataaar/Circle Created with python_avatars boostedsil40 says:

    if the alt doesnt work and cant do voltage drop could we just create a live circuit like hooking test light to the power so it lights up and thus creating current flow to test wire voltage drop?

  5. Avataaar/Circle Created with python_avatars boostedsil40 says:

    the more "common and routine" the more views as were most likely going to encounter it someday

  6. Avataaar/Circle Created with python_avatars Gary Waisner says:

    Always enjoy. Chasing grounds and repairing is what I have a hard time following. Just finding the wires looks confusing to me. But, you are the professional, that's why I watch.

  7. Avataaar/Circle Created with python_avatars Gregorio Carrasco says:

    You are my far the best mechanic on here. You positivity. Your attention to detail. You are down to earth and take the time to explain everything to us viewers

  8. Avataaar/Circle Created with python_avatars Manuel Amaya says:


  9. Avataaar/Circle Created with python_avatars Oby-1 says:

    We like everyday stuff because we have to deal with everyday stuff. Thank you for your honest videos.

  10. Avataaar/Circle Created with python_avatars Christopher Hernandez says:

    I have a 99 Silverado 1500 but It only has 1 wire and I feel like my alternator isn’t charging

  11. Avataaar/Circle Created with python_avatars Anil palitha Karunanayaka says:


  12. Avataaar/Circle Created with python_avatars Bob Moo says:

    Good review. Love the snap on meter. Had a similar problem, but it was diode in alternator,
    Does Chevy have a plug in to check alternator while driving, or a dash add on device,? Bo

  13. Avataaar/Circle Created with python_avatars DuB Jay Da 3rd says:

    This is for AL G. RITHIM

  14. Avataaar/Circle Created with python_avatars Moz Pogson says:

    Mr O, it's called a lack of attention to detail. We all make mistakes.

  15. Avataaar/Circle Created with python_avatars Ibe R says:

    The alternator of my car has a low voltage and is the second time that I change it .The mechanic doesn’t know what is wrong he just change it and is still the same problem 😭

  16. Avataaar/Circle Created with python_avatars Rick p says:

    Left the house Friday morning drove 360 km Picked up our175 lbs freezer order of Black Angus beef. my truck is an 06 gmc sierra LBZ Duramax. Even though my ol girl has 460,000 km I still use as main vehicle & always on top for maintaining reliability. From oil changes to glow plugs & axel seals, etc… + years of working on vehicles big & small with more tools I care to mention, I get by very well. Anyway, when leaving pick up area My heart started to pound as I looked at my BAT. dummy light on & low voltage on gauge + Battery not charging scrolling on info screen !! Now I'm fast pacing in my head do I have enough tools & the right ones packed to undertake a possible swap?? I Always pack a box of assorted tools for road trips. But next ? is, where will I find an Alternator on a bench waiting for me out here?? I'm about 300km away still on my way home with a whole lot of meat I can not afford to thaw out !! One attempt to salvage myself from a delay home I was rolling Hwy speeds, I shut the ignition off & back on. I would have to guess the surge created was enough to stir life back into alternator "somehow"?? & managed to get home voltage gauge maintaining @14v for remainder of trip except for last 20km of stretch home, the voltage started to slowly drop but still got home before it triggered dummy light. However once everything was unloaded & stored away, I lifted the hood & began my assessment. Individual battery load test had low 12v but still good so on to alternator output @ first start 12.5v humm not lookin good ??anyway, after a couple starts & more voltage drop finally the alternator died. no more output showing on meter. zero V. Guess you know what I ordered & waiting for?? Yep, a 4 pin 145amp alternator from Wilson. Watching your videos are always detailed to a T & had I not listened to details I would have no doubt done the same!! It's a 2 wire system lol! By the way, I just removed Alternator. No way I had tools required to do a road side fix!! Super happy I was home.

  17. Avataaar/Circle Created with python_avatars Nef says:

    I love how you go above and beyond your work ethic. It's all in the details.

  18. Avataaar/Circle Created with python_avatars Nef says:

    Thanks God for power tools.

  19. Avataaar/Circle Created with python_avatars Videos Gone Viral says:

    Awesome vids 👍👍

  20. Avataaar/Circle Created with python_avatars D Frohreich says:

    Not every bodies perfect. I need a 2 pin 2 wire 135 amp alternator for my V-6 . Birdy Serpentine belt remedy: trim 1 grove with razor knife. After, several attempt's to find the Birdy, water pump, tensioner, Alt.( I finally found a factory defect Harmonic balancer !)

  21. Avataaar/Circle Created with python_avatars Danny Bowen says:

    You make a lot of mistakes you forget in Spring a car

  22. Avataaar/Circle Created with python_avatars Danny Bowen says:

    Winners for best friends

  23. Avataaar/Circle Created with python_avatars Tristan Dominguez says:

    A 6 liter is a v8 ?

  24. Avataaar/Circle Created with python_avatars mike says:

    Try oriells lol

  25. Avataaar/Circle Created with python_avatars mike says:

    Vince fishelle was saying to amp clamp main wire and after its replenished the starter draw the alternator should only be about 8 amps. unloaded if your battery is good. If its too high it will return with a bad alternator depending on how hard the battery health calls for more voltage. The battery is a load.

  26. Avataaar/Circle Created with python_avatars mike says:

    The hertz is to keep the components from overheating. Thats how i heard it. Its engineering bs.

  27. Avataaar/Circle Created with python_avatars mark ket says:

    Yeah that's not a high amp alternator the high amp alternators have the metal between the two halves of the case, the low amp alternators have the case that is just aluminum

  28. Avataaar/Circle Created with python_avatars Collin Baroh says:

    Anti-seize – the herpes of the automotive world.

  29. Avataaar/Circle Created with python_avatars kevin bell says:

    I have a 2002 chevy truck with 305 in it been discharging after i restart after it was fine when i started it ,,then i run the rpms up to over 2000 and it starts to charge low diodes?? Bad

  30. Avataaar/Circle Created with python_avatars Angel Jarquin says:

    You are the best eric no matter what !

  31. Avataaar/Circle Created with python_avatars Ed Toro says:

    I’m pulling 14.77 on my alternator on a Dinali would that be the PCM that’s causing it to over voltage?

  32. Avataaar/Circle Created with python_avatars Doug Kubash says:

    On the old Delcotron alternators you could test them by going through the 'D' shaped hole on the back of the alternator case with a small screwdriver and touch the back of the regulator on the inside and touch the case and it would 'full field' the alternator.

  33. Avataaar/Circle Created with python_avatars Jeffrey Wilson Rubik's Cube on Beat says:

    Root prob wasnt fixed.

  34. Avataaar/Circle Created with python_avatars Chuck Huddleston says:

    Eric I thank you for taking the time to video all the details of this repair. You have no idea how helpful you are and how much you teach us DIYers. The Brake cleaner, the Ant seize, the liquid tape, the diagnostics with multimeter, not breaking off the 'ears' of the bracket (using pliers). Keep up the great work!! None of it is boring to me. !!

  35. Avataaar/Circle Created with python_avatars The bears den says:

    Get yourself portable land phone . Jus

  36. Avataaar/Circle Created with python_avatars MrMotofy says:

    Nope those are 105A alternators. Unless you happen to have the higher output one at 145A. But those aren't very typical.

  37. Avataaar/Circle Created with python_avatars Kyle Fowler says:

    This is an awesome video, I am just a DIY'er but bought a cheapo $40 oscilloscope from Amazon to test the questionable alternator on my 2004 chevy tahoe. It's good enough for broke people haha. However I really like the idea of buying a nice multimeter capable of reading Hz at 3v RMS and up. I have a nice Klein meter but apparently it can only read Hz and duty cycles at 8v rms and up. The duty cycle wire voltage on gm alternators of this vintage is 5v rms lol. So keep that in mind when multimeter shopping

    It felt great to definitely diagnose the problem before throwing a part at it and praying. Thanks for your help Eric!

  38. Avataaar/Circle Created with python_avatars wayne baker says:

    we related,,,??? I do that alot. found out, bring with you to the store, or, send a picture along with the order. Ho. ho,,ho,,,

  39. Avataaar/Circle Created with python_avatars Thomas Turner says:

    Do you use copper or silver anti size?