Low voltage on freeway

Discussion in 'GM Electrical Tech' started by Pikey, Jul 31, 2012.

  1. Pikey

    Pikey Moderator Staff Member 5+ Years ROTM Winner 5000 Posts

    Hi, I noticed yesterday while driving down the freeway that my voltage dropped to around 12 on the gauge. When I am in the city it is around 14 or above all the time. I had the battery check and it was bad, so it was replaced. I am still having the same issue. I had the alternator tested and that came back good. I have checked the belt tension and it seems good.(no slipping). I retested the alternator running the engine at a higher rpm (figuring that it as at 2300 when I am cruising) and it came back fine. Both places I went to said that their notes say to only test that alternator on the vehicle not on the bench. any ideas? it is the bosch style alternator, not the delco
  2. RayVoy

    RayVoy Rockstar 4 Years 5000 Posts

    Unless you had a lot of after market electrics, on the truck, the alt should be able to maintain 14v at idle.

    If you need to idle it at 2300 to get 14 volts, you may have a bad alt.

    However, you need to use a digital volt meter across the battery while the engine is idling to properly check the voltage. Do not rely on the dash gague.
  3. Pikey

    Pikey Moderator Staff Member 5+ Years ROTM Winner 5000 Posts

    I have a factory DVD player and the truck is loaded with everything but Nav. That being said, None of that stuff is on when this happens. It holds at 14 volts or above at idle and driving in the city. But, Longer trips (10 plus miles) at 65mph plus is when the gauge drops.
  4. RayVoy

    RayVoy Rockstar 4 Years 5000 Posts

    Without using a DVM across the battery, I can't tell if you have a battery/alternator/dash volt meter problem.
  5. Pikey

    Pikey Moderator Staff Member 5+ Years ROTM Winner 5000 Posts

    I will give it a check tonight.
  6. Sierraowner5.3

    Sierraowner5.3 Rockstar 4 Years 1000 Posts

    i think its a GM thing, my 11 does the same thing, full 14 volts in the city, dips towards 12 on the highway. i figure it has something to do with running less load on the alternator to help with MPG. cause itll be at 12 for a while, then jump to 14. dunno, maybe the e fans kicking on and off has something to do with it? anyways, you cant tell me when i bought my truck it came with a bad alternator with 17000 miles and only a year old.

  7. moogvo

    moogvo Epic Member 5+ Years 1000 Posts

    Bosch?! There's your problem! Those are for German cars! Put a proper Delco unit on that GM!
  8. Pikey

    Pikey Moderator Staff Member 5+ Years ROTM Winner 5000 Posts

    LOL, That is the one gm put on it. I believe that the connector is different. @Sierraowner5.3 , I have has 2 other newer trucks. A 2002 and a 2003, then this one. This is the only one that has ever dropped that low. The others always stayed at or around 14 all the time
  9. Sierraowner5.3

    Sierraowner5.3 Rockstar 4 Years 1000 Posts

    huh, i may have to get mine looked at, needs to go in for a oil change and some other stuff soon anyways.

  10. tbplus10

    tbplus10 Epic Member Staff Member 5+ Years 5000 Posts Platinum Contributor

    Pikey if you've got an owners manual look in it for a section on charging, I'm not sure when GM changed the system but newer trucks have a variable charging system that only produces the amount of volts the engine/charging system requires, thats what shows on the gauge. I'm not sure it was in use for 05 models but it could be. You cant test your alternator in the conventional way, you take a chance of damaging it doing this because it doesnt know what amperage it's supposed to be at.

    Alex your truck definately has this system, if they arent already turn you lights on auto setting and then watch the gauge next time your driving during the day and go through shadows dark enough to turn on the headlights, the gauge registers the amperage change almost instantly. Check your owners manual and it will explan the system much better than I can.

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