New alternator and batteries still reading 12.0 volts

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by Bubbamoore, Jul 22, 2019.

  1. Bubbamoore

    Bubbamoore Light Load Member

    Feb 20, 2013
    A while back i noticed my voltage guage on the dash slowly creeping south towards 12.1 to 12.4. went to TA thinking i needed 4 new batteries. They wouldnt warranty any electrical work unless i got a 70 dollar test of the batteries/ starter and alternator. 2 bad batteries, good starter alternator was fine but the voltage regulator (apparently IN the alternator) was bad. 2 new batteries and away i went. Fast forward to today. $337 Leece Neville 12.0V alternator installed, new belt, 2 NEW batteries and new power inverter cables installed. $700 dollars later, my power inverter/ the dash guage, and the multimeter all read about 12.0v to 12.4v. im super lost because I'm too stupid to grasp the concept of volts vs watts and all that. But with 2 bad batteries coupled with the old alternator (the one with the bad voltage regulator) it was an average reading of about 13.4v to 13.6v.
    How is it, with new batteries/alternator/ voltage regulator and inverter cables would the same load (mini fridge and cell phone charger) produce less voltage than with the old set-up??? Am i wasting money on batteries that will die as soon as i turn my truck off?? This is like trying to learn math with the Chinese alphabet to me..... Please help.
  2. Hulld

    Hulld Road Train Member

    Oct 19, 2015
    Upstate NY
    It sounds like the either the new alternator is defective ( which can happen often) or there may be a possible wire connection problem from the trigger wire or heavy wire on the alternator back to the batteries.
    reading 12.0 - 12.04 volts is not charging your batteries.
    You need a bare minimum of 12.5 volts ( and that’s to low) to begin to charge a 12.0 volt battery.
    Think of it as trying to fill a tire with 40 lbs of air in it to
    50 lbs with an air hose that only has 40 lbs.
    The tire will never get beyond 40lbs.
    I believe from your voltage readings that the alternator has zero output and you are reading the voltage of what’s in the batteries.
    When an alternator is charging correctly it should put out between 13.5 volts to 13.8 volts to push voltage in to a battery.
    You will need to go back to a TA and have them figure out what the issue is now. ( all should be under warranty in my opinion)
    You should get there quick as possible because I believe you are running on the reserve power of the batteries and when that’s gone the truck will quit.
    Bottom line is either the New alternator is no good or they missed diagnosed a wiring problem problem from the beginning when they sold you all the parts.
    Bubbamoore Thanks this.
  3. Heavyd

    Heavyd Road Train Member

    Feb 4, 2009
    Bad power feed to the cab.
    Bubbamoore Thanks this.
  4. Ridgeline

    Ridgeline Road Train Member

    Dec 18, 2011
    You may have a bad ground from the alternator to the truck.
    rabbiporkchop, 6wheeler and Bubbamoore Thank this.
  5. Bubbamoore

    Bubbamoore Light Load Member

    Feb 20, 2013
    Thanks to all yall who responded. I called the TA in Madison Ga (where the New Leece Neville was purchased) and was informed that there is absolutely no warranty on "over the counter" electrical parts unless they run the diagnostic test and install the part. The Pilot Flying J truck care in Blacksburg ran a test on it this morning only to find out that it was pushing 12.4v on high idle and 11.3 at low idle. They reinstalled my old alternator ( with bad voltage regulator) and its currently running at 12.8v at regular idle and at 13.4v high idle... I bought a $337 paperweight.‍♂️
    Story of my life... Yall be safe.
  6. 6wheeler

    6wheeler Road Train Member

    Aug 30, 2009
    I've been driving and working on trucks for over 20 years. The connections will loosen up. Positive, negative all of them. Pot holes cracks at 65 mph cause this. Even so much as a quarter of a turn of the nuts and bolts to the connections will leave a weak battery.
  7. mmk trucking

    mmk trucking Light Load Member

    Mar 6, 2016
    You should never go to truck stop mechanic
    rabbiporkchop and HoneyBadger67 Thank this.
  8. FoolsErrand

    FoolsErrand Road Train Member

    Sep 15, 2017
    An alternator is an electromagnet meaning voltage going into a coil of wire creates variable magnetism. This is called the field coil. The voltage regulator "looks" at the system voltage and "tells" the field how much magnetism to create, in order to dictate how much output to replenish the battery voltage level. 13.4 to 14v is a healthy output and thatll get your batteries up to say 12.6 or so resting. In the old days it was a dang lightbulb in series on the field wire that would burn out and stop your alternator from charging.

    As simplistic as it seems that bulb was quite clever. When not spinning, you click the key on and the bulb has zero volts [ground] on one leg and 12v on the other so it fully illuminates which tells you the bulb works. Now start the engine. The alternator produces 13ish volts and now that bulb has the same voltage on each leg so no current flows across it. The bulb goes out. When you had a slight charge issue youd have a slight current and a dim glow on the bulb to tell you of trouble. When the bulb fully lights while engine is running you know the charge just quit.

    An alternator that is charging will have a magnetic field around the back side that you can easilly pick up with a wrench or any other piece of metal. If your battery voltage doesnt jump up when you start engine, you arent charging. First look at the field wire. If it doesnt have 12v to the alternator it cannot excite the field to create magnetic flux to turn into volts.
    x1Heavy and Bubbamoore Thank this.
  9. Navtech

    Navtech Light Load Member

    May 9, 2019
    I may have missed what kind of truck this was when reading through, but I have had this issue more than once on internationals, they have a fuseable link on the starter solenoid that is the other side of the alternator wire, I’ve seen them rubbed through on the frame. And have bad connection
  10. x1Heavy

    x1Heavy Road Train Member

    Mar 5, 2016
    White County, Arkansas
    When I test my battery, I see about 12.4 to 12.6 on it. It's adequate. Now when I fire the engine up the voltmeter goes to 14 and change. That tells me that the battery is charging.

    Once in a while I have had situation where the alternator quit charging and now I see the volt meter start dropping with a red lamp on it. That tells me Im on batteries and I have about 3 hours to find somewhere to shop it before I drain the batteries so much that the fuel pump will quit working. (Pre computer trucks, air breathing engines) Computer trucks can endure a voltage drop to about 10 pounds and then brick off because it is a miniature computer that cannot stay booted on and functioning at or below a certain voltage, which would usually be at or less than 10 volts.

    I don't have much else to offer Except that when you have a good tester that tells you each battery has charge and that the alternator is creating a charge sufficient to feed batteries, around 13.6 to 14 and up. (But not too far up.)

    We had a truck once in 2000 that would go close to 15 volts on the voltmeter when shes running at 1500 cruise speed on the highway in the day time. That usually got pulled down when we turned on the inverter and fired up a 120 volt Mr Coffee behind it to brew a pot of coffee. Then she will come down to about 12.8 or so until the coffee pot was finished and turned off. (With inverter turned off) If it had kept falling down past 12 volts and did not come back to where it's normally at, you instantly know you have at least lost your alternator. OR you have at least one bad battery back there in the group.
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