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Voltmeter Reading Funny....or not?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Agar426, Jun 13, 2002.

  1. Agar426

    Agar426 Registered Member

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    I've got a '90 k5 350 TBI, and am concerned about the reading I'm getting from the voltmeter. It reads anywhere from 8 to 13 volts. Every other vehicle I've ever owned always hung out around 13 volts. My Blazer is at 8 more than it is at 13. I had the alternator checked, and it read a consistent 13.8 volts while at the same time the voltmeter was reading 8. Am I just paranoid, or do I have an electrical problem? The vehicle runs just fine otherwise.
     
  2. 350S10NJ

    350S10NJ Registered Member

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    Bad conection at the gauge.
    Bad gauge.
    Bad body ground.

    Just a few things to check.
     
  3. dyeager535

    dyeager535 1 ton status Premium Member

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    I'm not familiar with how the alternator was tested, but if it was at idle and putting out 13 volts, thats pretty good. Of course, you have to make sure the heater is on high, headlights are on and on high, etc., to determine if the alternator is "weak" or if it is, in fact, that gauge.

    The stock alternators even when "failing" will work fine typically at cruise, but at idle under load, voltage will drop a lot. If you are seeing that much REAL voltage drop at idle, you will be able to tell solely by the dash lights dimming and brightening as you increase and decrease engine RPM. Just looking at the lights isn't of course the "right way" to check, but with only 8 volts compared to the necessary 12V minimum, the lights will most definitely dim. Heater blower motor when on high will "bog down", headlights will dim, etc.

    Never seen a volt gauge fail, but it is certainly possible. (just odd in your case) If it is for certain not the alternator, I'd think something is either wrong in the gauge itself, or the wiring to it. Luckily, if thats the case, stock clusters and pieces are easily found at wrecking yards.
     
  4. Agar426

    Agar426 Registered Member

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    dyeager535,

    It's definitely odd. The lights don't dim, the dash lights up well, etc. The windows are slow, but I believe that's due to poor maintenance of all mechanisms involved. The alternator was tested at the local Auto Zone with their "shopping cart" tester, while the engine was at idle, and the A/C was running on high. I do notice the voltmeter will click in unison with the turn signal, but I've seen that on many vehicles. Like I mentioned earlier, the vehicle runs fine. I'm just a little paranoid about going on a long hunting trip or something, and the electrical system strands me in the middle of nowhere.
     
  5. Blue85

    Blue85 Troll Premium Member

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    Ahah! "I do notice the voltmeter will click in unison with the turn signal". Check your body grounds. Ground current from the turn signals is passing through the same ground as that for the voltmeter. This extra current creates a voltage drop that decreases the total amount of voltage drop across the voltmeter. Look at the ground straps from the battery and engine to the body and the ring terminals where wiring is connected to the body for lights, motors, etc..
     
  6. Itali83

    Itali83 1/2 ton status

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    Mine did the same thing, the guage was junk. I changed to autometer guages and that problem went. Well, I should say most of it. I guess the regulator was fried a little in the alternator, so at idle it was dropping volts, but no where near the 7-8 volts like the factory guage. SO I'm betting it's a combination of the two, the guage is junk like mine was and the alternator is in for a good rebuild also. I ran mine like this for 3 years on the same battery and never had a problem. Did the same test as you, tested it's output at the battery and kept coming up with 13+ volts with all acessories on. Just one of those things I guess. Hope this helps.
    Ben 87 Jimmy
     
  7. Blue85

    Blue85 Troll Premium Member

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    I think that these meters just show you what kind of voltage is available under the dash and not what the battery is doing. I have considered in the past seperating the voltmeter feed from the rest of the gauges and run some small sense wires right to the battery. I have also considered just running some heavy gauge wire into the passenger compartment so that I don't get that droop. With the engine off, I can roll down the windows and it drops from 12V to 10V even though a voltmeter right on the battery is reading 12V steady. The weird thing is that when the engine is running, I don't get the droop, it always seems pretty accurate.
     
  8. Sinful_Nature

    Sinful_Nature Registered Member

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    I had the same issue: Ended up replacing most the wiring- Postive & Negative Cables, replaced the stock gauge with a Phantom and upgraded to a 4-GA body ground. This seemed to address the issue, it also elimated a starting problem I had.

    ~ Russ
     
  9. dyeager535

    dyeager535 1 ton status Premium Member

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    I did some testing, I lost about .5v from the alternator to the fuse panel. Voltmeter is reading what the fuse panel is seeing, and there might be a few circuits that take a different path, (the starter is wired straight to the battery), and thats the most important item probably, due to the load it puts on the system. I'd imagine second would be the heater blower motor, especially in AC/HD heating trucks. Believe its power comes through fuse panel so you'd definitely notice the loss in fan speed as opposed to it getting the full output of the alternator.

    IMO, it doesn't matter where you are testing/reading voltage...if alternator output drops, so will the gauge reading. Since I consider the stock gauges "relative" I don't spend a lot of time trying to make sure that they read the EXACT same as alternator or fuse panel voltage, but that the reading does change when something happens like increased load or failing alternator, which it does.

    I know this is old news to you, but anyone else reading this thread may reconsider swapping gauges just to get "more accurate" readings. I know that if my voltmeter goes under 12v, I've got problems elsewhere. (and it proved it to me a couple weeks ago when the alternator failed)

    I think your window motor issue is the fact that they aren't seeing 12 volts (probably more like 13-14) but when its running, since the alternator increases output based on demand obviously. Your battery/system can't compensate for this without the alternator. I don't think the battery will reflect 10V coming out, unless its actually worn down to that voltage...right? (a 12v battery puts out 12v until the charge is gone, or something fries, regardless of load, correct?)
     
  10. Agar426

    Agar426 Registered Member

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    Thanks everyone for the responses. It looks as if though I've definitely got a path forward on troubleshooting this problem. Thanks again!

    Agar426
     
  11. Blue85

    Blue85 Troll Premium Member

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    The voltage on a lead-acid battery is a function of it's charge and the load applied to it. With no alternator and a constant load applied, the voltage will decrease linearly down to some voltage where the battery can't work the same anymore. The bigger the battery, generally the bigger capacity, but also lower internal resistance due to the larger surface area of the plates. So a given load will produce less voltage drop on a bigger battery (but you probably already knew that...)
     
  12. dyeager535

    dyeager535 1 ton status Premium Member

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    So given that an alternator varies output based on load (as is the case with power windows) there really is no way to increase the voltage your power windows (for example) "see" when the truck is off, short of using an enormous battery, correct?

    You say linear on battery voltage drop...so for example (whether simplified or not) with an alkaline battery, you will see very little voltage drop until the battery is just about dead, then a very quick drop to no charge, whereas a vehicle battery, it will drop such as 12.0, 11.9, 11.8, etc, until the charge is completely exhausted? I've seen that with the voltmeter, I just want to make sure that I understand alkaline and lead acid really can't be thought of the same way...

    Not trying to be a smart ass, just making sure I am clear and correct in the way I'm thinking here.
     
  13. Blue85

    Blue85 Troll Premium Member

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    That's correct. NiCd batteries stay even flatter in voltage across their charge.

    The other way to give more voltage to the windows is to run bigger wire and eliminate connections to lower the resistance from the battery to the motor. Or read my post about "turbo windows" /forums/images/icons/smile.gif
     
  14. dyeager535

    dyeager535 1 ton status Premium Member

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    Without searching for turbo windows because I'm lazy, lol, how large of a wire would you need to reduce volt drop? IIRC, the diameter needed to be quite large... On the smallest wires, like I mentioned before, I was seeing half a volt drop with the vehicle running. (at the fuse panel) Do you think that increasing voltage that .5V (if possible)will make a large difference? You mentioned 10V though didn't you? Would seem to indicate a poor connection somewhere, right?

    I've never measured my rear window, but I know it is slower when the truck is off, and according to the voltmeter, draws a fair amount of juice.
     

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