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Best Way To Disconnect An Amp Meter?

Discussion in 'Early CJ5 and CJ6 Tech' started by SoCalNickG, Oct 3, 2024.

  1. Oct 3, 2024
    SoCalNickG

    SoCalNickG Member 2024 Sponsor 2023 Sponsor 2022 Sponsor

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    I have a 1971 Renegade with the original amp and OP gauges, both still working. The P.O. installed a volt meter that is working. I want to disconnect the amp meter. I am not going to remove the amp meter I just want to avoid the fire potential that I have read about with amp meters.
    What is the best/safest way to disconnect it?
     
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  2. Oct 3, 2024
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

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    I have a drawing -

    [​IMG]

    Top is before, below is after. This is specific to my '82, but the idea is the same. Bulkhead is the firewall penetration connector. The wire labeled 14 ga is an '82 factory fusible link. The fusible links protect the rest of the car from the battery in case of a wreck or other disaster.

    Electrically, the ammeter acts like splice; in this case if you just removed it, you'd have no connection the dash and lights.
     
  3. Oct 3, 2024
    PeteL

    PeteL If it wasn't for physics, and law enforcement... 2024 Sponsor 2023 Sponsor 2022 Sponsor

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    The fire danger , if any, has nothing to do with the ampmeter itself. It is the fact that the circuit it monitors carries the full amperage used by the entire vehicle, and often is unfused. Thus damage to that wire anywhere on its path could result in a significant short circuit. That applies meter or no meter.

    [EDIT. See post # 5 below}
     
    Last edited: Oct 3, 2024
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  4. Oct 3, 2024
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

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    Maybe the round CJ ammeters are better in this respect. The ammeters used in the J-trucks and Wagoneers actually catch fire in the dash. I attribute this to the shunt in the meter aging and increasing resistance. The regulator only reads the battery voltage, and any voltage drop between the alternator and the battery is added to the output voltage of the alternator. The alternator increase current output to raise the voltage. Suggest that added current causes more heat in the ammeter, which raises the shunt temperature and its resistance, adding more current.
     
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  5. Oct 3, 2024
    PeteL

    PeteL If it wasn't for physics, and law enforcement... 2024 Sponsor 2023 Sponsor 2022 Sponsor

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    Oh. I never had an ammeter with an alternator. Sounds like another reason for me prefer generators...
     
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  6. Oct 3, 2024
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

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    Maybe. I don't know how a generator regulates versus an alternator. Both use some kind of regulator to change the current output based on the battery voltage, but generators don't have semiconductor components.
     
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  7. Oct 3, 2024
    PeteL

    PeteL If it wasn't for physics, and law enforcement... 2024 Sponsor 2023 Sponsor 2022 Sponsor

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    And most alternator vehicles use voltmeters, not ammeters.

    I suspect generators are somewhat self-limiting. My '34 Ford regulates the maximum output by how far one rotates the "three-brush" assembly relative to the field coils. I set it higher at night when I need the headlights!

    No ammeter fires. :D
     
    Last edited: Oct 3, 2024
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  8. Oct 4, 2024
    Rick Whitson

    Rick Whitson Detroit Area 2024 Sponsor 2022 Sponsor

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    I don't understand? One tells Volts, the other tells Amps, what is the problem? A short would cause a problem with either one, right? I have a 10 SI alternator no my Jeep with an amp meter like everyone else had in the old days, yes you have to Goose it start it charging, once you get it charging there is no problem? As soon as I give it the gas you can see the meter go up to what the amp meter is putting out.
     
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  9. Oct 4, 2024
    termin8ed

    termin8ed I didn't do it Staff Member

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    A volt meter is hooked up in parallel so not all the power is going thru it.- there is another path for electricity to take. They don't have much current at all going thru the meter, only a small fraction. Most current is going thru the bigger wiring(component) that is designed to handle the current.
    An amp meter is hooked up in series so all the current is flowing thru it. If you get 20 amps, 30 amps, 40 amps flowing thru a circuit, all that power is going thru the amp meter. If it's not designed for that much current, the magic smoke starts to go.


    Normally they work just fine, but the problem comes when the meter gets old and maybe some moisture got in corroding a contact creating more resistance.
     
    Last edited: Oct 4, 2024
  10. Oct 4, 2024
    jeepdaddy2000

    jeepdaddy2000 Active Member

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    Simply put. the ammeter sits directly between the battery and the rest of the load (minus the starter). It monitors whether the system is charging + (pushing electricity into the battery) or discharging - (power is flowing out of the battery and into the system).
    This is a source of potential issues since all the voltage flows through the meter.

    A volt meter simply reads existing voltage in the system.
    Take the + and - sides of the ammeter and connect then together with a small nut/bolt and hit the joint with some tape or shrink wrap is the easiest.
    Or
    Splice/solder the two together and insulate the joint.

    Insure your voltmeter is wired to the accessory side of the key. Meters left "on" continuously can drain the battery over time.
     
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  11. Oct 4, 2024
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

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    I think the main problem is the distance between the charge terminal of the alternator and the battery. Any resistance along that distance will make a voltage drop between the alternator and the battery, proportional to the resistance and the charge current.

    The regulator determines how much current to make by sensing the voltage at the battery. This is direct to the battery, with no cables and ammeter between. The voltage across the battery is held at a constant, say 14 volts. This includes the battery electrochemical potential of 12.6 volts and the internal resistance of the battery to ground.

    The load (current demand) increases, and the regulator responds by juicing up the field of the alternator to make more current. When the demand and supply equal, the voltage goes back to the ideal 14 volts.

    The regulator sees the battery voltage at the battery, not at the charge cable connection to the alternator. Any resistance in the charge cable reduces the supply current: turned into heat by the resistance in the ammeter and charge cable. The regulator increases the juice to the field to compensate.

    With more current, the ammeter gets hotter. Resistance increases as the temperature increase; the alternator increases the current to compensate. This is positive feedback ... not good.

    (A little speculation here). The regulator is also a feedback system. It tries to achieve the set point of 14 volts by feedback. Any feedback system with finite gain can only approach the set point, not match it exactly. I suspect any ammeter heating is fine as long as its gain does not exceed the regulator voltage feedback gain. Add a tiny amount more resistance, and the charge current increases a little... at some point the regulator says "meh, good enough" and all is stable.

    This is why the short wire direct to the battery is better than the long wire through the gauge, with all its extra connections. The wire has resistance gain just like the ammeter, but it's pfff. Nothing. The regulator gain is so much higher than the wire's gain that you never see it.

    One final comment - never connect an alternator capable of 60 amps charging to an ammeter that's not capable of handling at least 60 amps. The original ammeters in these Jeeps were like +/- 30 amps? You can't buy a 30 amp alternator any more; they are all replaced by alternators with much higher ratings.
     
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  12. Oct 4, 2024
    PeteL

    PeteL If it wasn't for physics, and law enforcement... 2024 Sponsor 2023 Sponsor 2022 Sponsor

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    Yet the last one I bought came with a warning that my new alternator could be ruined if I attempted to charge a dead battery.

    W. T. F. ?

    Another vote for The Good Old Days.
     
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  13. Oct 4, 2024
    Glenn

    Glenn Kinda grumpy old man Staff Member

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    That's because alternators are designed to maintain the battery charge rather than totally charge a battery. I know, it does sound crazy but that's the deal as I understand it.
     
  14. Oct 4, 2024
    PeteL

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    Undoubtedly true. But were they not at one time "designed" to do better?

    "Design" can also be to reduce cost, weight, or whatever the Head Office thinks is more profitable.
     
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  15. Oct 4, 2024
    Howard Eisenhauer

    Howard Eisenhauer Administrator Staff Member

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    I can't swear to this but I suspect that it might be that alternator regulators, at least the Delco 1/3 wire ones, don't incorporate current limiting in their circuitry- just voltage regulation. So, assuming a 60A 'nater, if there's an apparent load (mostly the battery in this case) of less than about a quarter of an ohm then the 'nater is overloaded. Which is to say running too much current for the sized wire user in the stator windings & perhaps the rectifiers.


    Note no current sensing resister between the output of the diodes & battery indicating no circuitry to monitor/ adjust current-
    upload_2024-10-4_20-8-41.jpeg
    Generator regulators do have a current limiting function, note relay "C" that's in a direct path between the genny armature output and the path to the "B" terminal to the battery through the relay "B" (disconnect to prevent backfeeding power when not running). When current exceeds a set value the contacts open removing the ground connection to the field coil.

    Genny-Regulator diagram_3.gif
    I know for a fact when my (nominally 30A) genny on Tonk hits 27 A output it sticks there as the field voltage gets reduced to limit output.
     
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  16. Oct 4, 2024
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

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    Yep. Seems clear. Maybe the OEM ones do.
     
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  17. Oct 5, 2024
    Rick Whitson

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    Thanks For explaining that, Howard and Term8ted, it makes Sense to me a little more now. I have little more now, I have never had much experience with DC ecstaticity, or AC for that matter. Rick
     
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  18. Oct 5, 2024
    colojeepguy

    colojeepguy Colorado Springs

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    I learned that in auto shop class 40+ years ago, it's not a new thing.
     
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  19. Oct 5, 2024
    Glenn

    Glenn Kinda grumpy old man Staff Member

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    I don't know but do wonder, were they? Were people just lucky that their alternator wasn't ruined when someone gave them a jump when their battery was dead? Or was the little bit of charge their battery would have gotten from the jump have been enough to not hurt the alternator?
     
  20. Oct 5, 2024
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

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    Could a dead battery be a shorted battery? Charging into a dead short would seem ill advised. Unless shorted, these batteries have internal resistance which would be current limiting to some extent.

    I'd suspect it's unusual for a battery to be completely discharged in normal service. You can imagine case with a parasitic drain and sitting for months, but most cars don't sit unattended long enough for the battery to be completely depleted. No start happens well before zero volts.
     
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