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Hot Rods Voltmeter question

Discussion in 'The Hokey Ass Message Board' started by Hellfish, Sep 12, 2016.

  1. Hellfish
    Joined: Jun 19, 2002
    Posts: 6,561


    I have an AutoMeter voltmeter hooked up to a '78 350 SBC with a internally regulated alternator. For years, it has always read a fairly steady 14v. However, on a recent long (700 mile) drive it started acting funny. I'm trying to sort it out.

    About halfway through the trip, the voltmeter started jumping between 12 and 14v every few seconds, but still mostly stayed at 14. I figured it just meant that the regulator was regulating. But then it started jumping from 14 to 10 to 12 to 14 to 12 to 14 randomly. Eventually it would drop to 8v or below for a few minutes and suddenly pop back to 14 and all up and down the scale. Sometimes it would pop back up if I goosed the throttle, but not consistently. It would also pop back if I turned on the headlights or wipers, but eventually it would start jumping around again.

    I couldn't determine if this was an alternator/regulator problem or a gauge problem, or both. I checked the connections and they were all tight. I did notice that the fan belt seemed a bit tighter than it probably should be while the engine was hot. How can I rule out/rule in a problem?

  2. Start by checking the Voltage at the alternator and see if it is putting out a steady 14 to 14.5 volts. Your alternator may be flaking out. If that looks good check the connections for the voltmeter itself and the take your meter and check voltage at the source you are using for your volt meter to read. Kinda sounds like you alternator may be starting to die or if its externally regulated, the regulator is acting up.
    jeffd1988 likes this.
  3. I'll agree and say it's probably the regulator. If you have or can rig a light bulb that's visible in the car (no lens over it), watch it; if you see the lamp visibly dim or brighten, you've likely got a regulator issue. Or hook up another voltmeter to eliminate the in-car unit as the issue.
  4. dirt t
    Joined: Mar 20, 2007
    Posts: 5,114

    dirt t
    1. HAMB Old Farts' Club

    Take a separate volt meter connect it across your present volt meter if it does the same look at alt stuff.
    If it doesn't jump around it your volt meter.

  5. Blues4U
    Joined: Oct 1, 2015
    Posts: 7,324

    from So Cal

    You should be able to clip the leads of a multi-meter to the terminals on the back of the voltmeter and go for a drive, see if the multi-meter agrees with the what the voltmeter shows. This is a case where an old fashioned analog multi-meter is handy to have, the needle movement can be easier to read than a digital readout.
  6. BJR
    Joined: Mar 11, 2005
    Posts: 8,245


    Sounds like the regulator or bad/warn brushes in the alternator.
  7. jeffd1988
    Joined: Apr 12, 2016
    Posts: 537


    Voltage regulator is going out. If it internaly regulated. Maybe about that time to get another alternator. But i agree with all these guys in the past threads. Test it with another meter hooked up and check.
  8. TerryT35
    Joined: Jun 16, 2006
    Posts: 37

    from Minnesota

    I had a similar problem with a voltmeter gauge.
    It ended up being a simple fix, a bad ground at
    the gauge.
  9. Hellfish
    Joined: Jun 19, 2002
    Posts: 6,561


    The ground is shared by all of the other gauges which all seem to work, but that is the second place I checked. It was barely finger tight. Maybe it loosened up again. The headlights, wipers, etc seem steady, which made me wonder if it was the gauge rather than the alternator. It is internally regulated.

    Thanks for the tips. I don't have another gauge, but I do have an analog multi-meter
  10. d2_willys
    Joined: Sep 8, 2007
    Posts: 4,230

    from Kansas

    Regulator or brushes is what most would say. I would take another meter as a previous post suggested and do the same test to see if it varied. Regulator is solid state and pretty durable. I would check all connections first, along with alternator grounding. Brushes may be making and breaking contact to rotor (field) slip rings, causing charging voltage to register, then battery by itself. Another possibility is the sense wire and field wires have bad connections.
  11. SouthernMn
    Joined: Jul 10, 2015
    Posts: 2


    I also have a voltmeter question. My voltmeter stays at 14v, until I turn on the lights at night. Then the voltmeter jumps up to about 30 volts. Any suggestions? What should I try first ? I've never had any battery charging issues, so I don't think its the alternator.
  12. 29FordMN
    Joined: Jul 11, 2016
    Posts: 9


    Usually when the voltage goes up past 14v its a voltage regulator issue.
  13. Ebbsspeed
    Joined: Nov 11, 2005
    Posts: 6,001


    OP, I'd bet on the voltmeter being bad. The assumption here is that when you say "jumps" it means it happens very quickly. If the voltmeter and wiring are good, and the voltage "jumps" from 14 to 10, or to whatever, then that means your battery voltage is following suit, and battery voltage won't change that fast unless it's got a huge load on it. Plus the fact that it never went over 14 to me says that the battery is staying very near that voltage. Bad gauge, or bad wiring to the gauge. As has been suggested, use a good voltmeter/multimeter and see if it's mimicking what the AutoMeter gauge is doing. But don't hook it up to the voltmeter as has been suggested by a couple replies, because if it's bad wiring it will prove nothing. Hook it to the battery for a real reading.
    trollst likes this.

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