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Ammeter issue?

Discussion in 'Modified Alpine' started by cnalbers, Dec 7, 2021.

  1. cnalbers

    cnalbers Donation Time

    I need help. My 3 wire GM alternator is putting out 14+ volts and 115amps at the alternator. However, when I measure the amperage across the +/- ammeter wires (disconnected from the gauge) I am getting 5amps. The gauge reads in the negative when the car is running, a pegs further down the more I turn on, but the lights don't dim and the output at the alternator does not change.

    What do I need to do to sort this out, apart from take it to an auto electric shop.... last resort, that.
  2. loose_electron

    loose_electron Donation Time

    You can't measure amperage "across" anything. Sounds like you are measuring things wrong.

    The ammeter option in the Alpine puts the current thru the meter and an associated shunt resistor. A more accurate way to monitor battery charging is to monitor voltage out of the alternator and at the battery. In a healthy system the two voltages should be very similar (within a half volt or less) and over 14V when charging.

    If you want to actually measure the current out of the alternator, you need one of these:

    My personal suggestion is to dump the ammeter in the car and rewire it to support a voltmeter instead. I doubt the alternator is putting out 115 amps of current, that's the max current limit of the alternator under full load I suspect.

    With the current clamp meter you can measure the currents and see where they go.
  3. cnalbers

    cnalbers Donation Time

    Voltage the alternator at idle (900) is 14.3; voltage at the battery is 14.1.

    I don't know if my ammeter is wired correctly... I recently rewired the car based on a rewire done by Tim Raymond in UK (pdf attached), installing fuse boxes, electric fans, relays for fans, lights & horns... I can't find a diagram for ammeter wiring, but mine is wired positive from alt to positive on starter solenoid, then to positive on ammeter, negative on ammeter is power supply for battery fuse box, which supplies power to the following: ign switch, high & low beams, light switch (which panel & parking are together), horns, cig lighter, map light, reverse lights, air fuel mixture gauge, and electric choke. I had to alter his wiring as it was based on a single wire alt, mine is 3.

    When the car is off, the ammeter reads zero, when it is running, it is just to the left; the more things I turn on, the more it moves negative. However, the output of the alt remains constant at the alt and battery.

    Attached Files:

  4. loose_electron

    loose_electron Donation Time

    With the wiring you describe in your text, the ammeter is measuring the current to the battery fuse box stuff. That will always be negative, as you describe. Your 14V numbers sound healthy.

    To measure the charge-discharge of the battery, the ammeter needs to be in line with the current going in-out of the battery. In cars, they leave the starter current cranking out of that, but everything else is included.

    The diagram you have on page 5 of the document you posted has this setup. All current but the starter current goes thru the ammeter, both in and out of the battery. That is correct.

    What you wrote: "negative on ammeter is power supply for battery fuse box"

    That's not the proper way to wire things up.
  5. cnalbers

    cnalbers Donation Time

    This is how it is wired... what do I need to do to correct it?

    Attached Files:

  6. hartmandm

    hartmandm Moderator Diamond Level Sponsor

    My thoughts ...

    Disconnect the alternator output from the starter solenoid.
    Change the Ammeter + terminal to be connected to the same starter solenoid connection as the battery.
    Then you need the alternator output to be connected to both the Ammeter negative terminal and the power input to the Fuse 1 block. Adding either red line in the diagram below will accomplish this. I.e. you could connect the alternator output to the Ammeter negative terminal, or connect the alternator output to the power input to the Fuse 1 block.

    Use sufficient gauge wire to handle the current requirements.

    I'm not sure how your car is running with only the green+white wire running from the starter solenoid to the ignition coil. With a stock solenoid, that green+white wire only goes hot when the ignition switch is in the "start" position. You must have another white wire that is not shown on the diagram coming off the ignition switch that goes to the tachometer (assuming you have an electronic tach) and then to the ignition coil.


    Last edited: Dec 8, 2021
  7. cnalbers

    cnalbers Donation Time

    Thanks for the reply. I am going to try your fix.

    The car is running fine. All the GRN/WHT wire from the solenoid to the coil does is ensure that the coil is getting full voltage from the battery when the ignition is activated; this is critical for my electronic ignition, as anything less than 11v will not send spark to the plugs. Yes, I do have the white wire from tach to coil as you describe.
    Last edited: Dec 8, 2021
  8. loose_electron

    loose_electron Donation Time

    Here is how I would change the wiring.

    1. In your drawing the coil was always powered up. Typically that gets turned on/off under control of the RUN output of the ignition switch.

    2. The ignition switch BAT connection gets it's feed from the Alternator so all current going through the AMP meter is all loads in the car except for the starter motor.

    3. Adding an inline fuse of about 35A so that all the ignition switch wiring is protected in a short. Might have to bump that value up a bit, but 35A ia a reasonable start point.

    4. Putting a high current fuse at the battery (100A to 350A) to protect the heavy battery cable from the back of the car where the battery is to the starter solenoid.

  9. loose_electron

    loose_electron Donation Time

    One addition to this -- get rid of the BRN wire from Ammeter to Fuse1 -- other than that it's essentially what I am suggesting.

  10. puff4

    puff4 Diamond Level Sponsor

  11. loose_electron

    loose_electron Donation Time

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