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  1. #1
    FEP Power Member fgross2006's Avatar
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    Default after market Volt meter question

    A few years back I restored my dash cluster with coverlays, LED's and I had the stock AMP gauge retrofit to be a Volt gauge.

    The instructions said to simply connect to a 12v keyed on source in the dash and ground it, which I did.

    What I have come to dislike about this set up is that the volt gauge reacts to everything I power up inside the car.

    Turn stereo on, the gauge drops, headlights, gauge drops, AC and it drops drastically, windshield wiper on and the gauge click back and forth.

    A few things I notice;

    On later model vehicles with stock volt gauge, like my Suburban, they don't notice or react to any power used in the vehicle. I also notice that when I hit the rear defrost in my 5.0 the volt meter doesn't notice it. That's because rear defrost gets power from the positive terminal of the starter solenoid. Not from inside the dash.

    SO,

    Does anyone know of a wiring technique where I can wire my volt gauge into the engine compartment, to get an actual reading on my alternator charging? That's supposed to be the purpose of the volt gauge anyway.

  2. #2
    FEP Supporter Broncojunkie's Avatar
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    I don't see why you couldn't feed it off of a relay that's connected to the battery (or starter relay post). Just use a switched 12v source to trigger the relay.
    79 Pace Car - 331, t5
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    82 Capri- working on 302, t5
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  3. #3
    FEP Power Member qikgts's Avatar
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    ^^^^ That's the way!^^^^
    '85 GT

  4. #4
    FEP Power Member fgross2006's Avatar
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    Quote Originally Posted by Broncojunkie View Post
    I don't see why you couldn't feed it off of a relay that's connected to the battery (or starter relay post). Just use a switched 12v source to trigger the relay.
    can you post a pic showing an example? Im not creative when it comes to electric. If I see a set up I can copy it.

  5. #5
    FEP Supporter Broncojunkie's Avatar
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    Quote Originally Posted by fgross2006 View Post
    can you post a pic showing an example? Im not creative when it comes to electric. If I see a set up I can copy it.
    Here's a simple diagram I found.
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    http://www.ado13.com/techs/relay.htm
    You would run a wire from 87 to your guage. *double check this...I can't remember right off the top of my head but this is the idea. I just can't confirm the diagram is 100% correct. A quick test with a light or meter can confirm.
    79 Pace Car - 331, t5
    79 Pace Car- 302, 4 spd
    79 Cobra - working on 351w, t5
    82 Capri- working on 302, t5
    82gt - working on 408w, c4

  6. #6
    FEP Power Member 87gtVIC's Avatar
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    That diagram will work....for visual purposes just eliminate the switch (the switch is there so one can control when the accessory is on/off ONLY when power is received from ignition switch).

    85 ground

    86 ignition acc/key on and run power (to trigger the coil within the relay)

    30 battery power always hot

    87 to you positive terminal of your volt meter. Negative terminal just goes straight to ground.

  7. #7
    FEP Power Member fgross2006's Avatar
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    Quote Originally Posted by 87gtVIC View Post
    That diagram will work....for visual purposes just eliminate the switch (the switch is there so one can control when the accessory is on/off ONLY when power is received from ignition switch).

    85 ground

    86 ignition acc/key on and run power (to trigger the coil within the relay)

    30 battery power always hot

    87 to you positive terminal of your volt meter. Negative terminal just goes straight to ground.
    The diagram makes some sense to me but I still dont see how the volt gauge would be getting a real time reading of alternator charging capacity to the battery.

    It looks like Im just tapping into another location for voltage and using a relay to turn it on and off.

    If I probe the pos and neg battery terminals while the car is running I see the actual voltage the battery is receiving from the alternator.

    Theoretically if I connected my voltage gauge to the positive battery terminal I'd see the actual charging capacity, but with key off the volt gauge would slowly drain the battery. So how can I get that real time voltage reading while insuring the battery wont bleed voltage when the key is off?

  8. #8
    FEP Power Member 87gtVIC's Avatar
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    You follow the diagram. Pin 86 is the TRIGGER. Triggered by the ignition switch. This will only activate the coil when the key is in ON, ACC or RUN. You connect pin 30 of the relay directly to the battery post. Pin 87 goes to the positive side of your voltmeter. Your voltmeter get a ground from where ever you choose.


    Key on, coil closes pin 30 to pin 87 and your voltmeter completes the circuit to ground.




    I done it differently in my car. I gave my voltmeter direct battery voltage. I have the ground running through a relay. Same end result.

  9. #9

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    I found the same thing when I installed my aftermarket gauges years ago. Connected the volt gauge to whatever switched 12V source I found in the dash, gauge moves anytime something is turned on/off. Been running this way for awhile, but would rather know accurate battery/charging system voltage. Plan to correct this issue with a dedicated relay while doing some other electrical projects this winter.
    Thomas

    1985 Mustang GT - Build Thread
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  10. #10
    FEP Super Member gr79's Avatar
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    Have a VDO voltmeter installed. Wired direct to dash wiring per instructions no relay.
    Reads high like 14v+ after start, then drops to 13.5 or so after driving.
    Gauge will move down a bit when heated backlight is turned on. Higher the drain, the more it drops.
    When sitting parked engine off listening to the radio, the aftermarket amp will drop the gauge as battery is used.
    When driving in winter with everything running, will read lower quicker as voltage drops than with everything off.
    The Ranger has factory voltmeter. It wiggles when items are switched on then rises to normal.
    Will drop and stay if loads are high, mostly with lights on and blower on high.
    Neither car or truck gauge react to w/s wiper sweeps. Car has 70a alternator, truck has 90a.
    Later vehicles have much higher amp alts that handle loads much better. More capy, less deflection.
    Felt early on the Mustang's stock 40a alt (no a/c) was too weak to handle added items.

    A digital voltmeter reads voltage as a number readout. Analog type is needle with marked overlay.
    Alternator amps can be checked with a induction amp tool placed on the alternator to battery wire.
    I always keep 3 LED volt checkers with leads plus HF digital multimeters onboard.
    And jump boxes.

  11. #11
    FEP Power Member fgross2006's Avatar
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    Quote Originally Posted by 87gtVIC View Post
    You follow the diagram. Pin 86 is the TRIGGER. Triggered by the ignition switch. This will only activate the coil when the key is in ON, ACC or RUN. You connect pin 30 of the relay directly to the battery post. Pin 87 goes to the positive side of your voltmeter. Your voltmeter get a ground from where ever you choose.


    Key on, coil closes pin 30 to pin 87 and your voltmeter completes the circuit to ground.




    I done it differently in my car. I gave my voltmeter direct battery voltage. I have the ground running through a relay. Same end result.
    OK but one more question. Relays are common items and are made to plug in to a fuse box to a mating female connection. I'd need some kind of female end to plug the relay into and run the wires off that.

    Or am I not visualizing this correct?

  12. #12

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    You can use spade connectors to connect the relay to the circuits. Or you can buy a relay pigtail.

    Something like this-

    https://www.amazon.com/ONLINE-LED-ST...dp/B01KVZ2MCW/

    Just make sure to fuse where necessary and that the wires are big enough to carry the load.
    Last edited by massacre; 11-09-2021 at 09:11 PM.
    79 Zephyr, 4v/4r70w swap, with team z front and rear suspension, 8.8 and upgraded brakes and coil overs.

  13. #13
    FEP Power Member fgross2006's Avatar
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    Quote Originally Posted by massacre View Post
    You can use spade connectors to connect the relay to the circuits. Or you can buy a relay pigtail.

    Something like this-

    https://www.amazon.com/ONLINE-LED-ST...dp/B01KVZ2MCW/

    Just make sure to fuse where necessary and that the wires are big enough to carry the load.
    How much of a load will voltage to a volt gauge gonna produce? Cant be that much

  14. #14

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    Quote Originally Posted by fgross2006 View Post
    How much of a load will voltage to a volt gauge gonna produce? Cant be that much
    I would ask the manufacturer of the gauge that question.
    79 Zephyr, 4v/4r70w swap, with team z front and rear suspension, 8.8 and upgraded brakes and coil overs.

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