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  1. #1
    FEP Power Member fgross2006's Avatar
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    Default rewire aftermarket Volt gauge direct to alternator

    I did a cluster refurbish a few years ago. When I had it out I sent the alt gauge out to a guy that guts the original gauge and converts it to a stock looking Volt gauge. The instructions said to wire it to a 12v keyed on source in the dash which I did. Although the Volt gauge powers up I don't like the way it behaves.

    Every time I power up something the gauge drops as if its losing power, even though its not. Turn on the A/C and it drops to halfway. Turn on the directional and the gauge flickers with the blinkers. Turn up the stereo and when its loud the gauge drops. Now I noticed one thing that does not impact the gauge at all. The rear defrost. And i know why. because the rear defrost is not wired internally in the dash. Its power runs direct to the positive side of the starter solenoid so the Volt gauge isn't aware or affected by its usage.

    Since the purpose of the Volt gauge is to show that the alternator is charging, I want to find a way to run its power line into the engine compartment and see if I can get a connection off the alternator, or if not can I connect a relay to the positive battery terminal that powers on/off with key on/off?

    If I just ran the power to the battery positive I'd get a direct reading of charging voltage but it would be a slow drain when the key of off without a relay to cut the power.

    Any suggestions for a custom set up to do what I want? On later model cars/SUVs they have stock voltage gauges and on my Suburban, they are not affected by usage of dashboard accessories. So with that concept in mind there has to be a way to hard wire an aftermarket Volt gauge in the engine compartment to get a realtime reading on actual charging status.

  2. #2
    FEP Super Member gr79's Avatar
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    I run a VDO gauge. Is wired to the dash circuit.
    After engine start needle pegs up to around 14v no matter what.
    After driving a bit, gauge needle settles at 13.5. night or day, light or near full load when driving.
    Radio, wipers, heat, rear defrost, lights, fogs, brake lights, needle still stays above 12v at idle.
    70a 1g alt. Five year old Exide battery, dual pulley alt belts in good shape and tension.
    Turn car off and to acc gauge needle is centered 12v or so.
    High amp JBL radio amp will slowly drop the gauge the most when parked. Lights too of course.
    Engine running, amp cranked up, 13.5v.

    The Ranger has a factory voltmeter and moves down at any little load then recovers. Sometimes fast, sometimes slow.
    Has alt light too. Alt light sometimes is falsely lit, gauge normal.; is a minor alt connection issue.

    I prefer real time readings if only for diagnostic purposes. If they don't move somethings wrong.
    Also have dvtms available in both vehicles for backup checks. Handy.
    I do weekly battery checks on the car. Truck has new Motorcraft battery.
    Batteries usually read low-mid 12's on the car and 12.6v+ truck after sitting a few days.

  3. #3
    FEP Senior Member OX1's Avatar
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    I did the factory looking volt meter about 8 years ago on my 86.
    Thread has some info on wiring.

    http://vb.foureyedpride.com/showthre...ght=volt+meter

    EIDT: I just noticed you were on that old thread. The way I did it and another person on that thread also did, was use a relay so voltmeter still switches on/off with ign, but then run source wires directly to the ground and pos ON battery. That will be your direct reading of what the battery is charging (or not), can't get any better than that.

    Also, what alt do you have? The stock alt is a POS that does not even keep up with headlights and rear defroster at idle.
    3G does everything you can turn on, still 13+ volts at idle.
    Last edited by OX1; 05-24-2023 at 04:37 PM.
    86 Capri, 5.0, 5Spd, A9L QH/BE, 47 lb Inj PMAS 3" MAF, Single T44 Turbo, Front Mount IC, TW170,
    Stock Cam, Explr Intake/TB, 1.7 Rockers, CF dual friction clutch, 3" DP, 2.5" full Exh, 3.27, 11.932 @ 115.78
    84 LTD, 331-10:1, TW170/Exprl Intake, 47 lbs inj/80 mm LMAF, Full Duals, Quarterhorse, Vortech 7PSI, Lentech AOD, 5 lug Mk VII brakes/rear, Eibach Sway bars, Cobra HB (dads ride, but I fix it )

  4. #4
    FEP Power Member fgross2006's Avatar
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    Quote Originally Posted by OX1 View Post
    I did the factory looking volt meter about 8 years ago on my 86.
    Thread has some info on wiring.

    http://vb.foureyedpride.com/showthre...ght=volt+meter
    I commented on the thread. I didnt find anything detailed I can use though.

    87gtVIC gave a description that sounds good but in his pics he has a relay box in his fox. I dont know if thats stock or he customized it to install.

    A simpler set up would be to mount a relay somewhere near the battery on the fender or radiator support and run volt and ground from the Volt gauge to the relay. Where I lose train of thought is how to turn that relay into a keyed on source.

    I see some comments about how its important to see if there's a voltage drain, but for my want and need, I prefer to see in real time what my battery is charging at. That's how the volt gauge in my suburban works, that's how I want my Mustang volt gauge to function as well. If the alternator fails I'll see that immediately.

  5. #5
    FEP Power Member 87gtVIC's Avatar
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    Quote Originally Posted by fgross2006 View Post
    I commented on the thread. I didnt find anything detailed I can use though.

    87gtVIC gave a description that sounds good but in his pics he has a relay box in his fox. I dont know if thats stock or he customized it to install.

    A simpler set up would be to mount a relay somewhere near the battery on the fender or radiator support and run volt and ground from the Volt gauge to the relay. Where I lose train of thought is how to turn that relay into a keyed on source.

    I see some comments about how its important to see if there's a voltage drain, but for my want and need, I prefer to see in real time what my battery is charging at. That's how the volt gauge in my suburban works, that's how I want my Mustang volt gauge to function as well. If the alternator fails I'll see that immediately.
    Hi.

    Just pick up a 5 pin relay and and follow what I originally had written.

    Wire your voltmeter positive DIRECTLY to the battery.

    Your voltmeter ground to pin 87 of the relay.

    Wire a keyed power to pin 86 of the relay

    Wire a ground to Pin 30 AND 85 of the relay.

    Put the relay anyplace you want as long as you wire it like mentioned above.

  6. #6
    FEP Super Member gr79's Avatar
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    The alt gauge connection is a very good idea.
    Imagine adding a triple pole rotary switch to check three connections. Alt, alt harness before and after regulator or battery.

    I use a vintage 70's amp tool to check the alt harness. Time to carry it in the car toolbox again, not in a home toolbox drawer.
    Came in handy doing a side of the road wiring repair to my Cobra in the 80's.
    Good alt was not charging good battery. Battery needed overnight charging.
    Was shorted wires in the alt to regulator harness. An unfused and fried electric choke caused that.
    Did not know could drive 50 miles on the battery (daytime).
    Problem was the tool should have been used earlier to check the harness. Even the dealer missed the damage.
    While using gauge, found an oddly swollen section of loom. Cutting in that spot, found wires melted and shorted.

  7. #7
    FEP Power Member fgross2006's Avatar
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    Quote Originally Posted by 87gtVIC View Post
    Hi.

    Just pick up a 5 pin relay and and follow what I originally had written.

    Wire your voltmeter positive DIRECTLY to the battery.

    Your voltmeter ground to pin 87 of the relay.

    Wire a keyed power to pin 86 of the relay

    Wire a ground to Pin 30 AND 85 of the relay.

    Put the relay anyplace you want as long as you wire it like mentioned above.
    Hey guys, I'm getting my ducks in a row to do this custom volt gauge wiring as per the 87gt wiring recommendation.

    I picked up a 5 pin relay and a relay pigtail. Its a Standard S654. The wires are 14 gauge. The auto parts store guy said they don't come in thinner gauge wiring. I assume I'd have to use 14 gauge on the everything I splice in? As for the draw, I dont want to link in to something thats fuse might be overloaded by the added amperage.

    Would keyed on power from the positive coil be a good source?

    As for the unused wire in this diagram, 87a, should I remove that wire from the pigtail and seal the opening with silicone or butyl? Rather than have a dangling unused wire tapped up.

    Also, is it advisable to put an inline fuse on the voltage run between the battery and the volt gauge? If so, what size fuse?
    Last edited by fgross2006; 06-29-2023 at 09:24 PM.

  8. #8
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    You're overthinking all of it. lol

    That relay isn't going to draw squat. Maybe 1/2 an amp. Pick a 12v switched or ignition circuit for pin 86 and feel good about it. Personally I wouldn't tag into the ignition coil circuit. If you put the relay behind the cluster you should be able to find switched 12v back there. It might be a red with yellow dots/hash marks.

    Wire gauge won't matter as 14 gauge is way more than technically needed for this project. Just remove or cut that 87a wire back an insulate it.

    I would fuse the wire going from the battery to the gauge. 5 amp or less will be fine.
    '85 GT

  9. #9
    FEP Power Member fgross2006's Avatar
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    Quote Originally Posted by 87gtVIC View Post
    Hi.

    Just pick up a 5 pin relay and and follow what I originally had written.

    Wire your voltmeter positive DIRECTLY to the battery.

    Your voltmeter ground to pin 87 of the relay.

    Wire a keyed power to pin 86 of the relay

    Wire a ground to Pin 30 AND 85 of the relay.

    Put the relay anyplace you want as long as you wire it like mentioned above.
    Just wanted to update on this thread. I worked on this all day today. I pulled the old wires that were powering the volt gauge, ran power and ground through the firewall, connected all the wires as listed and the first thing I found was no power to the volt gauge. Took me a few minutes but I realized it blew a fuse right away. And I used a 10 amp fuse. I bumped up to a 30 amp and its holding but I found that when I key off, there's still a draw at between 0.3 to 0.6.

    I am tapping into the same keyed on source I used before under the dash and when I checked that spot when I set up my volt gauge a few years back, I recall verifying my source was 0.0 volts keyed off. The only variable in this setup is the relay. I left the dash open and I'm going to look for another key on 12v source tomorrow.

    With that much draw I'm sure to eat up my battery life. The only other thing I'm not sure could be a problem is I piggy backed on the passenger side headlight ground on the radiator support for pins 30 and 85. Would that cause any ground loop or unfortunate side effects?

    ALSO, I just remembered, I did get to test the volt gauge and the first thing I did when I got power to the gauge was turn on the AC and I still saw a slight drop on the needle, but much less than it was doing before. Before, I was drawing 12v from a dashboard source and with the AC on my volt gauge would drop to the halfway mark. Now it drops 1/8 of a tick.

    Just FYI, the seller that sold me the gauge told me that do to size restrictions of the amp gauge, his retrofit would not see the mid point as 14 volts as per most cars. The max point, far right is 14 volts. So when i ran it off the relay I got 14.4 volts with the needle all the way right. Turning on AC it slightly dropped less than a quarter mark.

    I tested at the battery to see real time readings and turning on the AC dropped my battery voltage from 14.5 to 13.5.

    Its a 3G upgrade alternator from LMR.
    Last edited by fgross2006; 07-22-2023 at 09:07 PM.

  10. #10
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    Quote Originally Posted by fgross2006 View Post
    I bumped up to a 30 amp and its holding but I found that when I key off, there's still a draw at between 0.3 to 0.6.
    What type of "draw" are you referring to?
    A "parasitic" draw?
    ".3 to .6" of "what" were you measuring?

    Please specify how/where you measured this and what the exact results were in all of the scenarios.
    '85 GT

  11. #11
    FEP Power Member fgross2006's Avatar
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    Quote Originally Posted by qikgts View Post
    What type of "draw" are you referring to?
    A "parasitic" draw?
    ".3 to .6" of "what" were you measuring?

    Please specify how/where you measured this and what the exact results were in all of the scenarios.

    So it was a rough ride. The wiring to the relay was easy enough. It was all the bumps along the way that threw me off.

    The other day i was seeing a .03-.06 drain at the 12v keyed on source while I had all the wiring in place but not hard wired to the Volt gauge yet. As of now with the gauge installed I am getting .00 at the 12v keyed on source. The volt gauge powers up with the key in on position showing 12 volts and is fully lit up and showing 14.4 volts with engine running.

    Now, when I turn on the AC I do not see a drop on the gauge anymore. In fact I can power up everything and it doesn't affect the gauge at all. So I am finally getting a real time reading of my alternators charging output and not a dashboard voltage usage reading. This is what i wanted thanks so much for the instructions.

    This is what I dealt with during the process.

    The last time I hard wired my 12v and ground sources to the harness that plugs into the back of the cluster, replacing the wires that had been feeding the AMP gauge. I don't know why I was a stoonz this time but I tried to run my power and ground to the rear posts on the back of the volt gauge. The fabricator had marked the posts red and black for polarity. It drove me nuts because when I tested the volt gauge directly from the battery with alligator clips, it lit right up, but every time I connected my 12v and ground wires that I had snaked through the firewall, the volt gauge was dead. and yes, I tested the power lead and was getting 12v keyed on and 14.5 with engine running.

    I kept bringing the cluster to the battery to re check and every time it functioned at the battery but was not getting voltage I verified was there at the power lead in the dash. I had the wires on and off so many times that I damaged the mylar coating on the cluster by the volt gauge. I was sure I had ruined it, but got some krazy glue and re attached the round brass connection to the mylar and a few other leads that were lifting off the mylar.

    I noticed that if I lightly tapped the volt gauge with a finger thump it activated the gauge, but after a few key on key off cycles it stopped repeating. I finally realized that hard wired to the cluster harness was the way to go again. I attached my power and ground to the two slots on the harness for the volt gauge and it lit right up.

    I also noticed the ground post on the volt gauge would not tighten all the way which I assume is why I wasn't getting a good connection and I think the post was rotating in the gauge which is not good. It works now so I'm not gonna mess with it any more. If anybody else does the AMP to Volt gauge conversion like I did, I do not suggest using the posts on the back for connections. Trace to the correct pins on the cluster harness and tap into that for a direct and more efficient connection.

    My only minor headache is I have to pull the cluster one more time because one of my LED back lights is off.

    Heres a few pics.

    This is my 12v key on source. I don't know what this connector is for, but when the key is off, theres no voltage so my volt gauge and fog lights get keyed on power here.

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    This is the relay screwed into the inner wheel well. Near the battery would have obstructed the battery removal process.

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    And just to ask, is there a battery positive cable I can replace this with that has accessory connections? I'm daisy chaining too much direct to the positive battery. So far I have my stereo amp, my sub-woofer and now my volt gauge all leeching off the battery positive cable.

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  12. #12
    FEP Supporter
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    If you have it working fine now then I'm not going to drill down into minutia, but you say "drain" and never quantify that with an actual value. Did you use an ammeter to take the measurements? Amps/amperes are what you'll measure while looking for a draw.

    Here is a good vid with a bunch of tips about using a DVOM for most basic uses.
    https://www.youtube.com/watch?v=TdUK6RPdIrA

    To get those wires off your battery terminals, a buss bar is a easy solution. You would attach a short lead of the appropriate size wire between it and the battery terminal/battery+ stud on the starter solenoid, and then add on your fused circuits further away in a better location.
    https://www.ebay.com/itm/29552193760...mis&media=COPY
    '85 GT

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