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Thread: Voltage gauge reading low

  1. #1
    AKA "The Goat" Michael's Avatar
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    Voltage gauge reading low

    Problem that has annoyed me for a decade. My voltage gauge always reads low. My multimeter shows my battery at 13.4-.6v at idle and around 13.2-3 with AC engaged.

    My gauge however will show about 12.7 at idle and about 11.5v with AC on. Even less with lights on etc. It's always been this way but I know my charging system is fine and I know the gauge is known to be quirky. Is there anyway to recalibrate it or even pull the needle and reposition it to a more accurate location? At this point I just may buy a new gauge unit as my temp gauge sometimes will peg and get stuck at 12 o'clock when I turn the ignition off. I know these 2 share a ground. Is it possible a corroded ground could be the culprit?

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    The fuel pump shares a ground w/ the instrument cluster. You can reground it at the inertia switch to one of the bolts that secures the body to the frame by the drivers right leg. Probably won’t make a huge difference at the gauge though. Neither would a new gauge- the ground wiring across the whole car is so shitty that what the gauge gets at that location is accurate. As long as you know you are charging correctly at the battery/alternator, and you know your gauges baseline, so be it.
    5 speed, grooved hood, grey interior (Nov '81)
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    Administrator Ron's Avatar
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    Corrosion or any bad connection along the volt gauge's circuit, ground or feed, could cause the gauge to read low. (Wiggle the IP plugs, check the fuse/socket.)
    I would guess it has as much to do with our quirky gauges as anything else, as you say. So common that it wouldn't be worth trying a new unit, imho. There are ways to easily trick the needle to move lower (but not higher) so that it reads correctly at a given voltage, but that would throw readings at other values way off. So I'd agree with just live with it.

    Now if you knew a talented painter who could change the "13" to a "14"....

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    Dedicated ground wire spliced on to the binnacle connector for the gauge did the trick for me in terms of gauge accuracy (for an analogue gauge). Separately, Dave M's Idle ECU has the AC bump, which keeps the idle 100 RPMs above the normal setting while the compressor is engaged. That gives my car enough kick to stay above the 12.8 waterline with everything turned on at idle.

  5. #5
    Administrator Ron's Avatar
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    That's a good idea...bypasses all of the old connections etc. You might do the same thing for the positive side (but taken off of the (W)hite wire feeding Aux Relay 2 instead of the battery).

  6. #6
    Senior Member Bitsyncmaster's Avatar
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    Yes moving the ground on the inertia switch to frame ground will make your gauge read about 0.3 volts higher.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  7. #7
    AKA "The Goat" Michael's Avatar
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    Thanks for all the suggestions. I'm going to tack this onto my fall tune up list.

  8. #8
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    Quote Originally Posted by jackb View Post
    The fuel pump shares a ground w/ the instrument cluster. You can reground it at the inertia switch to one of the bolts that secures the body to the frame by the drivers right leg. Probably won’t make a huge difference at the gauge though. Neither would a new gauge- the ground wiring across the whole car is so shitty that what the gauge gets at that location is accurate. As long as you know you are charging correctly at the battery/alternator, and you know your gauges baseline, so be it.
    Quote Originally Posted by Bitsyncmaster View Post
    Yes moving the ground on the inertia switch to frame ground will make your gauge read about 0.3 volts higher.
    Not sure that any of us are planning to ever need this safety feature in the event of a crash, but would changing the grounding on the inertia switch change the way it is supposed to kill the fuel pump if the inertia switch goes off?

    Just wondering how that might work. Safety, or security too, if you happen to use the inertia switch plunger as an extra "don't steal my car" measure.

    My tach gauge does all kinds of wonky things bouncing around so getting in there to improve the grounding around the instrument cluster is on my winter to-do list as well.


    Sept. 81, auto, black interior

  9. #9
    Senior Member Bitsyncmaster's Avatar
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    Cutting the ground wire and moving it to the frame ground does not affect the operation of the inertia switch. It just moves the fuel pump ground current off the wire going to the gauges. The inertia switch still switches that ground open in a crash.

    You cut the black wire not the black/purple wire.

    It will also up the voltage your fuel pump gets by the same 0.3 volts. The problem is the OEM harness used a small gauge wire on the that ground connection to the gauges and when the 7 amps of fuel pump current passes throught that small gauge wire the voltage drop makes that ground point at 0.3 volts when it should be 0.0 volts.
    Last edited by Bitsyncmaster; 09-07-2019 at 08:40 PM.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  10. #10
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    Exactly what Dave said.
    Here’s a link to a guy discussing him doing it, for the same reasons as you:
    https://www.riumplus.com/delorean-fu...-re-grounding/
    5 speed, grooved hood, grey interior (Nov '81)
    Spax Shocks, DCUK springs, Delorean.eu LCA brackets, DPNW Poly swaybar bushings, DMCMW shock tower bar, Deloreana.com convex mirrors, DPNW Toby Tabs, DPI cat-less exhaust, C4 Corvette third brake light, PJ Grady tail light boards, Bitsyncmaster relays

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