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Thread: Digital Dash IO Adapter Resistance for Alternator

  1. #1
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    Default Digital Dash IO Adapter Resistance for Alternator

    I'm upgrading my charging system to a CS style alternator from a DN style. The way the alternator gets turned on is by having resistance on a wire from a light bulb that is connected to the harness, and ACC on ignition switch, which is connected to the alternator. The car is a 1964 Nova. I installed the Digital Dash and I removed the original gauge cluster and cut off all the light bulbs from the original cluster as I don't need them. Can I take the two wires that were connected to the light bulb and plug them into my IO adapter to mimic the same resistance the light bulb sent to the alternator in order to turn on the alternator. I was thinking IO 8, or 9 would work after looking at the instructions, but not exactly sure.


  2. #2
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    Just put a resistor inline.

  3. #3
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    Measure the resistance of the LSx light bulb in the charging circuit (or whatever it's called).
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: Dominator MPFI & DIS, A/C, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/X-pipe/shorty headers, 100HP progressive dry direct-port NOS, 160A 3G alternator, Optima Red battery, LenTech Strip Terminator wide-ratio AOD/2700 RPM converter, FRPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 tires.

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    I was thinking even if the light bulb is cut, the alternator will still be on & running and I can use the Digital Dash to read the volts. All the light bulb or idiot light does is light up when it sees no resistance. Am I wrong to think that?

  5. #5
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    It's an LSx thing, there must be information about this on the internet.
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: Dominator MPFI & DIS, A/C, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/X-pipe/shorty headers, 100HP progressive dry direct-port NOS, 160A 3G alternator, Optima Red battery, LenTech Strip Terminator wide-ratio AOD/2700 RPM converter, FRPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 tires.

  6. #6
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    It looks like I was wrong. The idiot light wiring ties into ACC ignition switch which is tied into the alternator. A break in that circuit will not turn on the alternator.

    Quote Originally Posted by 81 TransAm View Post
    Just put a resistor inline.
    This is the easiest way.

    Quote Originally Posted by Danny Cabral View Post
    Measure the resistance of the LSx light bulb in the charging circuit (or whatever it's called).
    I need a minimum of 35 OHMS and a max of 350 OHMS to create the resistance. It's seems like a poor design that if the light bulb goes out the alternator stops working, but I guess thats why after doing a bunch of research they say its best to have a light bulb for the idiot light and a resistor wire so that if the bulb does die there is still resistance in the circuit and the alternator will still work.
    Last edited by RPnova; 05-21-2019 at 01:09 PM.

  7. #7

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    Note: Some of this may be more than you want to bother with, but knowledge is always a good thing, and lets you make your own informed decisions. Possibly others can benefit also.

    If you don't want the idiot light just use a 50 Ohm, 5 Watt resistor in line with the power from the ignition switch, going to the alternator's L terminal. That will turn on a CS alternator. If you still want a "GEN" lamp, instead of the 50 Ohm, use a small 12 volt light bulb in parallel with a 150-300 Ohm 5 Watt resistor. That is equivalent to what you described. If the bulb blows out, the alternator will still turn on. That's how Delco did it back in the day.

    By the way, to be really correct, you should also run a separate wire, about 18 gauge, with a small fuse (3 amp) in line, directly from the 12 volt battery "+" terminal to the alternator's S terminal. That will ensure that the charging voltage at the battery is optimized. (Put the fuse at the battery end of the wire.) Common redneck shortcuts like tying the S terminal to the alternator output stud or to the starter solenoid will sort-of work, but will result in substandard charging performance and poor voltage regulation. The closer you get this wire to the actual battery, the better it will work. Do not leave the S terminal unconnected.

    Yeah, I know that just using a so called "1-wire" alternator is easier. 1-wire alternators suck in general. Lousy charging, lousy regulation. It pleases me that you at least show interest in doing things the right way rather than cutting corners.

    Also remember your grounds, which are just as important as your power wires. Run a separate piece of grounding braid or #4 black cable from the alternator mounting bolt head to the engine block, and make sure the engine block is grounded to the battery negative using a heavy gauge (2 AWG or larger) cable. (Make sure these are bare-metal connections, no paint, using a star lock washer.) Assuming that adequate grounds exist via the alternator brackets and motor mounts is not wise. A lot of people, especially the ones that use trunk mount batteries, use the chassis as the main ground conductor to the front. This is another hot-rodder redneck shortcut. Steel is a very poor conductor of electricity. Again, it will work, just not as well as it could. On my race cars I run a separate 0 or 1 gauge black battery cable from the rear mount battery negative, directly to the engine block. I also run separate grounding braid from the block to each cylinder head, plus the aforementioned ground cable to the alternator. I also run a ground braid to the steel chassis, but the chassis is not actually used as a ground conductor. I also run separate smaller ground wires back to the battery for all of the electronics. Guess who never has radio noise issues? Or charging issues? Or electronics interference issues? Me.

    Finally, when buying large wire for battery cables, steer clear of the cheap crap sold for car stereos. This stuff is almost always copper-coated aluminum, plus they lie about the wire gauge. Use real all-copper battery cable, or even better, flexible welding cable. Learn how to properly crimp battery terminals and use heavy adhesive heat shrink coverings. You'll never buy a premade cable again. Keith

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