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Thread: How To: GM 1 wire alternator to a 3 wire in a Toyota

  1. #1
    Administrator test MarcGSR's Avatar
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    Lightbulb How To: GM 1 wire alternator to a 3 wire in a Toyota

    A common modification on early Toyota Pickup's and 4Runner's is to replace the small factory alternator with a GM 2-terminal 10si or 12si case alternator. You can pick up a 100A or 140A Powermaster from http://www.summitracing.com (Manufacturer's Part Number: 47294, Summit Racing Part Number: PWM-47294). You can purchase the alternator bracket from http://www.trail-gear.com. Many people doing this swap replace the factory 5-wire Toyota alternator (Sense, Ignition, Alt Light, Ground, Output) with a 1-wire alternator for simplicity's sake - although all 10si and 12si alternators come 3-wire capable. The only real advantage to a self-exciting (one-wire) alternator is that it is very easy to wire - obviously. However, in my humble opinion it has many drawbacks including:

    - Inability to do remote voltage sensing likely to result in low system voltage and poor electrical performance.
    - Related to its inability to do remote voltage sensing a powerful self-exciting alternator can cause damage overcharging when re-charging a battery with a low state of charge.
    - When used in low RPM range it may not generating sufficient alternator RPM to get the alternator to self-excite.

    All it takes is a little research and a few minutes of extra time to wire it up properly as 3-wire.

    1) Once you have your new high output alternator installed you need to pick up an alternator wiring harness. I got mine from Partsource it says HP3870 GM 1963-1990.


    2) Match the pins on your new alternator to the corresponding plug on the wiring harness you just picked up. Don't connect it just yet but make a mental note what wire goes to what pin. It only plugs in one way.


    3) Cut the plug off your Toyota harness.


    4) Connect the wires coming off the new alternator wiring harness with the corresponding wires on your Toyota harness.
    #1 goes to the Toyota yellow "L" (light) wire.
    #2 goes to the Toyota white "S" (sense) wire.
    The Toyota red "IG" (ignition) wire is left unused.
    Connect the + post on the new alternator to the + post on your battery using 4ga high quality cable.
    Tape up and secure any unused factory wires.
    NOTE: you should also connect a 50 Ohm resistor in parallel with the #1 light wire so that if the bulb burns out the alternator will still be excited.


    That's it, your done. Connect the alternator wiring harness to the terminals on your new alternator. Enjoy being able to run your winch or your rock lights without worrying about your truck stalling.

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    Re: How To: GM 1 wire alternator to a 3 wire in a Toyota

    good writeup. is this the same alternator you texted me about?

    bmoney

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    How To: GM 1 wire alternator to a 3 wire in a Toyota

    One and the same

    Sent from my BlackBerry 9900 using Tapatalk

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    Re: How To: GM 1 wire alternator to a 3 wire in a Toyota

    I don't understand why this had to be done on your truck but not mine. I put that entire package in my truck with the original power lead to the alt and everything worked just as it should...? gauge and everything was the same as it was...

    Maybe the low idle switch or whatever didn't work, but I never had a winch so I never had a use for such a thing I guess...??

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    Re: How To: GM 1 wire alternator to a 3 wire in a Toyota

    Are you sure the gauge didn't stay in one spot when you turned the truck on? There was no way I'd get away with running factory gauge wire and not stalling out the winch. I had the alt out so it was the perfect time to re-wire it.

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    Re: How To: GM 1 wire alternator to a 3 wire in a Toyota

    My gauge worked perfect. Lower at idle, higher under engine load, hard bass hits made it drop etc.

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    Re: How To: GM 1 wire alternator to a 3 wire in a Toyota

    Then I don't know. Perhaps someone with more electrical knowledge than I can shed some light on how the factory gauge stays working with only one wire coming off the alt.

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    Re: How To: GM 1 wire alternator to a 3 wire in a Toyota

    since I'm not being paid enough to break out Mitchell and work backwards (), I'll surmise...MANY gauges in vehicles that old were not controlled or distributed from a central bus. Shit there was no bus of any sort lol. A voltage gauge in particular just laid in parallel with an ignition source and measured its voltage. I bet 10 bucks that if you took a good meter, you'd find solid continuity between the "signal" wire on the gauge and the main ignition lead on the ignition switch. If voltage is there, it reads it...just like aftermarket gauges do now.

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    How To: GM 1 wire alternator to a 3 wire in a Toyota

    The stock yota is a 5-wire set up I feel better running the GM as a 3-wire when it takes only a few minutes to do. Aside from better idle output performance vs 1-wire it also increases the lifespan of the alt.

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    Re: How To: GM 1 wire alternator to a 3 wire in a Toyota

    what amp 50 ohm resistor did u use

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    Re: How To: GM 1 wire alternator to a 3 wire in a Toyota

    The resistor should be equivalent to a 3-4 watt bulb. Using Ohms law to calculate the required value of the resistor as R = V^2 / Watts. In this case R = (14v*14v) / 4W = 49 Ohms. So a 50 Ohm resistor will do.

    You can find them on http://www.futurlec.com/Res5W.shtml for 20 cents.

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    Junior Member krackem's Avatar
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    Re: How To: GM 1 wire alternator to a 3 wire in a Toyota

    thanks i picked up a couple 2watt 51ohms for 1.29 at a local electronics store if thet dont work ill move up in watts im doing the swap beacuse mmy alternator will not put out charge steadly at first it put out 14 volt but just falls after that and my battery light wont work checked fuses, voltage at plug all give the same as the battery except the alt light (yellow wire dont no what it should read, and got power at the cluster, im stumped so gonna try your way first in if it dont work ill just 1 wire it but thanks man appreicate the info ur a life saver

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    Re: How To: GM 1 wire alternator to a 3 wire in a Toyota

    Quote Originally Posted by MarcGSR View Post
    The resistor should be equivalent to a 3-4 watt bulb. Using Ohms law to calculate the required value of the resistor as R = V^2 / Watts. In this case R = (14v*14v) / 4W = 49 Ohms. So a 50 Ohm resistor will do.

    You can find them on http://www.futurlec.com/Res5W.shtml for 20 cents.
    Your math is bang on little brother...but nobody asked why we need to put a resistor across this one wire? On older gm's, the battery light is actually a load. Turn ign key to run position, battery light comes on. Start engine, that light acts as a load to excite the fields in the alternator to get it to start charging. As the alternator gets going, the light goes out, indicating all is well as a side effect (remember, its first purpose is to get the alternator charging). Surprise surprise, that 194 bulb in the dash is about a 4 watter



    Thus, you guys are using a resistor instead of an indicator bulb to get the alternator up and off its ass when you fire up the engine.

    FYI - a resistor is rated with two values - WATTS and OHMS. Watts is a numerical indication of the amount of power the resistor will deal with (voltage x current). Higher wattage resistors are physically much bigger as they need to dissipate a bunch more heat (makes sense). OHMS relates to the fixed resistance the resistor introduces. Not to be confused with IMPEDENCE (also measured in ohms - think a speaker). Higher OHM value = higher resistance, or technically the more the device "resists" the flow of power (again, power being voltage x current). Resistors also have a TOLERANCE value - or how "accurate" the resistance value listed is actually reflected in the device. Cheap resistors are 25% tolerance, normal resistors are 10% and super high quality as low as 2-5%. Thus - a cheapo resistor's value can swing wildly....most new cars that work with computers and networks (multiplex) will work within 10%.

    Knowing the electrical system is extremely variable in an automobile (from low 12.x volts to high 14.x volts), resistor value derived as above is an approximation. Do the math again at 12.6 volts. Then again at 14.5 volts. You'll see what I mean.

    /basic physics lesson

    bmoney
    Last edited by bmoney; 07-26-2013 at 09:18 PM.

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    Re: How To: GM 1 wire alternator to a 3 wire in a Toyota

    Did the swap and nothing but issues. Getting the harness but bad thing is I don't have my old one after I did the swap. I got an 84 with a 22r motor. Thus is what I am working with. Can't figure out how to upload the picture but got 2 whites 1 red 1 yellow and 1 black. Also I can't view the original pictures
    Thanks

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    Re: How To: GM 1 wire alternator to a 3 wire in a Toyota

    Sounds like you need to pick up an alternator wiring harness from your local parts store. I got mine from Partsource at the time, it said HP3870 GM 1963-1990 then look up the electrical wiring diagram for an 84 pickup to see what those wires are. Regarding the lack of pictures, Photobucket recently went greedy and is now charging a monthly fee to link to pictures hosted with them, I'll have to upload them to a different site to get them to show up.

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