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  1. #1
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    Default 1978 Alternator light!!!

    Hey guys,
    This is my first post, I appreciate all help.
    I got this alternator light that won't go away. I replaced the alternator because it tested bad. Now with the new one the battery tests at 12 volts when the car is off, but 11.8 volts when the car is on... I could be wrong, but I think the battery is not charging and I don't know what else the prob may be. I replaced the belt and there is enough tension, alternator is good, brand new battery, what am I missing?

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    Well...it could be the battery.

    If the battery test's 12.0 v with the engine off, it's got a problem. It should test at 12.6.

    On the other hand, if it tests at 11.8 with the engine running at a fast idle, either the battery is problematic such that it will not accept a charge or the new alternator is bad. At a fast idle, you ought to see 13.8 volts or so.

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    Hope I remember all this correctly, it's been a long time since I worked on one that old: make sure that you have battery voltage at the main alternator stud and on the red wire at the two wire plug connector that plugs into the alternator. The smaller wire (brown?)on the two wire plug connector goes to the charge light bulb and should show low voltage when disconnected from the alternator, and the key on. You need these power feeds to excite the voltage regulator and start the alternator charging.
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    Did you have the battery tested? Also rebuilt alternators are notorious for being bad out of the box.
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    ClarificationSome additional checks that may help: With engine running / fast idle-
    1. Check voltage at the red wire on the rear of the alternator (large post) to ground, a reading from 13 - 14 volts is normal and should charge your battery, if it is good.
    2. The alternator warning light comes on when voltage from the alternator trio diode output is less than the battery voltage as seen at the warning lamp.
    3. Trio diode can be damaged if the car is jump started. Trio diode is located in the alternator, mounted on top of the regulator assy, it's output exits on the brown wire connector on rear of the alternator.
    4. A battery voltage reading of 12 volts with the engine off, pretty much indicates a good battery; if it read much less than 12, like the 10 - 11 V range than the battery is bad or at least in the need of a full charge and then load tested.
    5. A 12 v battery is made up of 6 cells, each one with an outout of ~2 V, connected in series to give you 12 volts out.
    6. Check both battery cables and their respective connections at the battery and at frame ground (negative) and positive cable at the starter / alternator for tightness, damage / corrosion, all these can have a degrading affect on the battery charging and alternator output.

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    Default Gonna try all the advice....

    Thanks guys, I bench tested the alternator and it was good, the battery was tested and was good, held a load and tested good. I got it narrowed down the wires and connections... We will see...

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    i cant believe it still 12 volts or lower when the car is on... power at red wire on 2 prong 12 volts, power at orange wire 12 volts, alternator benchtested good, but everything is pointing to the alternator....

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    Quote Originally Posted by Sal View Post
    i cant believe it still 12 volts or lower when the car is on... power at red wire on 2 prong 12 volts, power at orange wire 12 volts, alternator benchtested good, but everything is pointing to the alternator....
    You have 12 volts on all three alternator wires with the engine off and key (ignition) on?
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    Sounds like there might be a broken wire at the alternator. The brown (small) wire at the plug if notorious for breaking at the crimp.

    Key on, engine off. 12v testlight to all 3 wires. Hot post should light up (DUH!) big wire to the alt connector should light up too. The brown wire should just glow a little.

    And check the fuses too- I don't remember 78 but there is a choke heater relay that also controls the alternator field- could be either the relay or the fuse.

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    Well, I took it to my mechanic and he said its the alternator... i tried all those tested recommended but i have to get another alternator and i will advise... thanks you again for all of your help.....if its not the alternator, i don't know where the heaterchoker fuse is??

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    Quote Originally Posted by Sal View Post
    i cant believe it still 12 volts or lower when the car is on... power at red wire on 2 prong 12 volts, power at orange wire 12 volts, alternator benchtested good, but everything is pointing to the alternator....
    Although your mechanic said it was the alternator, as an FYI... you may want to check the extension harness that runs to the starter along the back of the engine near the windshield wiper motor. I'd do this just to make sure the entire charging system from individual components to wiring is working properly. I'd want to treat root cause and service the entire system as opposed to treating symptoms. A good alternator will ruin itself if it has bad connectors upstream that are creating resistance.

    I had a similar problem with a '78 that I owned years ago. IIHRC, there is a hot power wire that runs to the alternator (charging circuit) from that connector. I was going through an alternator every year or so.

    The wire I'm speaking of should be a thick red wire. Sometimes the GM/Packard connectors go bad, get corroded, or come loose. That is what happened to mine.
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    Thanks for all of the help guys...
    It

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    Thanks for all of the help guys...
    It ended up being the center console plug...
    the cooper connections that the plug goes into were bent slightly causing a bad connection for the alternator
    i appreciate all of the advice

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    Please clarify, what do you mean by center console plug? the plug that goes to the back of the guage cluster or your cigarette lighter plug... Just curous as to what exactly you are talking about.
    glad ya got it fixed.
    thx,

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    I realize you have it fixed. But I wanted to add for others from my experience. The wiring harness coming from the alternator in my car overheated near the place where it dropped over the valve cover. Shorted out and prevented the alternator from charging the battery. Went thru a battery and alternator needlessly. Because the wires were wrapped I never saw the short. A real nightmare running down the culprit.........

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