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OT-alt issue

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  • #31
    Carl I noticed in your charging system diagram that you do not show a 10 ohm resistance wire or indicator light in the circuit to the #1 terminal leading to the alternator.
    Not sure if this is an oversight on your part or you just decided to not show this, but this is a requirement on GM SI charging systems.

    Also your short driving cycles could be responsible for an undercharge condition.
    Another thing to keep in mind is that virtually all modern voltage regulation circuits are temperature biased, meaning summer time voltage output could be as low as 13.8, to as high as 15.8 volts at twenty below.
    Unfortunately I can verify this last point.

    Edited to add:

    Have a look here to help better explain the purpose of the indictor light.

    <H3>Dashboard Indicator Light

    If you have an alternator and are using the factory style indicator light on your dashboard, it is a pretty helpful thing. It helps kick-start the alternator into working at idle speeds when you first start the car, and it tells you if the alternator is putting out less voltage than the battery has in it, indicating a problem. The light is connected on one side to the field current system inside the alternator and to a switched ignition power source on the other side. When you turn the key on but have not started the car yet, the field acts as a ground and power flows through the light and out to ground - lighting the bulb so you know it works. Once you start the car, the voltage at the field is powered internally by the output of the alternator. If this value is exactly the same as the battery voltage, then you have the exact same voltage on each side of the indicator light and they balance each other out - kind of like a tug of war in reverse. If all goes well, the light never comes on, and you drive happily around knowing all is well with your alternator. If the output of the alternator should drop due to a slipping/broken belt or due to certain kinds of electrical faults inside the alternator itself, there will be less voltage on the field side of the light and more voltage on the switched ignition side of the light. The result is that some amount of electricity will flow through the light and into the field and the light will glow proportional to that voltage difference. This is how a slipping belt or an overloaded alternator will cause the light to glow very dimly, while a full-on failure will cause the light to glow very brightly. Note that if you disconnect (or forget to connect) the wire at the alternator, the light will never come on and the alternator will not charge properly.
    The dashboard indicator light circuit also typically has an extra wire with a calibrated resistance in it. This wire is run in parallel to the indicator light and has about a 10ohm resistance. It's purpose is to allow slightly more current to flow to the alternator field current system at initial start-up to make sure the alternator begins producing power as soon as the engine starts. About 1 amp total current is flowing to the field current between the light and the resistance wire, with the resistance wire supplying about 3/4 of an amp. This extra resistance wire does not affect the functionality of the indicator light in any way.
    NOTE: I've been informed by my readers that a Radio Shack 10 ohm 10 watt 10% wire wound ceramic resistor (part #271-132) has worked well on their GM vehicles. Use caution if you decide to do custom wiring work with resistors as they can get hot and melt stuff.
    Last edited by Willy; 09-22-2011, 01:56 PM.
    Home, down in the valley behind the Red Angus
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    • #32
      ecortech, I have both a Sun alt/gen, starter, 600A load tester and a Milton load tester and the battery appears to be good.

      Willy, I was not aware I needed the 10 ohm load in the #1 line. The alt seems to be charging correctly. When I start the Jeep it will charge from 20 to 30A and then in a few minutes gradually reduce to about a few amp charge.

      How much would the 10 ohm resistor drop the voltage and would it be the same as the approx .7v drop from the diode?

      I never have been good at using the ohm's law formula and the online calculators would not give me a voltage drop answer.
      It's only ink and paper


      • #33
        I've never used the resistor or a diode but I always use the idiot light when setting up custom applications. I use a #194 bulb in series on the #1 line. It should light when the key is turned to on and go out when the engine starts. The light will also indicate when the alternator isn't charging which is nice to know.

        The bulb and/or resistor act as a current limiter on the #1 line. The diode won't do that.


        • #34

          Transistor TR1 pulls current though the field coil. More current, more magnetic field, more charging voltage/current to battery.

          R1 and D1 feed current to the base of TR1, defaulting to "full on", or max charge current. Current can come from battery, in which case lamp will light, and current will be lower. or it can come from the rectifiers when alternator is running.

          TR2 "steals" base current from TR1 when on, to reduce charge current by reducing field current.

          When battery voltage is high, the output voltage of the "voltage divider" formed by R2 and R3 (and the potentiometer) is high enough to overcome the zener voltage of D2 plus base emitter voltage of TR2, so that TR2 turns on and reduces teh charge current as per the above.

          The "knee" is a little not-sharp" due to the need to supply not just voltage, but some current to the base of TR2, which has to come from R2 and the "pot", meaning that the voltage must be a tad higher than just the divider ratio indicates to really clamp down on TR1 base current (and charging).

          That no doubt accounts for the higher milliamp drain indicated for the unit, the lower resistance minimizes the error from the need to have base current.

          D3 prevents "flyback" pulses from damaging parts. C1 slows teh response a bit to prevent oscillation. R5 seems to provide a minimum load for lighting the indicator lamp when alternator is not running.. R4 keeps leakage from turning on TR1 (when TR2 collewctor has pulled down voltage), the same sort of thing I was mentioning with regard to TR2.

          Finally, D1 seems to be there to raise the voltage on TR2 collector, guaranteeing that it has full control of the base current of TR1.

          When alternator is running, indicator lamp will be off or very dim, since voltage will be pretty much same as battery voltage. if alternator quits generating, lamp will come on due to R5 and draw through TR1 and field coil (unless battery is so high that TR2 is "on).

          Originally posted by Willy
          JT, I didn't mean to imply that you were fundamentally wrong.
          And yes wide acceptance doesn't mean it's right, but after over 40 years of successful commercial application it does indicate that it is at least...good enough.
          And, the auto companies demand consistent parts.... they get 'em, too. If they spec a low leakage zener, they will get it, or else.

          I've had a contract manufacturer sub parts on me, when the original specified was guaranteed not to be able to cause a problem in that exact type circuit, but the sub was 50x more leakage and caused big headaches....

          it was simpler to add the base-to-ground resistor than to hold their feet to the fire after the fact. So I did.
          Last edited by J Tiers; 09-23-2011, 12:59 AM.
          CNC machines only go through the motions.

          Ideas expressed may be mine, or from anyone else in the universe.
          Not responsible for clerical errors. Or those made by lay people either.
          Number formats and units may be chosen at random depending on what day it is.
          I reserve the right to use a number system with any integer base without prior notice.
          Generalizations are understood to be "often" true, but not true in every case.


          • #35
            Nice explanation, JT. As you surmised, D1 is there to allow TR2 to bring TR1 into the fully off condition without having to go into saturation, which would require even more current to flow through the zener. It's one method of improving the regulation action, or 'sharpening the knee'.

            That's an elegantly simple regulation circuit overall. The reason that pin 2 goes directly to the battery + is so that the actual battery voltage can be monitored, and not the voltage at the end of some length of wire feeding another component. Having that pin go to the coil will have an effect- unwanted- whenever the points are closed, the coil will be drawing current and lowering the voltage at it's terminal. The regulator will think that the battery voltage is low and up the charge rate. In many vehicles there's a ballast resistor- it's there to lower the coil current once the engine has started. Only when starting does the coil get full 12v (minus losses in the feed wire), and so then the regulator gets to see the full battery voltage when the points are open, then only about 8 volts or so when the points are closed. It probably also gets to see an inductive kick from the coil every time the points open or close.

            Even if the battery is fully charged, the alternator is going to be pulsing it with full rated current every time a spark is initiated, although this action is moderated by the capacitor in the pin 2 circuit. I would not want this to be happening in my vehicle. If you're intent on wiring pin 2 to the coil, wire it instead to the 'hot' lead of the ballast resistor. The fluctuating charge action will still be there, but won't be as drastic. You still risk boiling the battery, but to a lesser extent.

            If the pin 2 current is responsible for the battery problem, I would rather see that circuit fed through a relay so it can be disconnected whenever the key is off. The typical bosch relay for vehicle systems would be fine.
            I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-