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  #16 (permalink)  
Old 06-30-2010, 12:36 PM
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i wanted to use the idiot light and add the diode to the ignition in side of the light,,, is that correct

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  #17 (permalink)  
Old 06-30-2010, 02:16 PM
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Quote:
Originally Posted by zephyrc
i wanted to use the idiot light and add the diode to the ignition in side of the light,,, is that correct
Don't use both the diode and the idiot light, use one or the other.
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Old 06-30-2010, 02:57 PM
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cobalt,, why can i not use them both
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Old 06-30-2010, 03:13 PM
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Quote:
Originally Posted by zephyrc
cobalt,, why can i not use them both
It seems to me that if the diode is on the same wire that the idiot light is on, that since the diode only allows current to pass in one direction there'd be a conflict. I could be wrong.
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  #20 (permalink)  
Old 06-30-2010, 04:27 PM
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Quote:
Originally Posted by cobalt327
It seems to me that if the diode is on the same wire that the idiot light is on, that since the diode only allows current to pass in one direction there'd be a conflict. I could be wrong.
The diode should be between the idiot light and the alternator terminal 1 I believe.
There is an article on the MSD website I believe about this
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Old 06-30-2010, 04:46 PM
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Quote:
Originally Posted by T-bucket23
The diode should be between the idiot light and the alternator terminal 1 I believe.
There is an article on the MSD website I believe about this
I was basing my "recommendation" on the idea that the diode would prevent the light from "seeing" a grounded condition, should the alternator fail. I know the diode will prevent current to flow from the alternator to the light- but since the light is activated by a ground, I guess the diode doesn't prevent the light from "seeing" a ground?

In any event be sure to orientate the diode correctly- band closest to the alternator.
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Old 06-30-2010, 05:34 PM
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Quote:
Originally Posted by cobalt327
I was basing my "recommendation" on the idea that the diode would prevent the light from "seeing" a grounded condition, should the alternator fail. I know the diode will prevent current to flow from the alternator to the light- but since the light is activated by a ground, I guess the diode doesn't prevent the light from "seeing" a ground?

In any event be sure to orientate the diode correctly- band closest to the alternator.
If the alternator grounded out the Idiot light would come on as the potential at the alternator would be the same "0" as when the car is not running providing a ground path for the light, I believe
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Old 07-01-2010, 09:29 AM
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As Cobalt327 has stated, use either the diode (installed correctly) or the idiot light. Do NOT use both.

You only need one or the other for it to work correctly.

If you were to use both with the diode and the bulb in series; when the ignition switch was turned to the ON position, current would flow from the ignition switch thru the bulb and thru the diode and continue on to the alternator and go to ground. This would enable the bulb (idiot light) to be illuminated.

Once the engine was started and the alternator was putting out current, the diode would prevent the backflow to the ignition switch (as desired) and the bulb would go out since that circuit is now interrupted.

The problem that I see is that when the engine has been shut off at the ignition switch, the diode will continue to block current backflow from the alternator, but since there no longer is current coming thru the ignition switch the bulb will also not be lit.

Perhaps I am over thinking this.

As I previously stated. I use and prefer an "idiot light". Using a diode instead of the idiot light will work provided that the diode is of the correct size and has been installed in the correct direction. Using a diode will give no indication if the alternator fails electrically or mechanically. There are other gauges that could show this condition, but an indicator light is very quickly observed over watching an individual gauge. Using a diode will only prevent the backflow of current when the ignition switch has been shut off and thus prevent a 'run-on' condition.
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Old 07-01-2010, 11:25 AM
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Quote:
Originally Posted by Frisco
Below is a photo of the wiring on the alternator on my old '30 Ford Coupe. It is wired the same as I posted above. It worked excellent. I used an idiot light. I have used this wiring method for Chevy alternators at least six times in the past with excellent results.
every hotrodder I've met has it hooked like you do and it's works great because the alternator is good and your wiring in the car is probably equally as good. However, the sense wire is ideal for telling the alternator to output more power on demand. Like, check the voltage at a brake like in the rear while everything else is on (fans, radio, ignition boxes, lights etc). Should the voltage all the way back there dip a little (and that dip can affect an msd box), the sense wire (hopefully connected far from the alternator) will let the alternator increase it's output to compensate.

I have mine connected to my power wire on my msd boxes which get their power from my fuse box. Lengthwise, the msd boxes are at least twice as far from alternator sense wire. If my msd power drops, the alternator should adjust for that. With the sense wire jumped I think the alternator output is just a function of rpm. The sense wire should increase output even at a steady rpm provided the alternator has a good amps output rating.
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Old 07-01-2010, 11:47 AM
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i thought an alternator had a diode internally that did exactly what i am needing to do,, surely the power only has to travel to earth at the alternator and the earth is cancelled when the alternator makes power, so power is not flowing both ways at the idiot light.. thanks guys for all the help and advice.. hoping to try the repair over the weekend
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  #26 (permalink)  
Old 07-02-2010, 08:29 AM
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Quote:
Originally Posted by zephyrc
i thought an alternator had a diode internally
An alternator produces Alternating Current (AC). There should be four diodes (there could be six) internally to convert the AC to Direct Current (DC). Those particular diodes are not in a circuit to prevent the 'run-on' you are experiencing. The indicator light (idiot light) is what the manufacturer designed in the system to prevent the run-on as well as to be a physical indicator of when the charging system was working. Replacing the indicator light with a separate diode will eliminate the run-on condition but will not show whether the charging system is functioning. The wiring for the alternator thru the diode or indicator light must go to a switched source to function correctly. Again, I suggest and recommend that source be the ignition terminal of the ignition switch.

Last edited by Frisco; 07-04-2010 at 09:20 AM.
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  #27 (permalink)  
Old 10-09-2010, 09:24 AM
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diode and idiot light

Use both. Or use a diode and a resistor.
If the idiot light goes to an IGN terminal (as it should), and your coil or HEI also goes there (typical), then you could have run-on without a diode in the idiot light line.
Buy the Speedway harness. Or I have just the diode -- see classified.
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