This is why folks say to check the light..... - Saab 900 Bulletin Board - Saabnet.com
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This is why folks say to check the light.....
Posted by adamK (more from adamK) on Fri, 9 Mar 2012 01:35:46
In Reply to: check alt. wiring, thresholdmusic, Thu, 8 Mar 2012 09:40:52
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The "field" trigger wire to the alternator is actually fed THROUGH the dash display light, and ends up as the lone small wire on the alternator. When you turn the key on, it puts 12V to that bulb, and since the alternator isn't spinning, the other end of that wire is 0v. The difference from 12v on one end, to 0v on the other end illuminates the bulb. When you start the car, you get 12v on the field wire of the alternator. Now you have 12v on one side of the bulb and 12v on the other side; a net 0v difference, and the bulb goes out. You'll hear of the display bulb starting to glow, or being faintly lit when an alternator starts to go (or voltage regulator most of the time). That's because the 12V on the alternator starts to drop down to 11v, 10v, 9v, and you still have 12V on the other side of the bulb, giving you a net difference of 1v, 2v, 3v, incrementally as the charging circuit starts to crap out, and those small voltages will begin to dimly light the bulb.
posted by 68.142.51...
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