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Old 01-10-2020, 01:54 PM
dcfirebottle dcfirebottle is offline
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Default Alt issue on a first gen

I'm running a mild build 400 in my 68, low miles. Switched over to the one wire internal regulated alt about ten years ago with no problems. Got an Autolight on it, also with ten years of use.
Gen light comes on a few days ago. Guage showing around twelve. I figure it's the alt but it tested out good. Put it back on and light is off. I check wiring and grounds just to be sure. Next day it's on again. Took it down again for another test. Still test good. Now I really start checking car closely. Harness' are all new. Can't find any nicks or anything. The only ground i can think of in the system to check and clean, is the battery ground and that's all good. Alt is either grounded through the brackets or through wiring I figure, not sure though. VR is inside alt, so that's checked at shop. Or is it? Are the diodes checked as well? I know the horn relay is in the loop but i figure if that's bad the car won't even run. What else could it be? I'm thinking maybe an alt issue that comes and goes, kind of like a starter can do. Probably just get another one anyway and see what happens. Bugs like these can make you feel like a total amateur and I've owned f bodies for thirty years. All comments welcome

  #2  
Old 01-10-2020, 02:20 PM
leeklm's Avatar
leeklm leeklm is offline
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I had very strange issues on an A body. Turned out that the main engine harness (2 yr old from M&H) had a cold solder joint where the fuseable link connects within the harness that let loose. It was a bear to trouble shoot!!

Are you getting 14v across the battery when running? If yes, that would tell you there is an issue with the light and/or connections as opposed to the charging system itself.

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Old 01-10-2020, 02:31 PM
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george kujanski george kujanski is online now
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Most likely a bad diode trio in the alt, especially if the bat voltage is "normal", about 14.0. The light goes on when the system voltage (battery, dash harness) voltage is higher than the terminal 1 voltage (idiot lite and excite terminal) on the alt. terminal 1 is driven by the internal diode trio, which normally is at the same voltage as the output. If the trio is bad, term 1 voltage is lower than the output (system voltage) and is enuf to light the light.

george

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  #4  
Old 01-12-2020, 03:43 PM
dcfirebottle dcfirebottle is offline
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Bought another alternator yesterday, put it on and it's charging at a little over 14 again and the light is out. I had the old one tested twice and both times the machine said."pass". (i was watching). Kind of makes you wonder. Appreciate the comments though.

  #5  
Old 01-12-2020, 05:06 PM
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bill ryder bill ryder is offline
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As George said," diode trio bad". Very inexpensive.

"Bill"!

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