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'74tii weird alternator "problem" ?? only red at 3200 rpm


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Here's a weird/unusual "problem" I've never seen before:  at  ~3200+/- rpm the red "alternator" light on the dash will flicker.  Below that rpm, no flicker.  Above that rpm, no flicker.  

     Light level goes from very faint to noticeable, not a constant level.  And sometimes it won't flicker as you accelerate/decelerate through that rpm, so it is not an "always" situation.  

     The light is never nice & bright which would indicate total failure to charge.  The light level is enough to be seen (or not seen).


HISTORY:  I installed a new alternator 12 months ago (FEB 2014).  Now only 3,000 miles on it.  Worked fine before all the engine work was done beginning in AUG 2014.


1)  I did notice it in December 2014 when I was putting 500 miles on the new engine.  Le checked it all out, couldn't find anything, his instruments said the charging system was A-OK.

    Last Saturday when I got the car back from Le, it was still doing it.

    Yesterday I removed the fan belt and sprayed it with "belt dressing."  It didn't exactly fix the problem, but the light didn't "light up" quite as brightly, but it is still there at that rpm, most of the time.


2)  An hour ago I installed a new voltage regulator:  no change.


3)  The fan belt certainly seems tight enough.  I was thinking there was some weird harmonic where the belt would slip at that certain rpm, but I don't think so.


4)  Is one of the brushes acting funny ??  Doesn't seem reasonable, but ...... 


SO, anyone ever seen anything like this ??


I'm thinking the only correct fix is a new alternator.





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Didja check the three prong plug on the alternator's backside?  It's supposed to be held in place with a wire bail, which is often missing.  There may be a vibration at 3200 rpm that's causing that plug (or possibly the plug at the voltage regulator) to loosen, thus causing the dim warning light on the dash.  


You might also doublecheck the ground wire that goes from the alternator's frame to the block.  Original ones can look OK but be badly corroded under the terminal insulators.  Again, sympathetic vibrations may be a cause.


cheers and happy troubleshooting



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So I head out this morning for a fun day .....



The red light is doing it's thing as expected and I'm in the land of denial to some extent.  After zooming through Carbon Canyon I notice the light is now occasionally flickering at 2000 rpm.  Uh Oh, I'm thinking imminent failure.

    I call Le and he says to drive to his shop.  I get there no problem and leave the car with him.


And I just-now learned that the problem is indeed the alternator, so it will be rebuilt.





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