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Ballast Resistor test?  Rating:  Rating
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 Posted: 10-11-2012 05:41 am
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answerman
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Joined: 09-10-2012
Location: Little Chute, Wisconsin USA
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Hello again.  Tonight, I decided, since I had an hour to kill, to try to figure out why Ms. Jenavieve's tach hasn't worked since I got her.  So, armed with the foldout wiring diagram from the owner's handbook, I started poking around (yes, I was finally introduced to the Lotus position).  I pulled the tach out and did some voltage testing on the wires... and they mostly seemed to test the way I expected them to (white and green were each 12v when the ignition switch was turned on, and the body of the tach was definitely grounded).

However, the white/slate (the one that runs from the tach to the ballast resistor) was only giving me about 0.7v.   From what I've been reading, that should be more like 9 to 10v.  Granted, I didn't have the engine running when I ran the tests, so maybe I am misunderstanding how the connection from the solenoid fits into the whole equation, but it appeared to me that I should have gotten some sort of backfeed through the ballast resistor to the white/slate wire connector.  Or is that connection only "hot" when the solenoid is actually engaged (i.e. during starting)?   Now that I think about it, that would make more sense...

Anyway, my question: what should the resistance of the ballast resistor be?  I haven't found any reference to it anywhere.  Should be easy enough to test if I knew what the resistance should be, and just trying to narrow down possibilities before I assume it's a faulty tach.

I had previously attempted a test by pulling the white/slate wire off the ballast resistor and trying to start her... and she would cough once and die, so I assume the resistor is doing "something".  Just not sure it's doing what it's supposed to.

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 Posted: 10-14-2012 10:11 pm
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h2oman
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Joined: 07-06-2010
Location: Mariposa, California USA
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I went out and measured my ballast resistor and it was about 2 ohms resistance with nothing connected to it.

The ballast resistor is in the loop to reduce current to the coil while the engine is running. While starting, the starter solenoid provides direct 12v to the coil with no resistance since voltage is lower while the battery is taxed with turning the loaded starter motor.

Hope this helps.

Bob Waterman

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