Non Functioning Battery Light

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ptrmzr

Well-known member
Joined
Jan 2, 2012
Messages
53
Reaction score
1
Location
NJ
My Car
1971 Mach 1 351C 4V 4sp 3.50L
It's been a while, but on a previous car the battery light would come on as I turned the key to ON and would go off as the engine started and rev'd enough for the alternator to kick in. The light on my current '71 M code does not come on at all. I changed out the bulb some time ago and I'm just about to dig into this problem again. Before I pull out that awful wiring diagram has anyone diagnosed this before? Thank you in advance!!
Peter
 
Lots of reasons why. On a 71, the light compares the ignition RUN signal to the alternator stimulator line, which is ACC power which should cut off when the key is in the CRANK position. There's a 15 ohm resistor in the circuit to bypass the light to stimulate the alternator should the light bulb fail.

The attached PDF file explains how the alternator and the wiring works, and is a wonderful guide (not written by me1)
 

Attachments

  • How Alternators work2.pdf
    3.1 MB · Views: 1
Midlife, thank you for sharing that guide! That is the clearest description of alternator function I have ever seen! One thing that struck me was it indicated that cars with ammeters may not have alt/batt lights. My car has the gauges including an ammeter. Do you know if '71's with the factory ammeter had the light?

Thanks again!!!
 
Factory ammeter and alternator indicator lamps have incompatible wiring, so you can have one but not the other.

Yeah, that write-up is wonderful! I wish I wrote it...
 
Midlife, thank you for sharing that guide! That is the clearest description of alternator function I have ever seen! One thing that struck me was it indicated that cars with ammeters may not have alt/batt lights. My car has the gauges including an ammeter. Do you know if '71's with the factory ammeter had the light?

Thanks again!!!
Yeah, it feels like I do not say it enough, but we have some great folks in this forum who help provide some solid info. I try to contribute info when I feel there is something worthwhile to put forth, and not (too) repetitive. Midlife has consistently been dead on in his suggestions and info. So, yeah, Midlife, thank you for being the person you are.
 
Yeah, it feels like I do not say it enough, but we have some great folks in this forum who help provide some solid info. I try to contribute info when I feel there is something worthwhile to put forth, and not (too) repetitive. Midlife has consistently been dead on in his suggestions and info. So, yeah, Midlife, thank you for being the person you are.
Can I get you to speak to my ex-Wife? Maybe she'd listen to you...
 
Can I get you to speak to my ex-Wife? Maybe she'd listen to you...
Sorry, MidLife, it sounds like that horse is already out of the barn. Then again, if she is an ex because of a misunderstanding, and you would truly like her back in your life, then yes, I will gladly speak with her. But, having two ex-wives myself I may not be the right guy for the task. In my case, I got lucky on wife #3, 24 years married and counting.

I got triple lucky with Lynda, and literally owe her my life as she is the one that kept me alive following multiple life saving operations about 8 years ago when I had a very serious bout with brain cancer. I was fortunate to have found the otherwise deadly, malignant tumor both by accident, and very early. Normally, for the type of tumor I had, once symptoms appear the victim has less than a year of life left. Anyway, for me the 3rd time married was the charm. It would have been nice if I were to have been able to avoid my first two wives. The only good thing from round one is having two great, well balanced kids.

Anyway, re: me speaking with your ex, if you dare take a chance on me doing any good, well, let me know. YMMV... What is the worst I can do? heh heh...
 
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