Classic Mustangs (Tech) Technical discussions about the Mustangs of yester-year.

electrical gremlin from hell

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Old Dec 3, 2009 | 07:16 PM
  #41  
lefty3382's Avatar
lefty3382
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From: Portland, OR
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I found a loose ground on the passenger side headlight circuitry. I will fix it in the next day or so. Plenty of time before my new alternator arrives next week.
Old Dec 3, 2009 | 10:12 PM
  #42  
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dirtydave289
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From: Newnan, GA
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high resistance in your alt wire could cause parasitic draw..
you checked your starter wiring also in the beginning right?

Also, someone a page or so back was saying a capacitor in your amp could cause this. How about the capacitor in the alt? Could it cause this?

IIRC something like 20-40 milliamps is acceptable battery drain with key off
Old Dec 10, 2009 | 05:38 PM
  #43  
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I wonder if one of the diodes in the rectifier shorted and is allowing current back into the alternator? It would essentially make the windings in the alternator a heater and create a constant draw on the battery when it is not running. Depending on the type of ohlm meter you have it wouldn't be had to test from the alternator out put lead to ground.
Old Dec 15, 2009 | 08:58 PM
  #44  
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lefty3382
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UPDATE

So I finally got the new alternator installed along with securing the loose ground wire in the headlight circuit and rearranging the wiring to the solenoid. I had to make some modifications for mounting the alternator since it is a "GM style case with ford mounting" according to summit. Its a Tuff Stuff 140 amp 1-wire unit, the lower alternator bracket isn't long enough and neither is the belt to install the alt comfortably. I jury rigged a striker plate for a door frame to extend the bracket, a temporary solution until I get a new bracket. I'm actually pretty proud how well I got it to fit. See pic below.

Anyway, I hooked up the alternator and checked for a draw coming off the battery with the car off. Nothing. The test light didn't come on. Yay! So that's a good sign. Started her up, and hooked up my new multimeter to test the voltage. I only checked the voltage at idle because at the time I had the wrong belt installed and was loose and squealing something awful when I revved it. At idle the voltage at the battery was a steady 13.4-13.5 volts. With the headlights on the voltage dropped to a flat 13. I'll test at higher rpm's tomorrow with the proper belt. Are those voltage readings normal? Isn't it supposed to be more like 14v? At least the reading was steady and not jumping around like it was before.


Old Dec 15, 2009 | 11:15 PM
  #45  
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From: Hills of California
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Those are good voltage numbers for idle...and props for getting a volt meter

at 1200 rpm and above volts should be around 13.5 to 14 and shouldn't drop off much with the lights on. By much I mean for example 13.5 vs 13.4

-Gun
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