Brushes are shorted to ground.

Electrical and Ignition

  1. 89on35s

    89on35s Inferno Red Duster FABO Gold Member

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    I assume if my brushes show continuity to the alternator case and body ground that my alternator would not charge properly? I was driving it on Friday and the voltage regulator connector slipped off and it was charging 16.5 to 17 volts for a few mi tues. I fixed that and ensured the connector would not fall off again, then it was charging at 14 volts as it should. I took it out today and it started charging as usual then after a while it was showing 11.5 v with the A.C. on and about 12 v with the A.C. off. It' a 74 Duster with the stock square back alternator and an adjustable voltage regulator.
    Am I on the right track with my thinking?
    Cley
     
  2. 89on35s

    89on35s Inferno Red Duster FABO Gold Member

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    I answered my own question.
    Cley
     
  3. Tooljunkie

    Tooljunkie King of cobble/master of the broken bolt FABO Gold Member

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    Brushes are not grounded. Only through regulator. Battery on one brush and ground through regulator is the circuit.
     
  4. 89on35s

    89on35s Inferno Red Duster FABO Gold Member

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    So if they show continuity to ground with the VR unplugged then the alternator is toast? That's kinda what I figured. Stupid reman
    alternators! Where does a guy get a decent alternator in Canada?
    Cley
     
  5. 67Dart273

    67Dart273 Well-Known Member

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    I don't understand how it could charge if the VR connector "fell off"

    The alternator may not be "toast" depending on whether you can get brushes and the insulation hardware. My last foray into NAPA did not end well. I ended up ordering 3 sets of brushes over 3 days, and at least two of the three had different hardware!!! I finally mixed and matched the 3 kits to produce 1 usable set, threw the rest of the parts in the 3rd box, and took it back and told them to turn it in as "defective." He had NO IDEA WTH that meant!!!

    But yes, the normal condition is NO continuity to ground from either brush, in other words "infinity." If you can't figure it out, remove the brushes one at a time and re-check, then LEAVE them out and check from ground directly to the slip rings. It is possible for the rotor to short
     
  6. 89on35s

    89on35s Inferno Red Duster FABO Gold Member

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    Well I picked up a new reman alternator and all is good. I now have 2 cores to play with and learn on. I will have at least 1 good core when I am done! I think the reason it was charging at 16+ with that plug off was that it was not all the way off. I think the voltage reference wire was disconnected but the other wire was making a connection. The factory tin retaining clip is missing, so I ty-wrapped it in place.

    Cley
     
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    • 67Dart273

      67Dart273 Well-Known Member

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      ^^I see^^ "Bebuilts" by the way have a long sordid history. In 1974 I went to work in a parts store, and they were crap then
       
    • 89on35s

      89on35s Inferno Red Duster FABO Gold Member

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      Yeah, that's why I am going to learn how to do it myself. Now I need to find a good source of quality rebuild parts.

      Cley
       
    • skykeith

      skykeith Well-Known Member

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      Download one of the factory manuals and it will have a section on rebuilding alternators. I rebuild about 2 dozen alternators a year. Half for airplanes and half for cars. I've bought a bunch of alternators from the Pull-A-Part and stripped them for parts, mainly the diodes. The airplane alternators require FAA approved parts and I get the bearings from Aircraft Spruce and Specialty (p/n 07-01829 and 07-01830) They cost less than $30 for both of them. I use them in all the alternators I rebuild. I'm not putting $8 dollars worth of Ching Chang bearings in anything I rebuild. As far as the brush sets go, 67Dart273 is right, buying brushes is a crapshoot. I have ordered a dozen at a time and mixed and matched them until I got the right retainers,hardware, and even brush lengths. If you rebuild one yourself, you know what you've got and not one that's been washed and sold as rebuilt.
       
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      • BillGrissom

        BillGrissom Well-Known Member

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        When you state "continuity to ground", please state the readings. If >100 ohms measured, I wouldn't worry much since that could just be carbon dust and such. A true short would be <10 ohm. It could have been in the insulators which secure each brush to the aluminum case. For those with the earlier boxy mechanical Vreg (~1971 and earlier), one brush should be shorted to ground. Indeed, there is only one field wire from Vreg to alternator, so ignore the diagnostics discussed here.
         
      • 89on35s

        89on35s Inferno Red Duster FABO Gold Member

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        It read 0 ohms to ground with my good Fluke multimeter.

        Cley
         
      • AJ/FormS

        AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s FABO Gold Member

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        I have seen quite a few shorted armatures in 45 years.
        I think more so in old-style starters tho.
        meh, IDK
         
      • KitCarlson

        KitCarlson Well-Known Member

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        The brushes go to rotor. The rotor has a spool type winding connected to slip rings. The brushes ride on slip rings. By removing the brush carriers, it should be possible to reach meter probes to the slip rings and check continuity to pulley. The rotor winding is only a few Ohms, so what you measure on each should be similar. The winding should isolated. If the winding is isolated, then the short is a problem in brush carrier insulaton, missing insulator washer..... or other problem. If the rotor winding has short to shaft, it is not easily fixed, but could be changed out.
         
      • 89on35s

        89on35s Inferno Red Duster FABO Gold Member

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        I have three to take apart and figure out. I hope to make 2 good ones at least. Thanks for all the insight.

        Cley
         
      • 67Dart273

        67Dart273 Well-Known Member

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        Make sure two things (or three or four)

        1...That you do not have an early (69/ earlier) which DOES have one grounded brush

        2...That the alternator is not some "rebuilt" which has had a grounded brush installed, or the insulating washers lift out or broken

        3...With both brushes removed, check from slip rings to the alternator case, it should be OPEN (infinity)
         
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