Assistance, please, in diagnosing charging system.

Early C Bodies - The Slab Side Years

  1. Crown of 66

    Crown of 66 New Member

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    Hey guys, can y'all please point me in the right direction on this? On my 66 Imperial, I am having an undercharging issue, with just under 12 volts at idle, just over 12 volts at high RPM. This measurement reads the same at the ALT battery post and directly at the battery. Below I've outlined the steps I've taken so far.

    I have a roundback alt with two fields. I think the original equipment was one field and grounded case, but understand the second "field" is really just a ground anyhow.

    Measured battery at rest with car off - got reading of 12.71

    Set up direct charging wire (10 AWG, with crimped 14 AWG fusible link) going from ALT BATT post to starter relay batt post to bypass potential charging issues at bulkhead and/or ammeter.

    Tested old style mechanical (black box with IGN and FLD) VR ground with test light - good. Removed VR and wire-wheeled VR bolthole to get shiny body metal and re-attached. Re-tested and still good.

    Verified continuity of green field wire from ALT to VR.

    Wired a jumper from ALT GND field directly to known good ground.

    Had alternator tested at AutoZone - they didn't give a reading, but it read PASS on two different tests. I know AutoZone is pretty cheesy and they have limited credibility, but they are literally a quarter mile away, so I used them for this stage.

    A FEW QUESTIONS:
    1. I thought the ALT FLD (green wire) should show hot or positive. Reading with my test light, though, it reads as a negative, as does the ALT GND field. What's up with that?

    2. What is an accurate way to test the VR?

    3. What are the next steps?

    Thank you guys for your help! Anxious to get this gal rolling again!
     
  2. Big_John

    Big_John Illegitimi non carborundum FCBO Gold Member

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    This is the best way I know of to check the charging system in a pre 1970 car.

    Disconnect the field wire at the alternator.

    Run a jumper wire from the field connection on the alternator to the 12 volt output on the alternator.

    Connect voltmeter to battery

    Turn on headlights (this creates a load)

    Start the car, and take a reading at the voltmeter at idle and one at around 1500-2000 RPM

    If the voltmeter reads 14+ volts, the alternator is good, but the voltage regulator (or wiring) is bad.

    If the voltmeter is still 12 volts, the problem is in the alternator.

    This duplicates the test procedure in the FSM, except you are using the headlights instead of a carbon pile to load the system.
     
    Last edited: Feb 9, 2019
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  3. Big_John

    Big_John Illegitimi non carborundum FCBO Gold Member

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  4. stubs300

    stubs300 Senior Member

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    It could also be as easy as having corroded or bad connections in the wiring. Clean, repair, and replace as needed. Good Luck
     
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  5. Jon O.

    Jon O. Well-Known Member

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    I can't offer much help in this field, but start with the basics. Make double sure that your battery posts and other exposed connections are clean. I once had trouble from that, and it was the last thing I checked.
     
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  6. Crown of 66

    Crown of 66 New Member

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    Thanks
    Thanks, John. I actually could NOT do the test you mentioned, because my ALT FLD reads grounded with a test light (which seemed wrong to me), so connecting it directly to the 12V output results in an exciting pyrotechnical display. The test you laid out for me confirms my thinking that the ALT FLD should not be grounded, so I am thinking something must be amiss in my alternator
     
  7. FURYGT

    FURYGT Senior Member FCBO Gold Member

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    Typically when a VR fails they either produce nothing or overcharge. You can always pull the cover off the VT and look inside for corrosion or burnt spots if it is a mechanical VR.

    Was your car converted to electronic ignition? If yes and if it has a mechanical VR that is another problem as electronic ignition systems and mechanical VR's do not work well together.

    Nonetheless, you have some good advice above and may be on to something.

    I think you just bought a VR from me - I will ship that out on Monday - Thanks!
     
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  8. Crown of 66

    Crown of 66 New Member

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    Yup, that was me that just bought that VR. I do see a few spots on the back of the VR that look burnt (see attached photo). I can't figure out how that might make the ALT FLD show as grounded, which seems like it might be a second different issue. But in any case, I'll put on the new VR, might run an additional ground cable off of it to be on the safe side, and see what's what then. Thank you guys for your help on this!

    IMG_8168.JPG
     
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  9. Big_John

    Big_John Illegitimi non carborundum FCBO Gold Member

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    Yes, the test light would show it to be grounded.

    So we are on the same page....

    There is no such thing as a "two field" alternator. It's one field for both. There are alternators with a internally grounded field that have one field connection and there are alternators with an isolated field (mistakenly called two field).

    With the first type, 1969 and older, one end of the field is grounded internally and the other end has power applied to it from the voltage regulator.

    The other type , 1970 and newer, is almost identical except the one end is connected to 12 volts and the other end has ground applied to it. This is the one you are using from your description.

    Using the newer type, 1970 up, as you have in your '66, just means you attach one end of the field to ground externally rather than internally.

    So... Yes, you will be able to light a test light because one end of the field is grounded by your external connection. That's the way it's supposed to be.
     
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  10. Big_John

    Big_John Illegitimi non carborundum FCBO Gold Member

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    This guy does a good job explaining how it all works. The first two minutes are just taking it off the car and then it gets good.

     
  11. stubs300

    stubs300 Senior Member

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    Answer the question that's been asked, DO YOU HAVE ELECTRONIC IGNITION? That makes a big difference!
    I for one do not like those little black regulators, do a upgrade and forget about it!
    Read the notes!
    charging system overview
     
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  12. Trace 300 Hurst

    Trace 300 Hurst Professional Tinkerer FCBO Gold Member

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    This a very good thread, guys. :thankyou:
     
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  13. Crown of 66

    Crown of 66 New Member

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    I have old points on this car, newer style "isolated field" alternator (post-1970), and old style mechanical regulator. I just bought a replacement from FuryGT. No electronic ignition.

    John, I appreciate your help, but think I must still be misunderstanding. Are BOTH the FLD, and the GND terminals supposed to show as negative with test light? I know that ONE should, but both are grounded on my alternator. I understood the test you laid out above to be running a jumper from alternator's 12 V output (the bigger battery post) to one of the terminals, but since both of my terminals are grounded on my alternator, I believe that test would have the same results as running a jumper from the positive side of the battery to the negative side. I may be missing something; please help me understand and thanks again for your responses!
     
    Last edited: Feb 9, 2019
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  14. Crown of 66

    Crown of 66 New Member

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    Thanks, Stubs, that link you provided on the Imperial site matches a diagram I was looking at for the newer style alternators, which seems to indicate that one of the alternator field terminals should be positive and one negative. Both of mine read negative.
     
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  15. stubs300

    stubs300 Senior Member

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    Honestly, I don't think it matters which one is + or -, at least I didn't care when I did mine and it works great. However, I grabbed everything off of a 73 dart and converted mine. Now? I don't know how a newer alt. would get along with the old style Mopar system, [the little black box]? This could be part of your problem? I have never like those boxes, either the new transistorized ones or the older mechanical ones as I have always had problems with them.
     
    Last edited: Feb 9, 2019
  16. Big_John

    Big_John Illegitimi non carborundum FCBO Gold Member

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    Think of the field as one long wire... Really that's what it is.

    It goes from the terminal - brush - slip ring - wire around the rotor- back to the other slip ring-out the other brush-to the other terminal.
     
  17. Crown of 66

    Crown of 66 New Member

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    Hmmmm, I think that makes sense, John, with the wire concept. I think that one end, or terminal, is supposed to show positive, and the load is in the middle (brush, slip ring, wire), and the other end, or terminal, is negative, whether that is the case itself or an external terminal in my case, right? That seems to be what I'm understanding.

    But on my alternator, both terminal ends are negative, which seems to vary from the FSM diagram and simple diagrams I see online like the one below (I know the diagram below shows an electronic VR, whereas I have the old one).

    moparpost70.gif
     
  18. Big_John

    Big_John Illegitimi non carborundum FCBO Gold Member

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    Quit looking at that drawing. It's confusing you even more.

    Take a wire. Hook one end to ground. Understand? One end is grounded.

    Now hook the other end to your test light and it will show that it's grounded. Got that?

    Now take that wire and wrap it around some iron. One end is still grounded and the other end shows it's still connected to ground. Right?

    Now, leaving the one side grounded and hook 12 volts DC to the other end. You've just made an electromagnet.

    The field is a basically a spinning electromagnet.
     
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  19. Crown of 66

    Crown of 66 New Member

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    Thanks John, that all makes sense to me, but doesn't the iron (or field) that the wire is wrapped around have to provide some kind of load to draw current? Otherwise, the 12 volts DC goes straight to ground and creates a short, with sparks and excitement. That is exactly what happened when I tried this part of your test:

    Run a jumper wire from the field connection on the alternator to the 12 volt output on the alternator.


    I think maybe that result happened because I ran the fusible link-protected direct charging wire from the starter relay to the 12 volts DC. I might disconnect that and give your test another go.
     
  20. Big_John

    Big_John Illegitimi non carborundum FCBO Gold Member

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    The wire is so long and small that it doesn't affect it like a short wire would.

    There should not have been any sparks... You did do all the connections without the car running... Correct?