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Pinbill

Voltage/ Resistance for Coil and Ballast Resistor 1971 240z

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    Hello,

    I would like make sure I am getting the proper voltage to my ignition system. I have the stock set up with the two post ballast resistor. Here are my measurements:

    Ballast Resistor: 1.6 ohms

    Ballast Resistor: 10.9v going in an 5.1v going out

    Coil: Primary 1.7 ohms  Secondary 8.95 ohms

    Coil (Car Running): 7.6v on the positive tab and 3.8v on the negative.

     

    Can someone confirm what voltage/ resistance I should be getting to the resistor, from the resistor, to the coil, and from the coil? 

     

    Thanks for the help,

    Bill

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    From the 72 FSM, but I don't think it's different from the 71:

    image.png

    I hope to assume that the actual reading you had on the secondary was 8.95K Ohms. Based upon that assumption, if you have everything wired correctly, you should have no problems. What is prompting your question?

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    Hey Steve,

    Thanks for the response. My car will idle, but it runs rough. I hit the throttle and it won't rev. It just bogs. I have been through the carb and vacuum system. Hoping to do some tests before  I throw parts at the ignition system.

    On the coil I got a resistance value of 8.95 from the contact to the distributor to the positive and 8.95 ohms from the contact to the distributor to the negative leads. Judging by the specs it looks like my coil is out of whack.

    Bill

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    Posted (edited)

    Should I be getting 12v from the tach to the ballast resistor or is 10.9v ok?

    Edited by Pinbill

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    Look at your wiring diagram. The feed to the Tach goes through the ballast resistor. The path should be: Ignition switch (BW) -> ballast (GW) -> Tach (BW) -> coil. 

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    Ok, got it, thanks. 

    Is 10.9v from the ignition switch to the ballast ok?

    The ballast is putting out 5.1v. Is that ok?

    I just checked the resistance on the ballast again and it is ranging 1.6-1.9 ohms. Spec is 1.6 ohms.

     

     

     

     

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    Well with the numbers you supplied*, everything looks fine to me.

    First off, you need to understand that the numbers you supplied were with the points closed. I don't know if you did that on purpose or not, but that's where you engine ended up the last time you shut it off. If the points were open, pretty much everything you measured would be full battery voltage instead of the readings you got.

    But with the points closed, here's some stuff you can infer from the readings you took:

    1) You're pulling 3.625 Amps through your ballast resistor and into the coil primary. Seems completely reasonable to me.

    2) (Assuming a loaded battery voltage of 12.5 Volts) You're losing about 1.6V worth of battery voltage in your ignition switch and the associated wiring. That means the total resistance of everything involved is just under a half an Ohm. That seems a little high to me, but for fifty year old wiring and maybe an original switch, it's probably expected. It won't prevent the car from running, but might be time to clean some connectors and replace the ignition switch.

    3) You're dropping about half your ignition voltage across the ballast and about half across the coil. Based on the resistances you measured, that makes perfect sense. A little off, but perfect for a reality check.

    Putting all that info on the circuit diagram, it looks like this:
    pointsignition3.jpg

    As for the numbers on the coil with the engine running? You're trying to take a DC measurement on something that is not DC and you need to be careful about how you interpret those numbers. Assuming you're using some new fancy fangled meter, it's probably providing some sort of "averaging of the signal" it's trying to read. If that's the case, then those number look OK too. You're using a DC scale to average a pulse width modulated waveform, and the "averages" look reasonable.

    * Except the coil secondary. I'm assuming you're reading the scale incorrectly and (as SteveJ mentioned earlier) it's actually 8.95K Ohms, not 8.95 Ohms. I think your meter is auto-ranging and you're not seeing the tiny "K" somewhere on the scale.

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    Wow. Thanks for the detailed schematic. It is a skill trying to communicate technical information typing through a forum. I really appreciate you taking the time to provide detailed information.

    I can't measure the coil with it running. My meter has an auto range for continuity.

    I ordered a new coil and ballast resistor today and threw in a coolant overflow bottle while I was at it. I don't mind replacing 50yr old parts even if the don't fix it. LOL.

    Bill

     

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