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Technical Ballast resistor replacement for flathead

Discussion in 'The Hokey Ass Message Board' started by wex65, May 14, 2014.

  1. wex65
    Joined: Dec 19, 2012
    Posts: 1,118

    wex65
    Member
    from WV

    Trying to get ready for the Jalopy Showdown...whilst moving the coil/ballast resistor on my '37 flathead (12v system) from on top of the engine to behind the firewall I rather stupidly managed to step on the resistor, breaking the ceramic portion.

    I measured it to try to track down a replacement and it measures 2.3-2.4 ohms. Not sure if I am missing anything but everything I see online is like 0.5 ohms to about 1.5 ohms...nothing above 2.

    Can anyone point me in the right direction or explain what I am measuring wrong?
     
  2. wex65
    Joined: Dec 19, 2012
    Posts: 1,118

    wex65
    Member
    from WV

    OK, researching this a little more it appears that the ballast resistor and coil combined "typically" add up to about 3 ohms. My coil measures out to 1.9 ohms on the primary meaning I need just about 1 ohm on the ballast resistor? Why I have a 2.3 ohm resistor is beyond me. The car did start fine with it though.

    Using the the formula (V = I x R) I seem remember from my distant memory it would appear then that an input voltage of 12v (I realize it is higher once the engine is running) would give 4 amps (12v = 4 amps x 3 ohms).

    If the engine is running and the 12v is more like 14.7v then this would move up to 4.9 amps (14v = 4.9 amps x 3 ohms).

    Is any of this remotely correct or am I barking mad.

    I had always read that the primary purpose of the ballast resistor was to lower the voltage but it appears in fact it is to lower the current being delivered and that the voltage reduction is a side product.

    If someone knowledgeable in these matters could stop laughing long enough to steer me right I would appreciate it. Just trying to get the right ballast resistor so I can get to the showdown.
     
    Last edited: May 14, 2014
  3. Grahamsc
    Joined: May 13, 2014
    Posts: 466

    Grahamsc
    Member
    from Colorado

    AC Delco part NO. 88951182
    This is a large ceramic encased resistor that pontiac used in their fuel pump circut on super charged GPs in the late 90s
    As I remember it is 1 ohm
    We used one in conjunction with our coil on our T speedster.
     
    Last edited: May 14, 2014
  4. old tom
    Joined: Jan 4, 2011
    Posts: 11

    old tom
    Member
    from il

    post on the early ford forum they willhave your answer
     

  5. flthd31
    Joined: Aug 5, 2007
    Posts: 584

    flthd31
    Member

    Here's a link to a thread that you may have already found while researching. Might help others reading this thread.
    http://www.jalopyjournal.com/forum/threads/ignition-coil-drama-from-bubbas.601339/

    I can tell you what I've been using for almost 15 years and have never had to service the dual points (8BA with Mallory dist).
    Mallory Coil #29217 (1.4 ohm)
    Mallory Ballast #700 (.75 ohms cold to 1.5 ohms cold)
    Mallory says that this coil can function without ballast, but I used the total 3 ohm rule that you referred to earlier. So I've used this variable resistance ballast from day one and have had no problems and great point life for many years.
    These Mallory parts were made in USA when I bought them years ago...not sure if there are the same quality today.
     
  6. Bruce Lancaster
    Joined: Oct 9, 2001
    Posts: 21,681

    Bruce Lancaster
    Member Emeritus

    NAPA Echlin ICR-40, FD-220 Standard, if anyone has them in stock.
    Echlin catalog lays .35 ohm...only lists resistance of a few of their devices, closest I see in that catalog is .56 ICR 23, Chrysler '55-72. Maybe just phone Joblot in NYC and beg them to toss one in express delivery realdamnfast. Where are you located?
    Never mind...I see your location, nowhere near here.
     
  7. wex65
    Joined: Dec 19, 2012
    Posts: 1,118

    wex65
    Member
    from WV

    Thanks guys, went to NAPA (no comment...) and managed to get a suitable item that will get me through the weekend at least.

    Has a 0.8 ohm resistance which when added to the 1.9 on my coil totals 2.7 which should be 'OK' till I get back from the Showdown and can get the right piece.

    Thanks for the part numbers and links, all useful and will be helpful in getting it right!
     
  8. Saxon
    Joined: Aug 9, 2008
    Posts: 2,157

    Saxon
    Member
    from MN

    You didn't mention your dizzy setup? Is it stock or electric. Then which coil? 12v to 6v on a stock setup you could run a 50s yblock resistor. Knocks it down to a 6 volt coil. Granted your best setup or a flathead stock setup is 3-4 volts. But whom knows what your ignition is....
     
  9. wex65
    Joined: Dec 19, 2012
    Posts: 1,118

    wex65
    Member
    from WV

    OK, so I finally did this morning what I SHOULD have done before opening the thread. I checked the distributor per Saxon's suggestion....

    I have a Mallory 5072101 (http://www.speedwaymotors.com/Mallo...thead-3-Bolt-Electronic-Distributor,7964.html)

    I was a little confused as it states that there is a need for a total 1.4Ω resistance.

    "...This distributor requires 1.4 ohms of coil resistance. If you are using a coil with less than 1.4 ohms, you must us a ballast resistor that will achieve the required 1.4 ohm total resistance. If you are using coil # 447-45001, no resistor will be required."

    My coil is 1.9Ω technically meaning I should need NO resistor, surely? Instead the car had a 2.3Ω resistor, making a total 4.2Ω...

    So it would appear that instead of trying to replicate what I had (I assumed it was correct) I am now replacing the coil with a 1.4Ω item and using NO ballast resistor.

    I am guessing the 4.2Ω I had meant I had a weak spark?

    New coil arrives tomorrow in time for the weekend.

    Lesson for the day...don't assume what you have is correct....
     
  10. Automotive Stud
    Joined: Sep 26, 2004
    Posts: 4,304

    Automotive Stud
    Member

    Good info and thanks for posting your results. I wonder if running too much resistance for too long could /would cause early failure of the module in the distributor?

    Posted using the Full Custom H.A.M.B. App!
     
  11. wex65
    Joined: Dec 19, 2012
    Posts: 1,118

    wex65
    Member
    from WV

    My guess is that too much resistance means a weak spark, low current etc. I am no expert on these matters so will bow to others.

    A slight update to ensure future visitors get the whole story. Mallory paperwork that came with the coil explains that there is no need to run a ballast resistor as the coil meets the 1.4Ω requirement BUT that is for performance driving and for street driving they DO recommend a 0.7Ω resistor which I added. I suspect the former configuration gives a slightly stronger spark but a shorter lifetime whereas the latter WITH the resistor provides a spark that is fine for street AND a longer lifespan.

    With the 1.4Ω coil and 0.7Ω resistor the car runs really well, fires instantly and has no issues.
     

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