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Subject: 841 Powermaster Electrical Help

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jrobin1    Posted 08-30-2022 at 13:31:26 [URL] [DELETE]        [Reply] [Email]  
  • 841 Powermaster Electrical Help
  • My 841 is converted to 12v. I used the JMOR 601/801 diagram I found on YT to do some rewiring from the original conversion, which I did not do. I have an external resistor and a 12v coil. When I switch on power, I thought I should get 12v at entry to resistor and around 7-8 on output side. When I check it there is 12v+ at every checkpoint all the way to the distributor. I put a new resistor in the circuit, and it checked the same. Both resistors test at about 3 ohms. And I have used the original one for years. Does the voltage only drop when the engine is running or am I missing something obvious.

    jrobin1    Posted 08-31-2022 at 09:42:58 [URL] [DELETE]        [Reply] [No Email]  
  • Re: 841 Powermaster Electrical Help
  • Thanks for the advice, guys. I think I will get the internally resisted coil and eliminate the external resistor.

    Bob in Ks    Posted 08-31-2022 at 09:02:26 [URL] [DELETE]        [Reply] [Email]  
  • Re: 841 Powermaster Electrical Help
  • "When I check it there is 12v+ at every checkpoint all the way to the distributor."

    Both your "12 volt, external resistor required" coil and the resistor should measure about 1 1/2 ohms. A 3 ohm resistor is too much and will likely cause harder starting and weaker spark problems.

    When you check those low resistances, short the 2 ohm meter probes together and note the resistance. Then measure the resistor and subtract the shorted probe resistance. That should give you the resistance of the resistor.

    With points open you should read near battery voltage all the way to the input of the distributor.

    With points closed you should read near battery voltage at input to resistor, about half that voltage between resistor and input to coil, and zero volts at side of resistor.

    Thus your measurements indicates the points are not closed with good contact when measurements were taken.

    The quality type of your meter can have an effect when trying to measure operating voltages.

    I would agree with Bruce's suggestion to get a direct 12 volt, "no external resistor required" coil (approx. 3 ohms across primary posts)and eliminate the resistor.

    Bruce(VA)    Posted 08-30-2022 at 15:46:26 [URL] [DELETE]        [Reply] [Email]  
  • Re: 841 Powermaster Electrical Help
  • Why are you using any external resistor w/ a 12v round coil?

    You will see battery voltage with the points open, 9v or so, points closed.

    jrobin1    Posted 08-30-2022 at 16:51:44 [URL] [DELETE]        [Reply] [Email]  
  • Re: 841 Powermaster Electrical Help
  • I am using the resistor because it says to use with external resistor. There is no internal resistor. I swapped out the old resistor with a new one I had on hand. Got the tractor started and while running it still has 12v at coil input side. So is that normal, or should I have 7-9 volts at the coil input side?

    Bruce(VA)    Posted 08-31-2022 at 06:43:55 [URL] [DELETE]        [Reply] [Email]  
  • Re: 841 Powermaster Electrical Help
  • No modern automotive coil has an “internal resistor”. Internal coil resistance is determined by wire gauge and the number of turns. Higher resistance is provided by smaller gauge wire with greater turns. If the coil is marked or sold with a notice “requires an external resistor” that simply means it has a low internal resistance and will be damaged by excessive voltage. It does not mean the coil has an internal resistor. Yes, plenty of coils are advertised as having an internal resistor. That’s called advertising, not electrical engineering. To be both accurate and precise, every automotive coil should simply state the internal resistance of the coil. But that would require a basic understanding of the Kettering Ignition system. So to keep it simple, ANY side coil with an internal resistance of 3.25 ohms will work fine on your 12 volt tractor w/o any external resistor. Like the NAPA coil, p/n IC14SB. Many folks just add a 12 volt coil rather than dealing with trying to find an external resistor to work with their existing 6v coil. Too much resistance gets you a weak spark; not enough and the coil melts. So, see if it's got 3.25 ohms or more. If not, you have a 6v coil. You're not going to get an accurate voltage reading w/ the tractor running. Measure the voltage points open, points closed, engine off.

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