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Ignition Resistor

R

Robert in Md

Member
Just got a junk Super A that had been converted to 12 volt. Resistor wasn,t any good so we started the tractor without it. It started but I could tell when I checked for plug fire by moving the points that there was to much current. Purchased new resistor and checked the voltage coming out of the resistor--12 volts. My question is if I still get 12 volts out of the resistor, What does it do?/? My son and I just assumed the resistor lowered the voltage to 6 volts... Please Explain
 
(quoted from post at 16:44:42 07/25/20) Just got a junk Super A that had been converted to 12 volt. Resistor wasn,t any good so we started the tractor without it. It started but I could tell when I checked for plug fire by moving the points that there was to much current. Purchased new resistor and checked the voltage coming out of the resistor--12 volts. My question is if I still get 12 volts out of the resistor, What does it do?/? My son and I just assumed the resistor lowered the voltage to 6 volts... Please Explain
Click to expand...
ou only get a voltage drop across a resistor when current is flowing thru it. no current=no drop.
 
The resistor limits the current flow to the points. If you put your meter across the resistor you should a reading. That is the am0unt the voltage is reduced/
 
This subject seems to come up on a weekly basis on this sight, but you don't learn until you are told or experience it, so that's a good question.
The voltage gets dropped as current flows through the resister. When there is no current and you just measure the voltage, you'll have 12 volts on both sides of your resister.
You need to measure the voltage on the points-side of the resister to get the accurate measurement of voltage when the engine is running.
 
My best stab at the question: If the points are closed, the distributor side of the coil will measure Zero volts, because it is grounded. If you measure (the voltage at the resistor side of the coil, engine off, points closed, ignition on) it will be around 6v. the resistor has ~2 ohms, the coil has about 2, and that series resistance. lets about 3 to 4 amps flow through. resistors do very little resisting when not loaded. Jim
 
Thanks for the replies. Most of my tractors still operate on 6 volts, My A , SA,and H have started just fine for the last 70 years on 6 volts.
 
Jim got it, the points must be closed.

Check with ignition on, engine off, points closed.

Should get 0 volts on the - coil terminal, around 6 volts on the + coil terminal.

Take your measurement quickly so not to overheat the coil or points.
 
(quoted from post at 21:05:35 07/25/20) Jim got it, the points must be closed.

Check with ignition on, engine off, points closed.

Should get 0 volts on the - coil terminal, around 6 volts on the + coil terminal.

Take your measurement quickly so not to overheat the coil or points.
Click to expand...
guess?, but he took a lot more words to say what I said right from the get go.
 
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