First of all, regardless if its a mechanical coil current switch (a set of points that open and close) or a solid state (transistor conducts or not) switch, the same coil that was previously used (provided its good and the right one and wired correct) should work just the same. HOWEVER often an elec switch has a higher current switching capacity then the old points (limited to not much over 4 amps) in which case a higher energy often less resistance coil can be used and many people do just that i.e. switch to a higher energy coil when they add an elec switch BUT ITS NOT NECESSARY AND THE OLD STOCK COIL SHOULD PERFORM THE SAME OR JUST A TAD BETTER (due to faster more positive switching characteristics)
NEXT as to whether you can use a 1.5 ohm or a 2 ohm or a 3 ohm coil, THE ANSWER DEPENDS ON THE CURRENT RATING OF THE NEW ELEC SWITCH. The coil MUST be suitable with the switch but often an elec switch has a greater then 4 amps (like old points) capacity meaning a 12 volt 2 ohm (draws 6 amps at 12 volts) coil "could" work HOWEVER not knowing the switches rating (and if no ballast resistor in the circuit) if I had to make a blind recommendation ID SUGGEST (if you stay stock) TO JUST GET A FULL TRUE 12 VOLT RATED COIL they will have from 2.5 to under 4 ohms "typical"
NOTE what I call a full true 12 volt coil is one designed to operate at 12 volts nominal WITH NO EXTERNAL BALLAST which will produce a sufficient spark as low as 10 volts when starting drags battery voltage down and still not overheat and work when the battery is being charged and at maybe as high as 14 volts. It may be labeled "12 volts" or "12 volts NOT for use with ballast resistor"
DOES THE TRACTOR HAVE A VOLTAGE DROPPING (12 to 6) BALLAST RESISTOR????? if so and you keep it in the circuit and then use a 12 volt (NOT intended for external ballast) coil THE SPARK WOULD BE WEAK !!!!!!!!
IF THE TRACTOR HAS A VOLTAGE DROPPING (12 to 6) BALLAST AND YOU KEEP AND USE IT, THEN YOU NEED A 6 VOLT COIL (labeled 6 volts or 12 volts for use with ballast) and they are typically 1.25 to 2 or so ohms.
Have you switched tractor polarity??????? I ask cuz the coil polarity should match the battery for best performance!!!!!! It can still spark at the wrong polarity but it will be weaker.
YOU GOTTA SEE IF IT HAS A BALLAST!!!!!!!!!!! if it were mine and I added an elec switch Id buy a new 12 volt coil (whose current draws is within switches ratings) and NOT use any ballast
Sure the elec switch is wired correct????
Sorry more questions then answers but we cant tell sittin here if you have a ballast or not and how things are wired and polarity etc. If the switch is at worng polarity it may be fried!!!!