resistor on coil ? yes no ? how can i tell

sharp farms

New User
I'm new to this page and tractor repairs in general my 706 coil gets red hot is it supposed to have a resistor ? How can I tell? its a 12 volt now I.m sure it was converted. I ordered a new coil with resistor and bought a universal resistor to use in the mean time . am I ok to run without one until new patr arrives
 
Your tractor will run OK with a non-resisted 12 volt coil for a while. The points will go bad very quickly. It should have a coil with an internal resistor or a separate resistor mounted before the coil. Good Luck.
 
Not really. It might just be a bad coil. they fail. and when they do they get hot. The excess current can ruin the points and
or burn up the internal resistor used (IIRC) by IH, which consists of a wire with resistance rather than a ceramic block. Using it
may damage the rest of the system. I would use only the coil from the source you purchased it from. Jim
 
thank you for the input , the one I bought says to use a resistor on 12volt system. so I should put the resistor inline between the coil and distributor ? The one on tractor now has no resistor but the mounting bracket is there for one? Sorry really confused as to how to decide if its required. I appreciate ant advice
 
706 should have a resistor bypass for starting.

Ideally you want a 6 volt coil or "12 volt external resistor required". Resistor goes between coil + and the switch, not between coil and side of distributor.

There should also be a wire from a small post on the start solenoid to the + post on the coil.

When solenoid kicks in it bypasses the resistor giving higher voltage and hotter spark. When solenoid kicks back out after starting the resistor is in circuit and limits current through the coil.
 
That makes sense Thanks I will add the resistor into circuit and see what happens . thanks everyone for the input. I will be back need to work on the shifting issue next
 
Since you say "it's 12V now, I'm sure it was converted", I would test for coil resistance (if you have a multimeter that will measure resistance) (ohms). Should be about 1.5 to 2.5 ohms if it was a true 6V coil. Then the 12V conversion should have the resistor. A true 12V coil will measure about 3.5 to 4 ohms if it's good and doesn't need the resistor.

Electrical guys, as John T says don't have a cow if you disagree with my resistance numbers. I've seen some fairly wide varying in those numbers depending on your multimeter and/or the coil be tested.
 
Good questions "I'm new to this page and tractor repairs in general my 706 coil gets red hot is it supposed to have a resistor ?

1) If it gets red hot and/or so hot you cant keep your hand on it THERES A PROBLEM such as:

a) If its a 12 volt tractor and it has a 6 volt coil with no series voltage dropping (12 down to 6) ballast resistor IT WILL OVFERHEAT BADLY AND THE POINTS BURN SOON. Perhaps the ballast is bad or is incorrectly shorted around in which cased a coil designed for 6 volts is seeing 12 constantly therefore overheating...??.

b) Perhaps its the right coil BUT ITS SIMPLY BAD MAYBE HAS AN INTERNAL SHORT causing it to overheat


"How can I tell? its a 12 volt now "

If you have a good ohm meter that's accurate at low resistances, a 6 volt coil will have a LV Primary resistance (between + and - terminals) of around 1.2 to 2 or so ohms, some average around 1.5. If it measure 2.5 up to near 4 ohms its a full true 12 volt coil no ballast used on a 12 volt tractor.

NOTE a 6 volt coil may be labeled "6 Volts" ALSO if a coil is labeled "12 Volts for use with Ballast" its in reality more like a 6 volt coil and on a 12 volt tractor needs that external series ballast just like it says or it will overheat. A full true 12 volt coil may be labeled "12 Volts" or "12 Volts not for use with ballast"

NOTE on a 12 volt tractor you can use EITHER a full true 12 volt coil orrrrrrrrrrr if you use a 6 volt coil you need an external series voltage dropping (12 to 6) ballast resistor

Hope this helps, post any questions

John T
 
I agree, as a used tractor dealer and past farmer I also saw a wide range of coil resistances

John T
 
All 706 tractors came from factory with 12 volt electrical system. Early ones had generator.
Later ones had alternator. All used coil that required external resistance.

Early ones had ballast resistor mounted right on coil bracket. Latest ones had a resistance wire
in the wiring harness AND DID NOT HAVE THE BALLAST RESISTOR MOUNTED BY COIL.

You might have alternator and still have the ballast resistor mounted by the coil or you could
have resistance wire in harness.

EASY WAY TO TELL THE DIFFERENCE. Look at the small wire terminal on the starter solenoid. If
that terminal has a double wire that tells you that your tractor has the resistance wire in
harness between ignition switch and the starter solenoid. Pay close attention to looking at that
feature.

You do not want to end up with a resistor at the coil and also have resistor wire in harness.

Also, the tractors with alternator had external voltage regulator that had a resistor mounted on
the regulator mount. That resistor may in some case look like a ignition resistor BUT, it is not.
It has a much higher resistance valve like 10 ohms or maybe even 25 ohms. A ignition resistor
will be very low like a previous poster already stated. Some where in the 2 to 3 ohm range.

You can always check voltage coming into coil but that is a whole different situation because you have to have current flowing through the coil and points or your test will be invalid.
 

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