Coil questions

Working on a Ford 901 series converted to 12 volt. It has a 6 volt coil. Resistance between + and - post is 1.5 ohm and from + or - to the tower if 9.09. I assume the coil is in need of replacement. Bought a new 6 volt coil @ Fleet Farm that has "Do not use with external resister" marked on it. Tractor wouldn"t start on it so I assume that although a resister is required to drop the 12 volts to 6, it does not allow the use of this coil. What"s the solution?

Thanks!

The difference is the resistor. If it is still in the circuit, the engine will probably not start. The coil you bought should be wired without the resistor. If you have removed it the coil might be bad. 9.09 ohms is small it might be in thousands of ohms rather than 9. Does a test light flash connected to the points when the engine is cranked? Jim

Freewheelin,Buy a Napa 12 volt coil, no resistor needed: part # IC14SB No resister needed for it and it's low priced to.Your coil problems will be overwith.Return the 6 volt coil if you can.

My MF 35 service manual says that points are designed to break 5 amps. Considering you are cranking, 12v/5 amps = 2.4 ohms total resistance from the voltage source through the ignition switch, starter switch, ignition coil primary circuit, ballast resistor, points, and distributor to the tractor frame where the battery (-) for negative ground systems is located. Less resistance and you burn up the points over time prematurely. More resistance and you don't have enough energy stored in the coil when the points break to fire the plug properly for combustion.

Voltage is just a mechanism to force the current through the circuit just like water pressure is the mechanism to force water through your water pipes. The resistor is just an orfice of sorts to regulate the flow.

The 6v coil you bought would work if it were still 6v.

Like Den said, get the 12v Napa coil, run it with no resistor. While you're at it, now would be a good time to do the electronic conversion, get rid of the point/condenser problems once and forever!

(quoted from post at 18:48:51 05/29/15) Working on a Ford 901 series converted to 12 volt. It has a 6 volt coil. Resistance between + and - post is 1.5 ohm and from + or - to the tower if 9.09. I assume the coil is in need of replacement. Bought a new 6 volt coil @ Fleet Farm that has "Do not use with external resister" marked on it. Tractor wouldn"t start on it so I assume that although a resister is required to drop the 12 volts to 6, it does not allow the use of this coil. What"s the solution?

Thanks!

Click to expand...

OK, you have a tractor concerted to 12 volt with a round can coil. Why did you buy a 6 volt coil? As others said, get the no external resistor needed 12 volt coil, get rid of the resistor and go drive your tractor. Oh and don't buy stuff like that coil at fleet. It's TISCO land of almost right imports for the most part. Get quality parts locally.

Rick

Why do you think the original coil was bad? Maybe you're chasing the wrong problem.

Lets take your questions one at a time:

"Bought a new 6 volt coil @ Fleet Farm that has "Do not use with external resister" marked on it."

That's exactly what Id expect if you bought a 6 volt coil for use on a 6 volt tractor. You would NOT want an external resistor in that case which (on a 6 volt tractor) could reduce the coils input voltage down to 3 or so volts!!!

HOWEVER if you use a 6 volt coil on a 12 volt tractor YOU MUST HAVE an external series voltage dropping (12 to 6) ballast resistor, otherwise the coil will overheat badly. Its designed to operate at 6 volts input NOT 12. But as above if used on a 6 volt tractor then you DO NOT use an external resistor WELL DUH


"Tractor wouldn"t start on it so I assume that although a resister is required to drop the 12 volts to 6, it does not allow the use of this coil."

I RESPECTFULLY DISAGREE

Many many 12 volt tractors use a 6 volt coil PLUS an external series voltage dropping (12 to 6) ballast resistor. If you now have a 12 volt tractor you can use EITHER a) A full true 12 volt coil (NO ballast required) or b) A 6 volt coil plus a 12 to 6 voltage reducing ballast resistor so the coil only sees the 6 volts it was designed for.

ARE YOU SURE THAT NEW COIL IS A 6 VOLT COIL????
IF ITS A 12 VOLT TRACTOR, WHY NOT USE A FULL TRUE 12 VOLT COIL SO YOU DONT NEED A BALLAST??

If you click the URL link below it will give you a step by step troubleshooting procedure to test the ignition and coil and determine the cause of non firing.

John T
John Ts Ignition Troubleshooting

You might want to put the old coil back on and do a little more troubleshooting. Those ohm numbers on the old coil are good.

Thanks. I was told the secondary should be 10,500 to 11,500 ohms. What is the correct spec? Also I was getting misfire when it got hot. Since the condenser was replaced I figured it was the coil.

The secondary resistance on the new 6 volt coil I bought at Fleet Farm was only 5,100 ohms and the primary was 2.2 ohms. If I removed the external resistor I would be getting like 5.5 amps at the points which would be too high. Correct? Anyway, I removed the ballast resistor and installed a 12 volt NAPA. She runs fine now.

Thanks John. I was told the secondary resistance should be 10,500 to 11,500. Apparently that's incorrect? What should it be? Was getting a misfire when it warmed up and with a new condenser I assumed it was the coil. Yes, I figured that the resistor would have to be retained with the 12 volt system and 6 volt coil. That's why I was surprised it wouldn't start. Secondary resistance on the FF coil is just over 5,000 ohms. I agree that Fleet Farm ignition parts are not quality stuff but this was for a friend who was anxious to get into the field and NAPA was closed. Anyway, I pulled the resistor and got a NAPA 12 volt. She runs fine now.