Ford 4000 3 cyl coil 6v 0r 12 v

bach939 · Apr 29, 2015

I have a Ford 4000 3 cyl tractor 12 v system. It has a 6 volt coil that is bad. I believe there is a resistor wire that is about 3 ft long that is plugged into a wire harnes of which goes to the ignition switch. By mistake, I purchased a 12 v coil to be used with an external reistor. Can I use this 12 v coil and if so do I need to add a ceramic type resister with or with out the current resistor wire?

Jerry/MT · Apr 29, 2015

Spark ignition systems on these older tractors are limited to about 4 amps max primary current. Thus a "12V" coil has ~ 3.2 ohms primary resistance and a "6 V" coil has ~ 1.6 ohms primary resistance. A 6V coil can be used on a 12V electrical system by using a resistor to limit the primary current. However, a 12 V coil should not be used on a 12V system with any additional resistance in the primary circuit (normal wire resistance excluded).

The exception to this rule is there are some so called 12v coils that say "must be used with external resistance". These are in fact a 1.6 ohm coil and must be used with additional resistance in a 12V electrical system. If this is what you purchased, then your existing resistor wire should be sufficient and you should not add any more resistance to the primary circuit.

I hope I haven't succeeded in confusing you.