MARTIN KERKER
New User
How can I tell if my 880 Oliver ignition coil has an internal or external resister?
- Thread starter MARTIN KERKER
- Start date
Janicholson
Well-known Member
None will have an internal resistor because there is no reason to put one inside. The windings of a 12v no external resistor
required coil are designed to operate at 14+ volts full time. They have more windings of smaller gauge. When disconnected from
the wiring, the resistance across the two small posts will be about 4 ohms for a coil that needs no resistor in the supply. It
will be about 2 ohms if it is a 12v external resistor required coil, or if it is a 6v coil. That will set them apart. Jim
required coil are designed to operate at 14+ volts full time. They have more windings of smaller gauge. When disconnected from
the wiring, the resistance across the two small posts will be about 4 ohms for a coil that needs no resistor in the supply. It
will be about 2 ohms if it is a 12v external resistor required coil, or if it is a 6v coil. That will set them apart. Jim
Steve@Advance
Well-known Member
Welcome Martin!
First, no coil has an internal resistor. Some coils "need" an external resistor, some don't. The difference is the primary winding.
There are two ways to determine if your coil needs a resistor.
One, look for a part number, or writing on the can which will tell what it requires.
Two, take the coil out of circuit and check the resistance across the + and - terminals with an ohm meter. If the resistance is around 1.5 ohms, it needs a resistor. If around 3 ohms, it does not. The goal is to have around 3 ohms, be it across the coil alone, or across the coil and a resistor in series.
Also on the tractor itself, with the coil in circuit, and the points closed, check the voltage from the + terminal to chassis ground. If the voltage is close to the battery voltage, there is no resistor in the circuit. If the voltage is down around 9 volts, there is a resistor in the circuit.
First, no coil has an internal resistor. Some coils "need" an external resistor, some don't. The difference is the primary winding.
There are two ways to determine if your coil needs a resistor.
One, look for a part number, or writing on the can which will tell what it requires.
Two, take the coil out of circuit and check the resistance across the + and - terminals with an ohm meter. If the resistance is around 1.5 ohms, it needs a resistor. If around 3 ohms, it does not. The goal is to have around 3 ohms, be it across the coil alone, or across the coil and a resistor in series.
Also on the tractor itself, with the coil in circuit, and the points closed, check the voltage from the + terminal to chassis ground. If the voltage is close to the battery voltage, there is no resistor in the circuit. If the voltage is down around 9 volts, there is a resistor in the circuit.
J. Schwiebert
Well-known Member
The 880 used a 12 volt coil
There's no clearly obvious way of telling what coil is attached to a 50 or 60 year-old tractor, as it could have been exposed to all sorts of "mechanics" over it's long life.
If you have access to a quality/accurate Ohmmeter and know how to "zero" it, check the resistance across the two primary coil terminals.
A "6 Volt" coil that needs an external resistor to live a long and happy life on 12 Volts will have a primary winding resistance of about 2.5 to 1.75 Ohms, and a "true" 12 volt coil will have a primary resistance of about twice those numbers.
If you have access to a quality/accurate Ohmmeter and know how to "zero" it, check the resistance across the two primary coil terminals.
A "6 Volt" coil that needs an external resistor to live a long and happy life on 12 Volts will have a primary winding resistance of about 2.5 to 1.75 Ohms, and a "true" 12 volt coil will have a primary resistance of about twice those numbers.
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