Posted by Steve@Advance on October 03, 2017 at 18:59:44 from (66.169.147.211):
In Reply to: 880 oliver posted by MARTIN KERKER on October 03, 2017 at 18:34:27:
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.
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