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Old 02-12-2013, 08:00 AM
oldred oldred is offline
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You should have two wires on the positive side of the coil, one from the resistor output and another from the "I" connection on the starter solenoid. The wire on the input side of the resistor should be full voltage with the switch in "run" with the output to the coil at about 7 to 9 volts and the wire from the solenoid should be "cold", basically it's only purpose is to bypass the resistor to supply the proper voltage for starting but it is "hot" ONLY in "start". If that wire (from the solenoid) is hot in "run" then that is your problem, this wire is supposed to be "hot" ONLY in the switch start position and dead at ALL other times. This wire will have to be disconnected from the coil when checking for proper operation, if left connected to the coil there will be a voltage reading from the resistor output.

To help understand how this works it's important to understand the purpose of the resistor which is to protect the coil from over-voltage. A 12 volt coil is actually designed to run on about 8 volts and the reason for this is because of voltage drop from starter motor load during cranking, if the coil was designed to run on 12 volts it would only get about 8 volts during start because of starter motor load dragging down the system voltage. Obviously the low voltage would provide a weak spark just when a hot spark is needed most, during starting. To solve this problem the engineers designed the ignition system to run on approximately the same voltage it would see while under the low voltage due to starter load. They did this by using the resistor or a resistor wire to drop the voltage when the engine is running to about the same as when it is starting. They then provided the system by-pass wire from the solenoid to by-pass the resistor and provide full system voltage (which will have droped to about 8 volts due to starter load) ONLY during the start phase but the voltage supply from that wire has to disappear when the switch returns to "run" otherwise when the system voltage rises after the starter is disengaged the coil will be in an over-voltage condition. This by-pass wire is necessary because without it the normally full voltage at the input side of the resistor would drop due to starter load during engine cranking and would result in even lower voltage on the out-put side of the resistor going to the coil, basically defeating the purpose of using the resistor in the first place.

Here is a good explanation of ignition systems and how they work, the first part deals in detail with points type and how/why the resistor is used/wired.

Last edited by oldred; 02-12-2013 at 08:21 AM.
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