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 Figure 1. Ignition switch off. |
Bypassing the Ignition Resistor
The ignition resistor limits current through the coil and breaker points to a safe level and it drops enough voltage so the coil operates effectively on around 10 volts. Battery voltage is going to be drawn down during cranking due to the high current demand of the starter motor. Since we start with a lower voltage at the ignition switch, there is going to be even less at the coil, . . . possibly enough less that it won't develop sufficient voltage to fire the spark plugs. Bypassing the resistor during cranking will put that lower battery voltage at the coil so it will be operating at its design voltage again.
Chrysler's method of bypassing the resistor was very simple and trouble-free. They just added a wire from the starter relay section of the ignition switch to the ignition coil. The ignition switch is actually two switches in one. The dashed line between them in Figure 1 shows they are connected mechanically so they both go to the same position, (off, on, or crank), at the same time when the key is turned. The lower switch has the two lower contacts tied together. That circuit provides voltage to the ignition coil with the resistor in the circuit.
The ignition resistor limits current through the coil and breaker points to a safe level and it drops enough voltage so the coil operates effectively on around 10 volts. Battery voltage is going to be drawn down during cranking due to the high current demand of the starter motor. Since we start with a lower voltage at the ignition switch, there is going to be even less at the coil, . . . possibly enough less that it won't develop sufficient voltage to fire the spark plugs. Bypassing the resistor during cranking will put that lower battery voltage at the coil so it will be operating at its design voltage again.
Chrysler's method of bypassing the resistor was very simple and trouble-free. They just added a wire from the starter relay section of the ignition switch to the ignition coil. The ignition switch is actually two switches in one. The dashed line between them in Figure 1 shows they are connected mechanically so they both go to the same position, (off, on, or crank), at the same time when the key is turned. The lower switch has the two lower contacts tied together. That circuit provides voltage to the ignition coil with the resistor in the circuit.
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The ignition switch is turned on in Figure 2. 12 volts is switched on to the resistor, as shown in red. That circuit also feeds many other things under the hood such as the alternator field winding, voltage regulator, and possibly an electric heating element for the automatic choke thermostatic spring. This is the normal condition when the engine is running. An average of around 10 volts will be found at the coil's positive terminal and resistor, but don't try to measure that with a digital voltmeter. If you remember from the previous page, the full 12 volts will be found there when the breaker points are open because with no current flow through the circuit, no voltage is dropped across the resistor. Roughly 4 volts* will be dropped when the points are closed and current is flowing. A digital voltmeter takes a "sample", or reading, analyzes it, then displays it while it takes the next sample. Some samples will find 8 volts and some will find 12 volts.
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 Figure 2. Ignition switch on. |
That will make the voltage displayed bounce around and be hard to read. In addition, a voltage much higher than 8 volts is developed in the coil's primary winding when the magnetic field collapses and that can confuse the voltmeter. The only time it's necessary to take voltage readings in this circuit is when it's not working. If the engine is running properly, there is no value in determining the exact average voltage at the coil.
* Did you catch the mistake? When the points are closed and current is flowing, expect about 4 volts to be dropped across the resistor leaving the coil to operate
on 10 volts. That adds up to 14 volts but this circuit description says it's being fed only 12 volts from the battery. 12 volts is used here for simplicity but once the
engine is running and the alternator is producing an output, system voltage will go up to around 14 volts. That's what the ignition system will actually be running on.
The voltages used here are to show how the circuit works. You will find different voltages on every ignition system you work on.
Figure 3 shows the ignition switch in the engine cranking position. Because the starter current draws the battery voltage down, it's shown here at only 10 volts instead of 12 volts. That 10 volts is applied to the resistor and an average of perhaps only 6 or 7 volts would be found at the coil. This is where the top part of the ignition switch comes into play. In the crank position, full battery voltage goes to the starter relay. That same circuit puts that 10 volts directly to the coil to allow it to create a nice strong spark. Even if this bypass circuit wasn't needed for the engine to start in warm weather, that weak spark might not be strong enough to effectively ignite the fuel / air mixture in cold weather.
* Did you catch the mistake? When the points are closed and current is flowing, expect about 4 volts to be dropped across the resistor leaving the coil to operate
on 10 volts. That adds up to 14 volts but this circuit description says it's being fed only 12 volts from the battery. 12 volts is used here for simplicity but once the
engine is running and the alternator is producing an output, system voltage will go up to around 14 volts. That's what the ignition system will actually be running on.
The voltages used here are to show how the circuit works. You will find different voltages on every ignition system you work on.
Figure 3 shows the ignition switch in the engine cranking position. Because the starter current draws the battery voltage down, it's shown here at only 10 volts instead of 12 volts. That 10 volts is applied to the resistor and an average of perhaps only 6 or 7 volts would be found at the coil. This is where the top part of the ignition switch comes into play. In the crank position, full battery voltage goes to the starter relay. That same circuit puts that 10 volts directly to the coil to allow it to create a nice strong spark. Even if this bypass circuit wasn't needed for the engine to start in warm weather, that weak spark might not be strong enough to effectively ignite the fuel / air mixture in cold weather.
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The lower switch is still sending battery voltage out on its circuit during cranking. It's unlikely you would notice any symptoms if that connection was missing but without it, during cranking, current would try to flow backward through the resistor to the alternator field and voltage regulator. The field winding, voltage regulator, and anything else on this circuit could total a lower resistance than the ignition coil, and the resulting higher current could stress the resistor and lead to it burning open, especially if prolonged cranking was necessary to get a poorly maintained engine started. |
 Figure 3. Ignition switch in the "crank" position. |
Diagnosis
Problems with this entire circuit are extremely rare and are more likely to be caused during other service work. You can troubleshoot it by unplugging the starter relay to prevent cranking the engine during testing. With the ignition switch on, there must be full battery voltage on the lower switch's middle contact and at the resistor. If it is missing, the switch is defective or the wire is broken. The symptom will be the engine starts and runs only while the ignition switch is held in the "crank" position. Current for the coil is coming through the upper bypass circuit. To troubleshoot the bypass circuit, the points should be closed so current flows through the resistor and it drops some voltage. You'll find around 8 volts at the coil positive terminal when the ignition switch is on. When switched to the crank position, the voltage must go up to full battery voltage. As an alternative, if the points are open, you can unplug one wire from the resistor, then check for voltage on the bypass circuit. Keep in mind the majority of the bypass circuit goes to the starter relay and just a short additional wire goes to the resistor / coil. A defect in that circuit is much more likely to cause a no-crank condition. If that additional wire was broken or removed, the typical symptom would be a long crank time and hard starting in cold weather, and a clue might be much easier starting is observed with a battery charger connected. The charger will raise system voltage so the coil will produce a higher secondary voltage and stronger spark.
Problems with this entire circuit are extremely rare and are more likely to be caused during other service work. You can troubleshoot it by unplugging the starter relay to prevent cranking the engine during testing. With the ignition switch on, there must be full battery voltage on the lower switch's middle contact and at the resistor. If it is missing, the switch is defective or the wire is broken. The symptom will be the engine starts and runs only while the ignition switch is held in the "crank" position. Current for the coil is coming through the upper bypass circuit. To troubleshoot the bypass circuit, the points should be closed so current flows through the resistor and it drops some voltage. You'll find around 8 volts at the coil positive terminal when the ignition switch is on. When switched to the crank position, the voltage must go up to full battery voltage. As an alternative, if the points are open, you can unplug one wire from the resistor, then check for voltage on the bypass circuit. Keep in mind the majority of the bypass circuit goes to the starter relay and just a short additional wire goes to the resistor / coil. A defect in that circuit is much more likely to cause a no-crank condition. If that additional wire was broken or removed, the typical symptom would be a long crank time and hard starting in cold weather, and a clue might be much easier starting is observed with a battery charger connected. The charger will raise system voltage so the coil will produce a higher secondary voltage and stronger spark.