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.

GM's method of bypassing the resistor was to use an extra contact and terminal on the starter solenoid.  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 top switch has the "on" and "crank" contacts tied together.  That circuit provides voltage to the ignition coil with the resistor in the circuit.

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 generator's wake-up signal through the warning light, and anything else that's turned on when the engine is running  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 10 volts and some will find 12 volts.  That will make the voltage displayed bounce around and be hard to read.  In addition, a voltage much higher than 10 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

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 starting circuit comes into play.  In the crank position, the starter relay and solenoid are energized, shown in green.  The magnetic field in the solenoid draws the spring-loaded contact disc, (blue) to the contacts which turns on current flow to the starter motor.

The resistor bypass circuit, also shown in blue, receives full battery voltage through the small additional contact when the contact disc connects to the battery terminal at the top of the solenoid.  That puts 10 volts directly to the ignition coil during cranking.  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.

The top switch's lower contact is still sending battery voltage out on its circuit during cranking.  It's unlikely you would notice any symptoms if that connection was not used but without it, during cranking, current would try to flow backward through the resistor to the generator warning light.  That circuit, along with anything else on it that could draw high current, such as the heater fan, could total a lower resistance

If the engine does not run, you'll have to look further with those voltage tests right on the starter solenoid.  The two smaller terminals are marked "S" for solenoid, and "R" for resistor.  Some people call the "R" terminal the relay terminal because the solenoid is acting like a relay by switching battery voltage on and off.  For convenience, clip a small jumper wire on the "R" terminal and connect the other end to your positive voltmeter probe so you can see the reading while cranking the engine.  If you find 12 volts on that terminal during cranking, there's a break in that wire going up to the resistor.  If 0 volts appears on that terminal, there's a problem inside the solenoid.  It can be replaced rather easily but the starter motor will have to be removed from the engine to do so.


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Basic Ignition System Theory of Operation (How it Works)