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.
Ford'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 left 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. 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 field winding, voltage regulator, 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.
Figure 2 shows the ignition switch in the engine cranking position and the neutral safety switch is closed. 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 right part of the ignition switch comes into play. In the crank position, current flows through the solenoid coil circuit, shown in green. The magnetic field draws the spring-loaded contact disc, (blue) up which turns on current flow to the starter motor.
The resistor bypass circuit, shown in blue, receives full battery voltage through the small additional contact when the contact disc connects to the battery terminal on the left side 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.
This second test should be done with the ignition switch off. See the Caution note below.
With the bypass wire still disconnected, use a jumper wire, shown in dark yellow, to momentarily connect terminal 1 to the large battery terminal. The solenoid will engage and the starter motor will crank the engine unless the starter cable is unbolted. Activate the solenoid just long enough to measure the voltage on terminal 2. Full battery voltage must appear there during cranking and it must go to 0 volts when the solenoid is turned off. If 0 volts is present during cranking, replace the solenoid.
CAUTION: The neutral safety switch is shown here open which means the transmission is in reverse or drive. This test bypasses that switch and the engine will start if the ignition switch is on. Be certain the transmission is in "park" before performing this test so you don't have to chase after your car!
The same test can be performed without the jumper wire by simply having a helper turn the ignition switch to "crank" while you take the voltage measurement.
Ignition System Theory of Operation (What it Does)
Basic Ignition System Theory of Operation (How it Works)
Test Your Ignition System