Here's how to test the power:
1. Identifying the Coil Terminals:
A typical automotive coil has two primary terminals and two secondary terminals. They might be marked, but often aren't clearly labeled. You'll need to consult a wiring diagram for your specific 1974 Dodge model to be absolutely certain. However, generally:
* Primary Terminals: These are lower voltage and handle the power from the ignition switch. One is connected to the +12V ignition system (often through the ignition switch and possibly a ballast resistor), and the other is connected to the ground.
* Secondary Terminals: These are high voltage and send the spark to the distributor. One goes to the distributor cap and the other is typically grounded internally within the coil.
2. Testing the Primary Circuit (Power IN):
* With the ignition key in the "ON" position (engine off):
* Set your multimeter to measure DC voltage (usually indicated by a symbol like "V---").
* Connect the positive (red) lead of your multimeter to the primary terminal connected to the +12V ignition system.
* Connect the negative (black) lead to a good ground (a clean, unpainted part of the engine block is ideal).
* You should read a voltage close to 12V. If you don't, there's a problem in the ignition system before the coil (e.g., ignition switch, wiring, ballast resistor). Note: Some systems only provide power briefly when the key is turned on to crank, so you may need to crank the engine for a brief moment to check for 12V if you only see zero voltage.
* Ballast Resistor: Many 1974 Dodges used a ballast resistor in the ignition primary circuit. This resistor limits the current to the coil when the ignition is on but the engine isn't cranking. This helps prolong the coil's life. During cranking, the starter bypasses the resistor for increased current. If you're only getting a low voltage reading (e.g. 9V) with the ignition on, this is normal operation with the ballast resistor in place. You should see approximately 12V during cranking, though this can be dangerous to test alone and only if you are absolutely certain of your process. It is highly recommended to check after the ballast resistor to be safe.
3. Testing the Secondary Circuit (Power OUT - Indirectly):
You can't directly measure the high voltage of the secondary circuit with a typical multimeter. Instead, indirectly check the coil's ability to produce a spark:
* With the ignition key in the "ON" position (engine off):
* Use a spark tester or a properly insulated spark plug wire. Connect this to the coil's secondary terminal that goes to the distributor cap. The other end should have an appropriate ground. You'll need to carefully ground this so the spark is created to a safe location, not to you.
* Have a helper turn the ignition key to the "start" position.
* You should observe a strong spark across the spark tester gap. If there is no spark, the coil is likely faulty, though problems could also exist in the distributor cap, rotor, or ignition wiring.
Important Considerations:
* Wiring Diagram: A wiring diagram for your specific 1974 Dodge model is essential for accurate terminal identification.
* Grounding: Ensure all your connections are clean and secure. Poor grounding can lead to inaccurate readings.
* Safety: High voltage is dangerous. Be extremely careful when working around the ignition system.
If you are not comfortable working with automotive electrical systems, it's best to take your vehicle to a qualified mechanic. Improper testing can damage components or cause injury.
