A customer complained he was down on power, and yes, confirmed he had less than 12 volts.
A drop in voltage to a 12-volt ignition coil has a direct and detrimental effect on its ability to produce a strong spark. This, in turn, will lead to a range of noticeable symptoms on a running engine, generally getting worse as the voltage drops further or as engine demands increase.
Here's what happens and the symptoms you'd likely experience:
The Core Consequence: Weaker Spark
- Reduced Primary Current: An ignition coil stores energy in its magnetic field. The amount of energy stored is critically dependent on the current flowing through its primary winding. This primary current is directly related to the supply voltage (Ohm's Law: Current = Voltage / Resistance). If the supply voltage drops, the primary current to the coil decreases.
- Less Energy Stored: With lower primary current, significantly less energy is stored in the coil's magnetic field.
- Lower Secondary Voltage (Spark Voltage): When the primary current is interrupted (by points opening or an electronic module switching), the collapse of this weaker magnetic field induces a lower peak voltage in the coil's secondary winding. This is the voltage that has to jump the spark plug gap.
- Reduced Spark Energy & Duration: Not only is the peak voltage lower, but the total energy and often the duration of the spark are also reduced. A weak, short-lived spark is less effective at igniting the air-fuel mixture.
Symptoms on a Running Engine (Generally from Mild to Severe):
These symptoms often appear or become more pronounced under conditions that demand more from the ignition system, such as under acceleration, high load, high RPM, or when the engine is cold.
- Subtle Loss of Performance: The engine might feel slightly less responsive or "flat."A minor reduction in power that might not be immediately obvious.Potentially a slight decrease in fuel economy over time.
- Misfires, Especially Under Load or at High RPM: This is one of the most common symptoms. As cylinder pressures increase (e.g., during acceleration, climbing a hill) or when there's less time for the coil to charge (at high RPM), the weaker spark may fail to ignite the mixture.Experience: The engine may stumble, hesitate, jerk, or feel like it's "missing" or "cutting out."
- Rough or Unstable Idle: The engine may idle erratically, with inconsistent RPMs, or feel shaky. The weak spark struggles to consistently ignite the mixture at low engine speeds.
- Difficulty Starting (Hard Starting): A significantly reduced voltage will make the engine much harder to start, especially when cold. The engine might crank for an extended period before (or without) firing.In severe cases, it might lead to a no-start condition.
- Engine Hesitation or Stumbling on Acceleration: When you try to accelerate, the increased load demands a stronger spark. If the voltage is low, the engine may hesitate or stumble before picking up speed, or it may misfire.
- Increased Fuel Consumption & Emissions: Inefficient combustion due to weak sparks means not all the fuel is being burned properly. This can lead to higher fuel consumption and increased emissions of unburnt hydrocarbons (HC) and carbon monoxide (CO).
- Backfiring (Less Common): If unburnt fuel from misfires accumulates in the exhaust system, it can sometimes ignite there, causing a backfire (a pop or bang from the exhaust).
- Engine Stalling: Particularly at idle or when coming to a stop, the engine may stall if the spark becomes too weak to maintain combustion.
- Illumination of Check Engine Light (on modern EFI vehicles): Misfires are often detected by the Engine Control Unit (ECU), which may trigger the check engine light and store misfire-related diagnostic trouble codes.
In summary: A drop in voltage to the ignition coil starves it of the electrical energy it needs to produce a healthy spark. The resulting weak spark leads to incomplete or inconsistent combustion, manifesting in a variety of drivability problems, from subtle performance degradation to complete engine shutdown.