Automotive Service Excellence (ASE) A8 Engine PerformanceIgnition System Diagnosis and RepairHard
A technician is diagnosing a vehicle with a P0353 (Ignition Coil 'C' Primary/Secondary Circuit Malfunction) code. After checking power and ground to the coil, which are good, they measure the resistance of the coil primary winding and find it to be 0.1 ohms. The manufacturer specification for primary resistance is 0.5-1.5 ohms. What does this measurement indicate?
- AA short circuit in the primary winding.
- BAn open circuit in the primary winding.
- CAn open circuit in the secondary winding.
- DNormal primary winding resistance.
Show answer & explanationAnswer & explanation
Correct answer: A. A short circuit in the primary winding.
A primary winding resistance significantly lower than specifications (0.1 ohms vs. 0.5-1.5 ohms) indicates a short circuit within the primary winding. This allows excessive current to flow, potentially damaging the coil driver and preventing proper magnetic field collapse for high voltage generation.
Why the other options are wrong
- B. An open circuit would result in an infinite resistance reading (OL on a multimeter).
- C. Primary resistance measurement does not directly diagnose an open in the secondary winding, which would typically be measured with a much higher resistance value.
- D. 0.1 ohms is well below the specified range of 0.5-1.5 ohms, so it is not normal.
Ignition Coil Primary Short
A short circuit in the primary winding of an ignition coil results in abnormally low resistance, causing excessive current draw and preventing proper magnetic field collapse, leading to misfires.
- Resistance reading significantly lower than spec.
- Causes high current draw.
- Prevents proper high voltage generation.
Memory trick: Low resistance, primary's short — no spark, a faulty report.