A technician tests a reciprocating engine's high-tension ignition harness leads using a megohmmeter and finds insulation resistance readings well below the minimum value specified in the maintenance manual. What does this result most likely indicate?
- AThe spark plugs connected to the leads are fouled with lead deposits
- BThe magneto's internal timing is out of adjustment
- CThe lead insulation has deteriorated, risking high-voltage leakage or arcing, especially at altitude
- DThe magneto points are burned and need dressing
Show answer & explanationAnswer & explanation
Correct answer: C. The lead insulation has deteriorated, risking high-voltage leakage or arcing, especially at altitude
Low insulation resistance on an ignition harness lead indicates the insulation has deteriorated (cracking, moisture absorption, or carbon tracking), which can allow high-voltage leakage to ground or arcing between conductors. This risk increases at altitude where reduced air pressure lowers the dielectric strength of air, making the lead more likely to arc externally. It is not related to magneto timing, plug fouling, or point condition.
Why the other options are wrong
- A. Spark plug fouling is identified by removing and inspecting plugs, not by megohmmeter tests on the harness.
- B. Insulation resistance testing checks lead condition, not magneto internal timing.
- D. Burned points affect magneto output but are checked separately, not via lead insulation resistance.
Ignition Harness Insulation Resistance Test
A test using a megohmmeter to measure the insulation resistance of high-tension ignition leads, verifying they can safely contain high voltage without leakage or arcing.
- Performed with a megohmmeter per manufacturer specifications
- Low resistance indicates deteriorated, cracked, or contaminated insulation
- Risk of voltage leakage increases at altitude due to lower air pressure
- Failed leads are replaced to prevent engine misfire or rough running at altitude
Memory trick: Low resistance, high risk — voltage escapes through cracked insulation.