Automotive Service Excellence (ASE) A8 Engine PerformanceIgnition System Diagnosis and RepairHard
A technician is using an inductive timing light to check the ignition timing on an older vehicle with a distributor. They notice that the timing mark appears to jump around erratically, and the engine runs rough. All spark plugs and wires are confirmed to be in good condition. What is the most probable cause of the erratic timing light reading and rough running?
- AExcessive end play in the distributor shaft.
- BA faulty crankshaft position sensor (CKP).
- CIncorrect spark plug gap.
- DA failing ignition coil primary winding.
Show answer & explanationAnswer & explanation
Correct answer: A. Excessive end play in the distributor shaft.
Erratic ignition timing in a distributor-type system, leading to a rough idle, is often caused by mechanical issues within the distributor itself, such as excessive end play or wear in the distributor shaft bushings. This allows the rotor and pickup coil to move inconsistently, causing the timing signal to fluctuate.
Why the other options are wrong
- B. While a faulty CKP can cause timing issues, this question specifically mentions a distributor-type system, where the CKP might not be the primary timing input or the issue is mechanical within the distributor.
- C. Incorrect spark plug gap would affect spark quality and misfires, but not typically cause the timing mark itself to jump erratically.
- D. A failing primary coil winding would typically cause a weak or absent spark, not necessarily erratic timing. The timing light would still show a consistent (though perhaps weak) flash.
Distributor Shaft Play
Excessive mechanical movement (end play or radial play) in the distributor shaft, which can cause erratic timing signals and poor engine performance.
- Affects timing of spark delivery.
- Leads to rough running, misfires.
- Diagnosed by checking shaft movement.
Memory trick: A 'wobbly dancer' (distributor shaft) can't keep 'perfect rhythm' (timing).