Ignition and Engine Waveforms

How do I check cam and crank sensor timing with a scope?

Capture the crank and cam signals at the same time on two channels, then compare their alignment with a known-good waveform for the engine to see whether timing has jumped or the chain has stretched.

What does a secondary ignition waveform show?

A secondary ignition waveform shows firing voltage, spark line voltage and burn time, which reveal problems such as worn plugs, wide gaps, lean cylinders, or coil and wire faults.

How can a scope test alternator diode ripple?

Short answer: Set the scope to AC coupling with a small voltage scale and measure at the alternator output with a load on; a healthy alternator shows small even ripples, while a bad diode shows a repeating drop or large hump.

Alternators produce AC and convert it to DC through diodes. Each diode contributes a small hump in the output. When a diode fails, the pattern shows a missing hump or a large dip at regular intervals, and output is reduced.

Connect the scope across the alternator output and ground. Use AC coupling to remove the DC voltage so the ripple is visible. Turn on headlights and the blower to load the alternator. A healthy pattern has small, even ripples.

Excessive AC ripple can cause electronic modules to act strangely and can drain the battery. Some battery testers include a ripple test that uses the same principle. Keep hands clear of the drive belt and fan, and secure the test leads before starting the engine.

What is an in-cylinder pressure transducer test?

Short answer: A pressure transducer screwed into the spark plug hole lets a scope show cylinder pressure over the engine cycle, revealing valve timing, exhaust restrictions and valve sealing problems without engine teardown.

A pressure transducer converts cylinder pressure into a voltage signal. With the engine cranking or running, the scope shows the compression peak, intake and exhaust events, and valve timing.

Experienced technicians use the trace to spot problems such as cam timing errors, restricted exhaust or leaking valves. Comparing it with a known-good pattern for the engine type helps interpretation.

Disable fuel and ignition to the test cylinder as service information describes, and make sure the transducer fitting is tight. Keep hands and leads away from moving parts while cranking or running the engine.

How do I test an ignition coil's primary current with a scope?

Short answer: Clamp a low-amp current probe around the coil's power feed and capture the current ramp; a healthy coil shows a consistent ramp to its limit, while a shorted coil ramps too fast and a weak coil ramps slowly or unevenly.

Coil primary current shows how the coil charges. When the driver turns on, current rises in a ramp. The shape and peak tell you about the coil windings and the driver.

A coil with shorted windings has lower resistance, so current rises faster. A coil with high resistance or a poor connection rises slowly or does not reach its normal peak. Comparing cylinders quickly highlights an odd coil.

On coil-on-plug engines, clamp around the shared power feed to see all coils or one coil's feed to isolate it. Use a low-amp probe on an appropriate range and zero it first. Keep hands away from the coils while the engine runs to avoid shocks.