Using attenuators to protect your scope

So using an attenuator won't protect scopes from spikes? Is it recommended to use only secondary probes for ignition primary/secondary's? I'm still learning my scope.

An attenuator WILL protect your scope from spikes. Andy's caution:

Andy.MacFadyen wrote: *********An attenuator is not suitable for measuring ignition secondary (HT) voltage,*********

is referring to Secondary VOLTAGE ( The visible spark that jumps the Spark Plug gap. ) This voltage is can peak over 40,000 volts. An attenuator is not suitable to measure this.

Is it recommended to use only secondary probes for ignition primary/secondary's?

To measure PRIMARY VOLTAGE, we back-probe the control side of the ignition coil using regular test leads attached to the scope. When the coil is turned on, it creates a magnetic field. When it is turned off, the magnetic field collapses and creates a voltage spike. This voltage spike can peak over 400 volts. This is where an attenuator would be used. (Snap-on scopes can handle a 400v spike and do not require attenuation.)

As the magnetic field collapses in the Primary winding, it induces a voltage in the SECONDARY winding that is about 100x higher in voltage. This SECONDARY VOLTAGE is NOT measured, directly, by the scope. This is where a Secondary Probe is used. The probe measures the magnetic field around the plug wire and converts it to a low voltage output. The scope, then, reads this low voltage. No attenuator is needed.

And secondary ignition voltage pattern usually mirrors the primary pattern? I was told that by another tech but wanted to verify it. Sorry if these seem like stupid questions, but if I don't ask, I won't know.

al514 wrote: And secondary ignition voltage pattern usually mirrors the primary pattern?

Close. The Primary pattern mirrors the Secondary. Sounds backwards but, the Primary pattern, after coil turn-off, is feedback from Secondary. Without the secondary winding, the Primary wave would look like a fuel injector wave, without the pintle hump.

I have moved this thead to the "Diagnostic tools and techniques - Lab Scope Discussion" section.

That's what I thought too, it sounded backwards, but that makes sense. Thanks again my friend.

Attenuators depend on electronic surface mount components resistors acting as voltage deviders and diodes as over voltage protection I have seen ignition secondary produce 65,000 volts would you really want to trust these to parts to protect your scope ?

Increasingly it is becoming more difficult to scope ignition voltages it used to be easy scoping secondaries that started to become more difficult so we started using the primary voltages as the way in but even that has become difficult so increasingly curent raming is the way in.