Quick Tip for Testing Ignition coils

Here's a quick tip for diagnosing a misfire condition. This Honda Odyssey came in with misfire codes on all cylinders however it felt as if only one cylinder was misfiring. This was a dead miss under all driving conditions. Fairly routine diagnosis but we must ALWAYS do our due diligence and test the vehicle properly. 

This vehicle has a 3 wire COP ignition coil system which includes a constant B+ (with the key on), constant ground, and a digital square wave signal from the PCM to turn the coil on and off. This is a pretty typical setup on most vehicles today. 

Since this vehicle has a shared B+ circuit among all 6 coils, you can easily and efficiently measure the voltage drop on that circuit. Every time a coil fires, the shared voltage will naturally fall ever so slightly. This is a normal condition that can be used to our advantage in diagnosis.

Using Pico-scopes vertical scale enhancements, an invert Math channel, and a little filtering, you are able to quickly come up with a capture that is very similar to using an amp clamp. Using an amp clamp is a very effective tool for measuring the current for all coils but sometimes accessing that fuse for the circuit or a portion of the wire upstream of all 6 coils may be tricky to get to. Using and manipulating the voltage capture can be a very efficient method for relative coil analysis. 

You can see in this capture here I have my connections on Channel A in blue on the B+ circuit and on channel B in red I am using an amp clamp on just #5 Ignition coil to show you how close the patterns can be. I zoomed in here to show how similar to two captures become with a little pico magic. Channel A was inverted, vertically scaled, and filtered on the Orange Math channel that you see. Its very easy to see in both the voltage pattern and the current pattern that the coil is shorted. Also, by scoping the shared voltage wire, I am able to have a view of the condition of ALL of the coils. This is very powerful. Not only do I have proof of the condition of the coil that I am sampling from BUT I have evidence of the condition of all the coils simultaneously. 

If this vehicle had a coil that wasn't exactly "failed" yet but had evidence of future failure, I can properly advise the customer to avoid a Boomerang situation. None of us like boomerangs! 

I find it incredibly efficient to just find the B+ wire on any of the ignition coils and work with my scope to display what I want it to. With the power of pico's "creature comfort" functions it becomes very easy to perform.

If you are using another brand of scope, simply putting your scope into AC coupling may also give you the results you are looking for when measuring the B+ circuit.