Inside the Wave - Using a Digital Storage Oscilloscope to test fuel injectors adds depth to your fuel system diagnosis. - Motor Age - Automotive training, certification & parts info
The present and future — gasoline direct injection — is high pressure, multiple injection events of extremely short duration.
Modern scan tools, both OEM and aftermarket, have come a long way. More data parameters to use, faster refresh rates and better
graphing all have played a role in making scan tool diagnostics easier. But there are still those occasions when I want to
see what is happening directly without going through a "middle man," and those are the occasions I use my scope.
(Figure 1) This is a typical, saturation style, injector pattern.
A Digital Storage Oscilloscope (DSO) is a high-powered voltmeter that allows a user to monitor a voltage signal over time,
viewing those results in a graph format on the viewing screen. It is excellent for checking electrical circuits for intermittent
failures, voltage drops and even mechanical function. It is also a good choice when monitoring a circuit that operates in
milliseconds, like ignition systems or today's topic, fuel injectors.
(Figure 2) Section A is prior to the injector being commanded "on."
A DSO is nothing more than a voltmeter that graphs the voltage signal over time. To set up your scope for viewing an injector
pattern, set the time divisions (horizontal axis) at 2 milliseconds/division, and the voltage divisions (vertical axis) at
20 volts/division. One advantage of the scope is the ability to adjust these two scales to take a closer look at any particular
part of the pattern, and we'll do that as we go along. Now, connect your scope's leads. The negative lead should attach to
the battery negative post as close to the battery as possible.
Attach the positive lead to the groundside of the injector you want to test, as close to the injector as you can get. Does
this lead setup sound familiar? It should, as it's the same lead setup you would use in checking this circuit for voltage
drop. With the leads attached and the circuit running (engine idling), you should get a pattern similar to the one shown in
Figure 1.
(Figure 3) Section B is the commanded pulse width. There is a lot to see here.
Let's break this pattern into sections and take a closer look at what is going on in each. In Figure 2, the highlighted section
(A) is a measurement of system voltage to the injector and through the injector winding. Since the circuit is not closed at
this time, there is no current flow and no voltage drop through the injector itself. This voltage should be within 0.50 volts
of system voltage measured at the battery. If not, take a closer look at the power side of the injector circuit up to and
including the injector coil windings.
Peter Meier is an ASE-certified Master Technician, member of iATN and full-time tech in Tampa, Fla. His experience reaches back over 30 years, and his contributions to Motor Age reflect a wide variety of experience with almost every make and model.
Articles by Peter F. Meier
