Using a scope to see fuel injector waveform?

[madan1]

Hello folks.
What is the correct way to view the waveform of an injector ( petrol engine )?
I have one of those kit scopes and would like to hook it to a GM Ecotec engine and see the injector waveforms... so how should I connect it?
The injectors have constant B+ and the B-/ground goes through the control module/computer.
Thanks in advance.

[eccerr0r]

Re: Using a scope to see fuel injector waveform?

Just connect it as you would think it should be connected. Note that the sweep speed needs to be fairly low at low RPM so if you don't have a storage oscilloscope that slows down samples, you will just see a ghost on the screen.

800 RPM = 400 intake cycles per minute ~= 7 pulses per second. You'll need a sweep of 1 second or so to see the 7 pulses, if you have a typical 10 division scope, it will be 100 milliseconds (0.1 second) per division. That's very slow, you can see the sweep across the screen.

With a storage oscilloscope, likely digital, you can record the full second and keep it lit on the screen. Digital scopes with "scrolling" function look really neat for slow signals such as this.

You only need to connect the scope across the injector, note that you will likely get ringing/spikes when the injector shuts off just like any other inductor. If you do it this way, make sure the scope isn't touching the car body and the car isn't otherwise grounded.

You can also connect the GND of the scope to the car body and hook the signal to the ECU output. This will give you the inverse of what you'd expect (high = closed, low = open) but usually scopes have an "inverse" function, perhaps needing to be used on the B channel on a 2-channel scope (make sure you trigger off of B if you do this.)