Why won't scope work well on car's injectors?

Actually, they do, but...

In order to stabilize a pattern, you need to synchronize the signal with something. Sychronizing can be via a second trigger input from another event such as the #1 sparkplug, a primary ignition event or by the signal you're measuring itself. In the case of an injector waveform, you'll probably want to synchronize off of the injector pattern (internal trigger) since they (injectors) don't always coincide with anything else happening electronically. Internal triggering can be labeled many different ways, but it usually has provisions for adjusting the trigger level to a certain voltage so that when the event you're trying to view reaches that voltage level, the scope begins displaying that event. One would normally adjust the volts range and time base so that the waveform fills about 80-90% of the screen (height and width), then adjust the trigger level to about 25-33% of the screen height, this will help keep the trigger from triggering on noise or transient spikes in the case where the scope or probes being used are overly sensitive.

One of the advantages to using an automotive dedicated DSO is that they often come with the ability to use pre-set test screens for a particular component test, this gets you close to where you can then fine tune the screen settings depending on where you want to focus on the waveform.

Try:

and: for some good information to help you get started. (please don't hammer the hell out of these guys sites!)

If you can point to a manufacturers, vendors or users site for this particular brand of scope, I may be able to offer more assistance.

lastly... Measuring pulse width is all well and good but, after 20 years, I still have yet to find any definitive correlation to pulse width and rich/lean. Measuring oxygen sensor voltage is still the best way to determine mixture status. Also, merely measuring the injector voltage signal will not give you an exact account of how long the injector is actually open and spraying fuel because there is a lag time between when the circuit turns on and when the injector pintle actually opens and -is- spraying fuel. (electrical versus mechanical actions) A low amps probe will help in this case since the movement of the injector pintle can actually be seen in the amps waveform.

If you meant that I should have hook up to a second trigger, I am unsure if my scope has a second trigger I can hook up to. The scope is a 20MHz 2 Ch, Dual Trace & Function Generator Model OS-5020G Manufacturer: EZ Digital (Formerly Goldstar / LG Precision)

Got it.

Thanks, very great information.

I once had a dead cold sensor and the smog station failed my car. He said, "Your car is running rich." I couldn't find what was causing a rich mixture until I found it by accident. If only I thought of an amp probe below.

I think this is a very great idea. Any amp prode will do? Do they make digital and analog amp probes? Rick

Triggering is mentioned in the product specs...

[snip]

I don't believe digital versus analog matters. Here are a couple of places to get them..

(second one down, or the Fluke on the bottom if cost is not an issue)

and,

(same as the AES generic) I believe these come with 1/4" bananna jacks, BNC adaptors are available to fit it up if your scope has BNC connectors.

The Ease price is $10 cheaper but AES often ships for free and their service is excellent. AES has many other products you may find interesting for automotive work.

A customer of ours (John Pavlick) had a similar problem tuning a motorbike.

The issue of triggering is critical, particularly in automotive applications as engines tend to be less than perfectly periodic and good trigger sources can be hard to find. You need a scope that has a seperate trigger channel or which can derive a reliable trigger from the waveform itself.

Alternatively if you have a digital storage scope, you can capture one or more complete cycles and look at them in detail (in which case a steady trigger is less important).

In the end he solved his problem using our BitScope with a notebook PC

John Pavlick's website is

You might like to ask him how he did it.

Best Regards, Bruce Tulloch.

-- Bitscope Designs

Those products are great. But before I get one, I am thinking of hooking the car's injector signal to activate a tone which will be inputed to a sound-card on my notebook PC. The hard part is how do I use a current meter to activate a sound.

Do you mean most generic scope like mine does not have a reliable trigger. Yes, I noticed that the injector pulses tend to be less than perfectly periodic. Even with perfectly periodic low frequency square waves generated by the scope itself will give the trigger a hard time. Or is just a lack of knowlege? Rick

Approximately 10/22/03 17:52, Ricky Spartacus uttered for posterity:

It kinda depends on what you mean by using the injector signal to activate a tone that will go to a sound card.

It is possible you can directly just record the injector signal with reasonable fidelity and just look at it with your favorite audio editing program.

Kinda depends on the scope and the signal being sync'd and what the baseline of that signal looks like.

Would guess for an automotive environment, I'd probably start with AC coupling. Then the trick is to trigger reliably when there IS a signal, but not when there h'ain't. Use the frequency/time knob to widen out the pulses so you can see their leading edge.

Then grab the sync level adjust and move it up the left side of the wave. You may need to flip a polarity switch for the trigger if the scope has one. As you move the sync level adjust, you'll notice that the left side of the wave will begin at higher and higher position on the display. A good place to start is to adjust the sync such that the wave begins roughly 2/3rd of the way up the wave. If you can get that reasonably stable, then move the freq/time knob to your preferences.

Not really sure an external sync would work that well in the sloppy mechanicals of a car engine, say using the crank position sensor to trigger the sweeps.