Steve,
I think the sensor will need to be powered to have a noticeable effect. I remember once hooking up an oscilloscope to a Croma sender (and a very good Tektronix oscilloscope at that!) and getting a very confusing single spike, noisy, which decayed exponentially. The scope auto-triggered, but it didn't really tell me whether it was correct or not. I was expecting a nice digital square-wave (I suppose 80's electronics are often not like that).
Nevertheless I will try, I guess as long as there's any sort of output (even a single spike) that probably means it's OK.
Right, now to discuss your question
A bench-oscilloscope from Jaycar is always good value but most are old-tech CRT scopes. I learned to use the Microtek digital-type at Uni - a computer samples the signals, takes the measurement, and gives you the figures you want to know... the display on the backlit LCD is only for a graphical idea of what's going on. There's even an auto-set button that acquires all the signal characteristics and sets the timescale & volts/division accordingly. Once you've learned with this type (i.e. not learned), it's hard to be bothered with a CRT type. Bit like getting a drivers' license for automatic-only.
I bought the handheld type mainly for the novelty of it and because it only cost me $160 or so, and because being a digital device I hoped it would have some of the capabilities of the Microtek units (which, incidentally, start at about $1800 educational-price, over $2000 retail).
It turns out there are two Velleman models mentioned in the manual; Jaycar sells the cheaper one. The display is quite limited in size and resolution, but that's OK because you're not having to read the measurements off the waveform. As with the Tektronix, the scope takes the samples and the measurement, and the display is for interest-only. However it means that even the figures displayed are tiny and the LCD is not backlit. The waveform is coarse and the LCD is quite slow. However, given the price, you can forgive all these problems considering that it gives you an insight into something you otherwise wouldn't see at all
The probe is excellent-quality and the unit feels robust enough for workshop use (these things are probably important long-term). You'd have to watch that the five (!) AA batteries don't leak in disuse, though.
What is really nice is (as I alluded to before) the ability to press a button and see various measurements taken from the signal. Some of the measurements are quite advanced! For example (and this is great fun), you can connect to the speaker-outputs of a car stereo, with the speakers connected, crank the volume, and the scope gives you the actual power readings once you enter the (known) speaker impedance. This could be useful for troubleshooting sound quality, or comparing head unit vs. amplifier, etc. and discovering that "45W x 4" really means 18W.
The display has several possible layout settings (within the confines of the small LCD) so you should be able to see all the measurements you need. There's a memory capability, which I haven't used. There's even an 'Auto' button, so you really do get many of the Tektronix capabilities.
However, you would have to consider the bandwidth (sample rate)... the lesser model is much more limited than the more expensive model (that Jaycar don't sell).
To be specific, the manual says that the HPS10 has maximum bandwidth of 2MHz, with "sample rate of 10MS/s for repetitive signals, 2MS/s for single-shot events". I'm really not sure what those units mean! I think they mean million-samples per second, so if the signal goes high for 0.5ms (one 2000th of a second), it can read it. Or if the signal repeatedly goes high for 0.1ms (one 10000th of a second), it will catch it. That is a frequency of 10kHz - not very high! You won't be reading any computer data buses - and perhaps since you have specific knowledge of such, would you care to say whether the sensors of modern engine management work faster than this? (It wouldn't be an issue for things like the oxygen sensor of course, since those things switch at more like 1Hz...)
A standard bench oscilloscope has bandwidth of 20Mhz or 40Mhz (for many models), and so you'd expect the sample rate to be ten or twenty times higher.
The more expensive Velleman handheld model, HPS40 (if you could get it) has bandwidth of 12MHz (40MS/s repetitive, 10MS/s single-shot). It also has interfacing capabilities, which might be useful if you wanted to log values while driving.
In short, the Velleman HPS10 (like I bought) has limited capabilities but is a robust and well-made instrument - it feels trustworthy and the documentation is excellent - meets relevant approvals. It doesn't feel at all like the dodgy multimeters that Jaycar sells
I think that it is a lot better than nothing in trying to assess the output of sensors etc. For example, the LCD would show the spikes in a 'noisy' flap-type AFM, where a digital multimeter would find this difficult. And, the 'Auto' button would have you up and running, when you'd still be twiddling the knobs on one of those CRT units (which always remind me of the WWII radar sets my grandfather worked on...)
Cheers,
-Alex
