"Sounds like you know a bunch of stuff I want to know"
Great. Just ask. I'm also learning some things from you.
"What is 'dwell' "
Before the existence of modern electronic ignition (found on all cars today), ignition systems used to use points (a kind of mechanical switch). They would open and close rapidly, as the distributor spun. They would cause the plugs to fire. It was important to adjust how long the points were closed (as compared with the time they spent being open). Because this had to do with rotation, the name given to this concept was "dwell angle" or "dwell time." It basically means the points are closed a certain percentage of the time, and open the rest of the time.
More stuff about this here:
http://www.jetav8r.com/Vision/Ignition/CDI.html
It turns out that this concept (something being in one state a certain percentage of the time, and being in another state the rest of the time) has a general name: duty cycle (see here:
http://en.wikipedia.org/wiki/Duty_cycle).
It also turns out that the concept of duty cycle is a very important part of how modern EFI works. Each injector is open a certain percentage of the time, and closed the rest of the time. (This is called either duty cycle or pulse width.)
If you can observe your injectors' duty cycle (or pulse width), you are essentially monitoring instantaneous fuel use.
You can buy a fancy automotive multimeter, which has a special feature called "duty cycle" or "pulse width," that is mostly intended to monitor injectors (although it also has other applications).
But you don't need a fancy automotive multimeter. You just need an automotive multimeter that has a "dwell" feature. Why? Because "dwell" is really just another term for "duty cycle."
So it turns out that a concept (duty cycle) that used to apply with regard to ignition (points and plugs) is now being applied with regard to fuel (EFI injectors).
The term "dwell" is archaic, because cars don't have points anymore. But it's really just another name for "duty cycle," so you can use a dwell meter to monitor your injectors.
If you look around a place like eBay, you'll find lots of dwell meters, cheap. They are often called "tach/dwell meters" because they also do tach. They are typically old and analog, but I think they would probably work sort of OK, for this purpose. But I was happy to find a cheap new DMM with a dwell feature.
Actually, the digital dwell is much better. That's because your injector duty cycle % is often very low (a number less than 10, on a scale of 90). This small value could be hard to read on an analog meter, especially if you want precision. Whereas on a DMM, no problem. Also, the DMM responds much faster than a mechanical needle.
"links to HF for the DMM and timing light? I think I found them"
Yes, you did. I decided I didn't need the fancy timing light.
If someone wants a DMM and doesn't need dwell, HF has them as low as $4! Amazing place.
"I'll be right near a HF store on Friday"
I've never been in one of their stores, but I just found out there's one not too far, so I intend to visit.
"spend money I don't have on stuff I don't need"
I can relate.
"Anyway, can you provide hookup details"
I could be very specific if you had a Honda. You don't, but I think the general principles still apply.
"What and where do I probe to read injector duty cycle?"
My ECU has 4 pins, one for each injector. I just picked one. Positive lead of DMM attaches here. Other lead of DMM attaches to chassis ground.
"What setting do I use on the DMM?"
A dwell meter has choices for number of cylinders (3,4,6,8). It almost doesn't matter what you choose. You should get a precise and meaningful result, at least in relative terms, even if you pick the wrong scale.
I set mine to 4 cylinder. That means full scale is 90 degrees (a quarter of 360). When you're in the world of dwell, you're speaking degrees of rotation.
When my dwell meter indicates 10, let's say, it thinks it's telling me that my points are open 10 degrees out of each 90. But here's what it's really telling me: my injectors are open 10/90th of the time.
90 degrees just happens to be a number that's conveniently close to 100 per cent. So when my meter tells me 10, instead of thinking "10 degrees," I just think "10 per cent." Easy.
If I set the meter for six cylinders, then full scale would mean 60, instead of 90. Because on a 6-cylinder engine, a cycle for the points (for one cylinder) is 60 degrees of rotation, instead of 90 degrees.
But understanding this part isn't that important. Even if you picked the wrong dwell scale, you would still see the numbers rise and fall, in a very precise way, as you manipulated the throttle. And you would figure out how to translate those numbers into more meaningful units (like gallons per hour).
"Where is a good place to probe the o2 sensor?"
If you don't have a wideband sensor (I think you don't; do you?), you might find that reading the sensor is not terribly interesting or helpful. Anyway, to find the answer to your question you could consult a shop manual or wiring diagram to understand which ECU pins are attached to the sensor.
"What setting do I use on the DMM?"
With the O2 sensor, it's simple DC voltage. You have several scales to choose from. You probably want something like a 2V scale.
"I have a Ross Tech Vag-Com"
That's interesting. Never saw that before. Thanks for mentioning it.
"I think they make similar products for non-VW cars too"
Their web site says only VW/Audi.
"maybe it would work on a plain OBDII"
My car is OBD1.
"The Dataq data acquisition kit doesn't look like something that can be figured out by someone like me, let alone people who are less technically inclined. Could it be done by non-techies with a DIY document?"
Yes. That would be the concept. You would hopefully just need some simple instructions about how to hook it up.
Did you get the Vag-Com specifically because you liked the idea of getting this kind of data onto your PC?
Thank you for all the data.
Linear Mode
