Re: Capacitive & inductive pick-ups for o-scope ignition?

Not to say anything against the above advice, but... The way that you acquire a signal has nothing to do with your use for the signal. What I'm saying is that a capacitively coupled probe and an inductively coupled probe will both obtain a signal, and whether you use one or the other for sync is irrelevant. It happens that an inductive probe is handy for obtaining the sync from a particular cylinder (typically #1), because you can just hang the probe around the #1 spark plug wire.

It is worth remembering what the two types of probes really measure; the capacitively coupled probe measures voltage at the point of attachment, while the inductively coupled probe reads current in the wire placed through the probe's window. Being able to read both current and voltage can be very useful; you can diagnose a mechanism that's binding while watching the current draw on the motor.

For a 20 MHz bandwidth, both capacitive and inductive probes are not tough to design, and should read with equal accuracy. (And you already have the good habit of thinking about what signals will be encountered, before sticking your probes somewhere, so I don't need to issue that old warning. )

You will find it very interesting to see what happens in your nominally 12 VDC electrical system. With a 20 MHz bandwidth, you will be able to easily see events that happen in only a millionth of a second (IF they happen repetitively at a reasonable rate; if they are one-time, random, or very low repetition rate events, you will need a digital storage oscilloscope, and that's way beyond you right now.)

Both a capacitive probe and an inductive probe can't measure DC at all. If you want to measure DC voltage, you will need a direct coupled probe (and oscilloscope input, but that's usually just a front-panel switch selection). If you want to measure DC current (without breaking into the circuit), you will need rather exotic technology, like a Hall-effect device or a magnetoresistive sensor. In the automotive world, that kind of stuff isn't really needed.

Ed

I've tried it both ways (obtaining a secondary ignition pattern as the OP intends to do) the capacitive probe gives the better waveform, the inductive (for trigger purposes) is all that's needed. And since Sun Electric, Snap-On, Bear, Allen, etc. all specify and use capacitive pick-ups to aquire secondary ignition waveforms, inductive pick-ups for synch purposes, I felt it appropriate to pass on which type was correct for the intended purposes. I would also note for your above paragraph that -both- the inductive *and* the capacitive probes [keeping the OP in mind] are non-intrusive connections and for the OPs original intended application, merely "hang" on whichever wire is appropriate to place them on. (coil or #1) Capacitive pick-ups are also available for a variety of "coil in cap" configurations (GM, Toyota, Mitsubishi, Honda, etc.) as are capacitive pick-ups available for "coil on plug" applications. All are non-intrusive.

He needs to walk before he runs. Since the OP is interested in obtaining secondary ignition -voltage- patterns, then as you say, the capacitive probe is the appropriate piece of equipment, choosing the correct piece from the get-go will save him a lot of screwing around and mis-diagnosis.

The OP wants to view/measure secondary ignition waveforms, these signals are normally aquired via non-intrusive methods... The only caution would be about connecting to severely leaking ignition cables.

-I- find it interesting on a daily basis.

I own three labscopes, two are DSOs, I've had them for quite a few years, I've taught automotive driveability diagnostics with all three of them for a number of years also. #4 is in the future, it will likely be a Picoscope for my laptop.

Yes, I've posted to that effect a number of times over the years.

coupled probe (and

Yes, I own four "Hall Effect" type low amps probes, I use them (at least one of them) every day. Three were given to me to beta test for a major automotive equipment manufacturer.

You're kidding right? I would never dream of attacking a driveability problem

-without- having a low amps probe available. Injector current (rise time, duration, peak amps, current control circuitry, peak/hold values), (voltage waves are more or less useless if the injectors are wired in parallel as many are). Ignition coil primary current (rise time, peak amps, secondary capacitance are all useful and needed information) Electric fuel pumps (RPM, shorted comutator segments, bouncing brushes, etc.). Power window motors. Wiper motors. Cooling fan motors. Ignition off current draw (remember, it is very easy to trash a theft lock radio by merely opening a battery connection on a vehicle nowdays).

Given the cost of components and the labor involved in replacing same, it is better by far to test by -every- means available than it is to shot-gun parts, and considering the 'gottchas' that can arise performing what used to be simple ho-hum service operations, one had better be well equiped to address the work arounds needed lest he find himself creating more problems than the ones he's supposed to be repairing.

Can you elaborate please?

97T

You've bought a used 2001 Pontiac whatever with a theftloc radio, the previous owner entered a passcode into the radio, the dealership taking the trade and selling you the car hasn't a clue what the radio code is, so they can't pass it on to you. Six months after purchase, you swap out the battery, the radio because it is in theft mode due to the loss of 12 volt input will no longer play, you now have to pay a GM dealer to un-lock your radio.

Sheesh, thanks for the info.

97T

What confused me, at first, were distinct references wrt to synch leads vs inductive PUs: I'm now garnering, for this app, they are one in the same. I definately want to be able to view single cylinder, parade, and possibly superimposed patterns

I've tested the cables per the 1st half of

There was some arcing between the grounded lead and the distrib cap boots, but nothing observable between the boots themselves (2d set of new ignition wires)

In both experience and cost :-)!

I definately want to ck the injectors - wired in parallel - for lean misfire (will also be checking the O2 sensor w/the scope [older Fluke DMM _may_ be showing a slight lean shift, during the bucking problem I'm trying to resolve, but don't think the Fluke is anywhere's near fast enuf to see what's going on])

But thought I could newbieish just ck injector PW, so will look at getting a low-range probe, too (think I'm going to be spending more $$ on probes, than the scope!, but it appears necessary).

I have a few o-scope books coming from the local library: the ones wrt automotive use are pretty dated (late 70s and early 80s). Have also found a good waveform URL and some other good info on the TEK site. I'm sure there's tons more info out that I'll stumble into as I go

Have been aware of scopes for some time, but not of their dx potential. Feel like light is just turning on - is pretty _amazing_ what can be done w/them. Unfortunately (and surprisingly) most of the shops were I live don't use them

I think this is going to be a kick (hopefully figuratively and not literally)! A piezo-crystal cylinder pressure inducer, and even a vacuum inducer also seem very intriguing...

I appreciate the help/insight

I should have said "transducer"...

Meant to ask this earlier - would a pin-tip low-current probe do most of the above? or do you use clamp-ons for certain apps (like?)

Lastly ;-), are clamp-ons broken down into inductive vs Hall-Effect?

Thanks again

Your synch lead can be nothing more than the inductive pick-up lopped of of an old timing light (rummage sale) that doesn't work anymore because the strobe tube went south.

Viewing single cylinder should be no problem, parade can get pretty crowded if the scope screen is smallish, superimpose will depend on the scope itself and whether it has that capability.

You can do a lot worse than listening to Bohdan...8^) Sharp guy!

I don't know... What would have cost $30,000 fifteen, twenty years ago can be had for a few hundred dollars on e-bay today.

The lean misfire will show up on secondary.

Given the reaction time of an O2 sensor, it doesn't take a very fast meter to see a problem, but not all problems are easily seen thru the O2 sensor.

Pulse width can be measured by viewing injector voltage, shorted injectors are best spotted with a low amps probe. between the two, there are lots of things that can be analyzed. You can set yourself up pretty good for probably around $500 and be lightyears ahead of where diagnostics were 15 years ago in both capability and cost of equipment.

Lots of good stuff out there.

That's because they still don't "fix the car," (old scope joke) and shops are reluctant to charge for scope diagnostic time, plus there is some iniative needed on the mechanics part to become proficient.

There are lots of ways to skin a cat...

Stuff like this looks so much better on r.a.t than washer nozzle LEDs. 8^)