O-scope square waveform interpretation?

On Thu, 21 Aug 2003 11:55:08 -0700, Lance Morgan wrote (in message ):

Look *very* closely at the inside of the cap. Any trace of a crack or residue indicates HV arcing.

I presume you've replaced cap and rotor since this problem cropped up?

Good luck,

My experience scoping (Tektronix 465b) a car was with a 90' Nissan Pathfinder with the 3.0 engine. I scoped the distributor signals to see if the timing pulses from the unit were bad. (and they were) The waveform did look like a square wave that you pictured and varied due to rpm (obviously). The pickups and other stuff in mine were burned from a car fire and we were troubleshooting the various components to limit the "swap till you drop" mentality along with the additional non-returnable cost that is associated as well. We used parts from more commonly available 4 cylinder Nissans as our donors and basically fixed the components that were burned out or fried from electrical shorts. As far as I remember the waveform did not jump around from the distributor. It was steady in speed and amplitude. Your's would be similar if it's a optical pickup like ours. You'll have to figure out the ckt to understand what is going on. We had the factory service manual which did help for some understanding of the computer functionality along with the blinking lights on the unit as well. You would be well off to invest in the wiring diagrams and technical descriptions if available to you. It helps a lot when using a scope for diagnosis on a car as it can become complex very quickly just trying to understand why signals may vary (like yours is). A reference car that works would be helpful as well. Good luck !

Steve m.... Ps. That 90 Pathfinder was a very nice design in the electronics dept. I really enjoyed figuring that one out !

"Lance Morgan" wrote in message news: snipped-for-privacy@posting.google.com...

Set the scope trigger on "auto", and adjust the level a little - you may be close to the edge of trigger.

Andrew

Steve m... wrote:

Somebody will correct me if I'm wrong here, but in my understanding, if you plug wires fail the wet arcing test anywhere then you've got a problem. This would easily explain the bad running you're experiencing.

Fix that first. If the boots on the new wires aren't on perfect, take them back and either get a replacement or maybe consider a different brand. Are they OEM or a well known brand? Are they "high performance" wires? Better wires help spark plugs fire w/o problems, but they don't fire better than OEM.

I hope this doesn't sound insulting, but it's very easy to break plug wires if you pull on the wire instead of on the boot. And even then it's not hard with some brands.

Bob

I've made mistakes w/the best of them, but pulling on the wires instead of the boots hasn't been one of them. All 2dary ignition components have been twice replaced w/OEM - no change w/bucking prob - and 2dary resistance is still excessive, w/problem across all 4 cylinders. But the root cause has been elusive, so far. Accel is strong from 3500 RPM to redline

I'm waiting on some inductive & capacitive o-scope p/u s for some more definitive testing (will be looking for pos or neg burn slope, stable and snap kV, etc.). Got a nice square crank (Hall-Effect) sensor pattern, throughout the RPM range, both cold and hot-soaked (probed so complete wiring harness was tested, too), thanks to some help from the NGs and some reading

I recently did a dry compression ck - wish I had had a transducer! - which was fine (not excessively high)

Lance