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O-scope square waveform interpretation?

I recently scoped the Hall-Effect sensor on my 91 Saab 900 (4 cyl, electronic ignition w/one coil, distrib, rotor), which has an narrow-band RPM related bucking problem

The pattern _appeared_ to be good to my untrained eye - just wanted to ask about one possible glitch

It essentially looks like the lower horizontal trace pair here

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[my new/used scope is a very similar predecessor of the above, btw, and am using 10:1 passive probes]

Quite consistent square corners (the vertical portion of the trace is missing, but I've been somewhat reassured that that's normal, given the short time of the switching event), w/some horizontal movement/creeping corresponding w/RPM changes.

But occasionally I get an clear/abrupt horizontal/left-to-right(?) shift

Is the square pattern all's that really important (input Vref is stable), to ck a H-E sensor, or should the immed above be cause for concern?

If not, I plan to proceed to getting some other appropriate probes to run the ignition system (knock sensor appeared to be good, but may also need to get some 1:1s to see better)

Reply to
Lance Morgan
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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,

Reply to
DaveC

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...

Reply to
Steve m...

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:

Reply to
Andrew Paule

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