O-scope square waveform interpretation?

[Lance Morgan]

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

www.tequipment.net/LeaderLS8022.html

[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)

 

[DaveC]

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

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,

 

[Steve m...]

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 !

 

[Andrew Paule]

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

Andrew

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 !

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

www.tequipment.net/LeaderLS8022.html

[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)