Timing randomly drops 4-8 degrees

[95 Tahoe]

Did some data logging and put it in spread sheet. Turns out the distributor reference pulse drops out randomly. Now to figure out why. It did similar things with 3 different distributors. The old one, a cheap replacement, then a better parts store replacement over the period of several months. I suspect there is a bad wire somewhere.

 

[95 Tahoe]

I'm glad I figured out how to get this in excel. Now I can plot what is happening. Note that the engine RPM's is on the right scale and everything else is on the left. Data labels at the very bottom. I dropped it in gear at about line 600 and it started running really rough. The reference pulse can be dropping in and out. That can be seen in light blue at the very bottom. 1 it occurred. 0 it hasn't. Dropped in a probably good ICM and it did the same thing. Therefore, the problem is most likely not with it. I assume that leaves wiring.

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[Erik the Awful]

My inner nerd is singing. Nice work.

 

[95 Tahoe]

I plugged my oscilloscope into the reference wire and it confirms the pulse dropping out as the ECM shows. I pulled the distributor and connected to the pick up coil lead an rotated the shaft by hand. It pulses consistently. Is 2 bad ICM's in a row possible?

 

[someotherguy]

Check the harnesses that plug into the ICM. They are known to get brittle over all these years.

Richard

 

[Road Trip]

Ignition as perpetrator? Or Ignition as victim of intermittent power feed from tired Ignition Switch?

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BTW, nice scope sleuthing! If you have your scope handy, possibly channel 1 on the power in and
channel 2 on those reference pulses? And if both go missing simultaneously, then you can
pursue the flaky ignition switch theory with confidence. And if this section of power is OK, then
I'd check the PCM power as well. Sometimes it isn't the parts or even the wiring, but just the
lowly ignition switch with countless mechanical cycles visited upon it.

FWIW I've included a couple wiring diagram pages for your viewing pleasure. Didn't want to wallpaper
this thread, but if you have downloaded a TBI-era FSM for the electrical & emissions stuff there's
a lot of good info in there.

Best of luck. Rootin' for ya!

Just some troubleshooting food for thought.

 

[tayto]

have you monitored knock counts? maybe it's dropping timing because of knock

 

[95 Tahoe]

Ignition as perpetrator? Or Ignition as victim of intermittent power feed from tired Ignition Switch?

You must be registered for see images attach

BTW, nice scope sleuthing! If you have your scope handy, possibly channel 1 on the power in and
channel 2 on those reference pulses? And if both go missing simultaneously, then you can
pursue the flaky ignition switch theory with confidence. And if this section of power is OK, then
I'd check the PCM power as well. Sometimes it isn't the parts or even the wiring, but just the
lowly ignition switch with countless mechanical cycles visited upon it.

FWIW I've included a couple wiring diagram pages for your viewing pleasure. Didn't want to wallpaper
this thread, but if you have downloaded a TBI-era FSM for the electrical & emissions stuff there's
a lot of good info in there.

Best of luck. Rootin' for ya!

Just some troubleshooting food for thought.

I appreciate the thoughts and the diagrams. My scope only has 1 channel. However, I can I will tie into coil power wire and see if I see the power dropping out. If, so I may run a hot wire directly from the battery to the coil + and see what happens.

 

[Road Trip]

I appreciate the thoughts and the diagrams. My scope only has 1 channel. However, I can I will tie into coil power wire and see if I see the power dropping out. If, so I may run a hot wire directly from the battery to the coil + and see what happens.

That's exactly what I would do. If you temporarily hot wire the ignition and get it to run
2-3x the failure interval with no more problems, then you will know with confidence what has
to be done. AND if hot wiring does NOT change the behavior, then you'll also know with
confidence that the ignition switch is not the problem.

Either way it's well worth the extra step to do so.

Looking forward to your update with what you discover.

Good stuff!

 

[95 Tahoe]

That's exactly what I would do. If you temporarily hot wire the ignition and get it to run
2-3x the failure interval with no more problems, then you will know with confidence what has
to be done. AND if hot wiring does NOT change the behavior, then you'll also know with
confidence that the ignition switch is not the problem.

Either way it's well with the extra step to do so.

Looking forward to your update with what you discover.

Good stuff!

I put a pin in the back of the coil on the 12+ in. I attached my scope to it and ran it for about 10 minutes. I never saw a drop in voltage. I think the supply voltage dropping out is a dead end. It was worth a shot. Tomorrow I'm going to investigate the ECM power dropping. I'm waiting on a current clamp so I can check the coil wire to the distributor. Later next week I may try to find a mechanic that knows more about TBI engines than I do. If I do not come up with anything soon I am going to put a Holley and HEI distributor on it. I know how to make the conversion just don't know if I want to do it.