Erratic timing

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gasburner

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Hi,
I'm chasing a timing issue and I need your help.
The truck is a 93 silverado 3500drw, 2wd, 454 auto. Brand new stock rebuild on the engine by reputable shop, they've done 10 or so remans for me with great results.
. Initial startup is done but hasn't been on road yet to break in rings.
I have a video of the timing jumping around.--
Timing. Video taken with tan/black ECT wire disconnected.
Disconnecting ECT wire gives corresponding dtc code, which clears when re-connected. Distributor is new, old one had a little runout but same jumpy timing.issue.
Both connectors on ignition module replaced.
Harmonic balancer looks great. No runout, no shift from factory position, rubber not rotted, etc.
I'm a little bit stumped and could use your input. Thanks in advance!
 

1998_K1500_Sub

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You said the distributor is new but should I assume the timing module is new, too? You didn't say explicitly.

Does the timing advance (I think it's 20deg, and I know it's a fixed amount of advance) when you have the tan/blk wire disconnected and you rev the engine (I think it's got to go above 1200 or 2000 RPM to advance)? If so, is it still jumpy?

Did you try connecting the timing light to some other spark plug wire, e.g., #6 (the light should be stationary on the balancer regardless of which cylinder is used)?

Is the timing light being triggered by some spurious signal (spark jumping in the cap or between wires)?

Is the timing light known to be "good"... have you tried it on another vehicle?

Is the ground between the timing module and the ECM compromised?

Is the pulse from the pickup coil in the distributor to the module "good" (confirming this may require an oscilloscope)?

I did watch the video.

The image below is for reference, it's from a 1989 FSM.

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Schurkey

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As was so accurately said, the plug wires and the timing light are suspect, and should be verified. Screwy plug wires can cause the timing light to go crazy, especially if it's a dial-back "timing advance" light.

If the plug wire(s) and timing light check out, verify the ESC/knock sensor system, and the ignition module.

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That's a seven-pin module, used on older computer-controlled V-8s, and was still used on the craptastic 4-popper "Iron Duke" in the S-10. (Note the four poles on the pickup coil.)

The 8-pin module used on the TBI V6 and V8 engines is slightly different but operates in essentially the same manner. I'm surprised that this diagram doesn't have the one-wire timing connector on the Bypass wire illustrated.
 

gasburner

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Thanks for the replies.
I'll confirm theses things today.
Already tried a new module before I replaced the distributor, new distributor is a complete assembly (cap to gear) with module #3.
Plug wires are brand new, heavy blue streak wires with ceramic boots, routed and separated.
My wiring is slightly different than the above diagrams, having a 2 wire and a 4-wire connector. The 2 wire connector goes to the coil (non HEI). Grounds on thermostat housing and passengers side head were replaced during engine installation.
I'll post back here what I can find today. Thanks!
 

1998_K1500_Sub

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You must be registered for see images attach

That's a seven-pin module, used on older computer-controlled V-8s, and was still used on the craptastic 4-popper "Iron Duke" in the S-10. (Note the four poles on the pickup coil.)

Yes, unexpectedly the 1989 FSM I have covers the S10s and GMT400s and I just happened upon the wiring diagram for the 4-cyl S10's ign module while perusing it. I quickly grabbed the image and posted it w/o a thought about it being for a 4-cyl. It served its purpose :patriot:
 

Schurkey

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My wiring is slightly different than the above diagrams, having a 2 wire and a 4-wire connector.
Plus two more, kinda hidden, that the pickup coil plugs into. 8 pins in total for V8 and V6 in the TBI era. 7 pins for the four-popper, at least through '89.

The 2 wire connector goes to the coil (non HEI).
Pretty sure that when the truck was new, it was considered a version of the HEI system. There's no ballast resistor, and it was designed by GM who owns/owned the HEI trade name.

"HEI" is now obsolete, eradicated from the official language by the SAE who prefer "Distributor Ignition", as if points and condensers and ballast resistors never existed.
 
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