Just an update for anyone that somehow has this same issue in the future. I replaced the ECM and it seems to be working perfectly fine now. All of the issues mentioned before are gone. There was no visible damage to the ECM, but it definitely was the problem.
Greetings sdryluth, belated Welcome to the GMT400 forum!
I know I'm late to this thread, but first of all thanks for coming back and
sharing the final fix that got your '90 K1500 to straighten up & fly right.
If everyone who joined and asked a question would follow your lead,
then this forum really would be a repository of the 'tested-good
troubleshooting strategies'.
Reading through your thread, I had the following questions pop up:
Q1: Do you still have the flaky ECU? Or did you exchange it?
I ask this because the observed behavior mimicks what a realtime
computer would do if the power required for proper operation was
dancing just above & below the threshold where it initializes and runs, versus
shutting down as the power crossed through the threshold.
I currently do not have the documentation on the processor inside your
ECU, but in general terms there would be a microsequencer that has
several inputs to it. One signal of interest is the one that it would continuously
monitor whether or not the +5v DC power has crossed the threshold
that allows initialization and start fetching the program out of memory and
start instruction decode, etc.
In English, we're finally at the point in the ECM's lifecycle where the weakest
link internally in the ECM (electrolytic capacitors) are drying up, losing their
ability to provide their rated capacitance (electrical storage) ...and this leads
to the +5v voltage levels to start sagging. And once this critical voltage sags
below the threshold, the computer will shutdown.
In a perfect world, the computer would act like an light bulb. It either works
or it doesn't. A binary failure mode if you will. But because the designers
wanted the computer to initialize and run without any additional steps
required by the driver, once +12V is applied to the ECM via the ignition switch,
it comes up automatically.
In a way, between the marginal capacitors + any RFI picked up & riding on top,
this can give us a 'picket fence' version of DC power, and between all of the
above the computer is responding as if the driver was switching the key on &
off very rapidly.
This is how I would explain the flickering SES/MIL/CEL light. If the computer
is cycling on and off rapidly this is how I would expect the light to look. The
cool thing is that with a 2-channel oscilloscope we could monitor the +5v
line vs the SES light, and see if there is correlation. No guessing required.
And of course rerun this check after the new caps are installed.
Q2: All of the above is to set the table for this. IF you still have the original
computer, would you be willing to open it up and look for evidence of leaky/puffy/failed
electrolytic capacitors. And if you found evidence of this, you would take some
sharp photos & post them here?
Q3: IF you found failed caps in Q2, would you be willing to replace them & see
if the old computer could be restored to fully functional status? (If you don't
normally work on electronics, possibly send the computer & caps up to me
and I'll give it a shot?
****
My theory is that thanks to the quality of the components used in the GM
ECM (ECU/PCM/VCM/etc) just about all of the internal components have an
indefinite useful lifespan.
On the other hand, the electrolytic capacitors have a finite lifespan, they lose their
functionality, and when the internal power regulators that rely on these can't
maintain a steady voltage, these types of failure occur. (
ECM cap failures)
If you still have your bad ECM, possibly we can come up with a fix. You would
end up with a 'known-good' spare on the shelf (the new part) ...while running
your vehicle on the repaired unit.
And meanwhile, this forum will end up with some valuable info that could help
us keep these machines on the road and out of the crusher for a fraction of
the cost of buying an entire ECM.
Just a thought. Again, thanks for coming back and sharing the fix. Good stuff!
Cheers --
