Quote Originally Posted by spfautsch View Post
I performed a few tests tonight, and think I've been laboring under flawed assumptions.

I wasn't able to determine pins readily at the opti pigtail connector, and though my 22 year old harness is a bit discolored I wasn't seeing anything similar to blue so I just determined the pins by trial and error. With a DMM I found ground and +12v with key on. The other two pins went to +5v which I made the assumption were the ECM's opti inputs. I removed the male connector end from the pigtail side of the connector.

The first one I disconnected appeared to be the low res pin, because it cranked but nothing was happening - no RPM change in the ALDL, and it immediately set DTC16.

I cleared the DTC, reconnected low res and disconnected high res. This time i was seeing RPM changes in the ALDL data, but no codes, no fire, nothing.

I would love for someone to confirm or disprove. I wonder if the presumption that the engine would fire without high res signal came from the pre-94 LT1s that were batch fired and didn't need to sequence anything. In the case of my having witnessed it start with the high res code, it would have to have been a case where the high res phototransistor was working well enough during cranking for the ECM to determine sequence. As I think I mentioned, mine would only re-set the code when the engine got to temperature.

What I'd like to find out is what happens when the high res signal is removed while the engine is running, but I don't currently have the connectors to allow this without cutting wires I don't want cut.

At any rate, I'm going to labor under the assumption that we need high res to determine sequence. I hope to post a sketch soon.

That`s happening because when high res dtc is set eside processor is trapped at infinite do nothing loop.
If you unset high res dtc bit at 12029 bit $10, you may try again runnig with high res disconnected.
I am pretty sure it will at least fire up and all the calculations will be time based on the low res signal. It will work just like the older distributor system with coil pick up signal. Of course spark scatter will be higher and timing will be not very accurate and we will lose the great starting time of less than a second.