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Discussion Starter #21
Resolved!!! After 3 months of infuriatingly frustrating troubleshooting, the issue was (perhaps predictably) an unconnected ground wire, specifically from the ignition control unit. Because there was no ground there, the ignition control unit wasn't getting power, so the Hall Effect sensor wasn't getting power, so I guess the ECU wasn't getting the engine speed signal.

What I still don't understand: I was seeing 2-3VAC at Pin 1 of the ECU upon engine cranking, which the Bentley says means the ECU is getting the engine speed signal. Additionally, I was getting spark from the ignition coil, which I thought was an indication that the Hall Effect sensor was working, which is why I didn't chase down this issue earlier. If anybody is still following this thread and can illuminate anything about these factors, that would go a long way in resolving my lingering confusion, but I appreciate the input everybody had!

The car starts. It still needs some work to get it roadworthy, but hopefully we're downhill from here!! Fingers crossed
 

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Not to be a downer, but don't count your chickens before they're hatched. A disconnected ground could be the issue, but as you note some things worked in spite of the missing wire... so maybe it wasn't that missing

That said, the ECM requires a very consistent signal to operate, so it could just be the effective ground was intermittent. 2-3vac is an approximation of what the DC square wave signal looks like, but it's not the same thing. The Bentley test allows a mechanic to get a good idea of whether things are working with basic automotive tools, but it's not a slam dunk. These days, mechanics are equipped with scopes to do accurate testing of these type of electronics.- but they weren't in the '80s so this was the test developed to get close.
 
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