I dream of rain

The problem child, two weeks later

The truck's still out of commission, and the mechanics are stumped.

The runs-then-it-doesn't problem happened to them too; after they replaced the distributor and the computer (the first time), it ran fine (if a little rough), only to refuse to start the next morning when they tried to adjust the timing.

Two more computers and a consult with a heart surgeon later (seriously, they called in someone who specializes in diagnosing computer issues), they're pulling their hair out. At this point, all signs point to a short or broken wire that feeds into the computer.

Armed with the only meaningful symptom (the computer isn't generating the reference signal to the distributor) and the list of stuff they've already checked, I've spent quality time tonight with the GM shop manuals, enumerating all of the wires in the computer's harness. There are a total of 56 wires across two harnesses, of which 21 appear to be unused. Only a handful of them are actually inputs into the computer:

• Ground signals (two)
• Battery supply (two)
• Ignition "on" signal (unlikely, as the fuel pump kicks on and the check engine lamp illuminates)
• Throttle Position sensor (No signal == no run!)
• Manifold Absolute Pressor sensor (unlikely; should result in running rough rather than simply not working at all)
• Engine Coolant Temperature sensor (No signal == no run!)
• Cranking signal (unlikely, but worth checking)
• Knock sensor (also unlikely, but again, worth checking)
• Fuel Pump run (unlikely, as it's normally off when cranking)
• Oxygen sensor (irrelevant)
• A/C signal (irrelevant)

The good news is that testing these wiring runs is actually pretty simple; all you need is a multimeter, a length of wire, and some time. Assuming the wiring is all good (unlikely!) the sensors themselves need to be tested.

The ECT sensor itself is easy, as it's just a measurement of resistance. The other sensors are more involved, needing a reference signal. It's a shame the truck is locked up in the mechanic's lot over the weekend; I have the tools (and time) do all of this grunt work myself.