Hi Chris,

It sounds to me like you are talking about the fuel injection harness, with the gray plug at the firewall. You are fighting an ignition problem; that harness is between the ignition control unit (ICU) and coil, with a branch to the Hall sender.

The symptom you report, where it sparks once when you turn off the key, indicates the coil is OK, but getting a constant ground to the #1 terminal. The tach behavior supports this. As you say it isn't a hard fault, and sometimes comes back to life, we might be thinking connection problems or a short in the harness.

The plug at the ICU is a known trouble spot, but the small wires and connector on the side of the distributor is even more infamous. What I can't tell you for sure is whether a shorted output at the Hall effect (frayed wiring there) would allow the ICU to keep the coil grounded constantly. Is the coil getting hot?

If you can get it to run again, wiggle the ignition harness to simulate the fault again. If not, examine those red, green, and black wires at the distributor, and the connector on the side.
--
Art Benstein near Baltimore

Women and cats will do as they please, and men and dogs should relax and get used to the idea. -Robert A. Heinlein