[Dave at Pelican Parts]

The seatbelt interlock relay has a lot of things that run *to* it, but not much that actually goes *through* it. Most of what does relates to the specific seatbelt-warning related systems.

The only thing the seatbelt relay gizmo does that is of any interest to the running of the car is that it allows the current to go from the ignition switch to the starter motor. That's it. It acts as a big switch in the big yellow wire from the "crank" position on the ignition switch to the starter solenoid.

I suspect that the unsheilded bus mentioned by JP was also shorting out some of the wires that go to the relay but not *through* it.

The tach doesn't have anything to do with the interlock system. The tach signal wire goes from the tach, to the relay board, through the board (doesn't connect with anything else) to another wire that runs to the (-) side of the coil where the green points wire connects. The tach can fail and internally short its input to ground, which means the points have no effect and so no spark.

The fact that the tach goes straight to zero indicates to me that you are losing spark. It probably isn't the tach causing it, but unplugging the tach signal wire from the coil is VERY inexpensive and easy to do.

One thing that often gets overlooked: Check the ground braid inside the distributor. It should go from the points plate to the distributor body, and provides the ground path for the points. If it disconnects, you will lose all spark. See if you can move it and disconnect it.

Someone on this forum just recently reported this problem.

--DD