Now here is an example of where I think I'm missing the original question from the email side. I only use the web-based forum.

This is the first message in the thread that I see from Emery:

I know the tach wire goes to the end of the distributor but I cannot find it. Can someone tell me the color?

It is not shown in the wiring diagrams.

Emery Stora
emerystora@mac.com


So I'm not sure what Emery is hooking up... is it just a Tachometer, or is at an EFI system, or is this some piece of test equipment. Regardless, the tach connection is usually to the points side of the ignition coil. On HEI systems, the TACH connection is marked on the distributor cap and the connector is parallel with the HEI module's open collector output to the coil. OEM, the GMC had nothing connected to the TACH point.

Be careful that what you are hooking to the Tach connection is compatible with 12V switching to ground. In the case of a Howell EFI, there is a small circuit board in the wiring harness that level shifts the 12V Tach signal to 5V. This circuit board gets removed when a spark-controlled distributor is added because ESC output is 5V.