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.

Richard,
The GM 7747 ECM used in the Howell (and EBL) was designed for use with a CCD for spark control. The output of the CCD module to the ECM is a 5V pulse so that is what the ECM is designed for. Howell designed a little board to reduce the 12V pulses from either the points coil or the HEI Tach connection terminal down to 5V. The Howell board might work with a 5V pulse input, but I would not think it would. Its just a Schmidt trigger NAND gate with a voltage divider at the input (if I recall). When you pulled the 4 wires required for the CCD, you likely pulled out the single Tach wire with the circuit board in heat-shrink from the wiring harness.

The Tach connection for the CCD distributor is the same for both standard and ESC distributors. Was it the CIP tach that was reading 4000RPM intermittently or the Tach on the EBL What's up Display (WUD)? If it was the WUD, there is a filter to smooth the electrical noise from the terminal. Look for "RPM-Filter for 12.5 RPM, mine is set to 81.57%

FYI, if you a Vehicle Speed Sensor (VSS), there is a noise smoothing filter for it as well. A couple of years ago I noticed my engine would randomly seem to cut out, so with the WUD I noticed the Speed indicator was jumping by 20-50MPH. Turns out the speed limiting was coming ON even though it was set to some high speed. I replaced the reed switch in the VSS, then Randy Van Winkle pointed out the filter setting called "VSS- Filter, Filter %" mine is set now to 75.29 .