Question about '7427 with large cap HEI tach signal.

**mihela** · 11-15-2016

I've started the process of building my wiring harness for my BBCad pickup project. I have installed a 7 pin module from a 94 Deville into the original '76 500 large cap HEI distributor. I want to know what to do for a tach input to the PCM. Do I use the tach output on the coil, or is it supposed to be the connection labelled "reference" off of the module? It seems to me that the tach output may damage the PCM. If it is supposed to be the "reference" signal, what do I do with the tach lead on the coil, do I leave it unhooked?

**dave w** · 11-16-2016

Maybe the information on this site will help? http://chevythunder.com/ignition_sys..._operation.htm dave w

**lionelhutz** · 11-16-2016

You should have 4 wires from the PCM to connect at the distributor. Which module did you use?

The harness should have a 4-pin connector that would plug into the larger connector on module "B" in the link Dave provided. A remote coil setup also has a 2-wire jumper that would plug into the other connector (it connects between the module and the remote coil). If you use module "B", you can simply plug the harness connector into it. You can also route the 2 wires from the jumper harness to the coil in the cap instead of a remote coil.

If you use the module "A" then you have to take the B, R and E pins to the PCM wires as well as a ground connection for the 4th wire. You'll have to come up with a connector for it since you won't want to wire it directly.

**mihela** · 11-18-2016

The two wire connector to the cap is pretty obvious. The other 4 I was planning on running to a 4 pin square weatherpack connector in place of the big 6 pin flat weatherpack. So I take it my tach signal to the PCM is the pin labelled "reference" as I asked.

**uncabob** · 11-18-2016

Originally Posted by mihela

It seems to me that the tach output may damage the PCM. If it is supposed to be the "reference" signal,what do I do with the tach lead on the coil, do I leave it unhooked?

Yes. The input to the PCM will be "reference". The "tach-lead" is used to feed your tachometer not your PCM.