72 Tach issue with HEI

[brandnew72]

Just rewired the truck, everything works except the tach. Drives great, all things functioning. The distributor cap has a Batt and TACH terminal. Ignition to BATT and the green wire from the tach itself hooks to the Tach terminal. No rpm displayed. Tach lights up, needle goes through it's preset motions and then displays 0 rpm.

Has a signal filter, just installed, still nothing. Glowshift gauge, set to cylinder setting 8.

Should their be varying voltage on the Tach terminal if read with a multimeter? Just trying to narrow it down because it seems so simple but doesnt function correctly. The wires can only go so many places.

[Dashman]

Are you using the old coil wire? Did you replace it with 12 gauge back to the bulkhead harness? Without the tack are you getting 12 volts on the power wire?

[PbFut]

Two points in above two posts. First the HEI is likely providing a square wave 12 volts for the tach feed, or whatever it is seeing at the battery connection. If you put a multi meter on that tach feed wire it is going to jump around relative to the point it samples on the sqare wave.
Second point. That Batt connection on your HEI needs a good 12 volts. If you used the yellow wiRE from the factory harness, it is a resister wire and will be providing about 9 or 10 volts while your system is at 12 to 13. If ordered correctly, an aftermarket ignition harness has aready been swapped. This yellow wire is a resister wire designed to protect the original factory coil. That reduced voltage does not play nice with aftermarket HEI or ECU requiring the wire to be replaced with 12 gauge copper auto wire. Some aftermarket techs require a shunt resister on the tach feed. Glowshift tech support can walk you through that if need be.

[brandnew72]

Man yall are quick, lol.

I used an AAW harness, Classic Update. Their diagram has the pink ignition running to batt and I verified it's receiving 12.8v when keyed.

As far as a shunt resistor, Im unfamiliar with that. I did install Glowshift's signal filter, to no avail.

[brandnew72]

Spoke with Glowshift again and the 2nd fella I talked with said 99.9% chance it's a faulty tach. Even without the online signal filter, he said something should happening after it powers up. After discussing what was checked and such, this conclusion was made.

So returning it and getting a new one sent out.