Aftermarket tech issue

[65chevysub]

I put in an aftermarket tach in my 84 Sierra, 350/TH400, but it doesn't work right. At idle it doesn't read, but as soon as I start to drive, it goes to 3500 rpm and stops there. I have it connected to the TACH terminal on the distributor. What can be causing it to do this?
Thanks

[raceman6135]

Not all aftermarket tachometers can 'use' the trigger signal from an HEI distributor, and require an adapter like MSD part number 8920.

Find the instructions for whatever tach you've bought, which should show you a wiring schematic for the different ignition systems.

Since the engine seems to be running fine, the only thing distributor related that could cause your problems would be if the wire inside the distributor cap for the tach terminal could be broken / corroded. Otherwise, there's a problem with your tach or the wiring to it.

[65chevysub]

Thanks for the reply. The guy at our local auto electric shop thinks it may be a bad tach. I replaced the cap, rotorcoil, plugs and wires last year, so hoping that stuff is ok.