I have a tachometer that I believe fits an early 65-68 Buick with the Consolette. It has 6412531 printed on the back. How do I test it?
The case is grounded to the chassis. The blue wire to negative post on coil, white wire is light. Test on any 12v coil ignition.
You will have a hekkin' time trying to create the collapsing field of a coil in rapid succession that the tac transformer samples to energize the field windings that swing the needle. Or more simply, I don't think you can do it without a pretty sophisticated bench setup. That does not mean someone hasn't done this because they were told it was a lot of work.
Scott Moody (87GN_70GS) can test it and if necessary repair it. PM him. https://www.v8buick.com/index.php?members/87gn_70gs.1236/