I have an Autogage tach with serial number 233904 hooked up to a MSD 6A ignition and the tach will drop to 0 when starting but once the engine starts the tach goes back to the shift light setting. I went through all of the wiring and it all looks good. I also have been looking on MSD tech forums and checked the tach output terminal when the engine is at idle and the voltage is higher than what MSD said it should be. It should be at 1 to 2 volts and I get a reading of about 6 volts. I heard that Autogage tachs need the tach adapter but I would like to know for sure before I go and buy it just in case the tach output is messed up. I also tried hooking up a brand new Autometer tach and it just stayed at 0 which makes me think the tach output is messed up.

It should have worked with the autometer tach. Autometer tachs don't need a adapter. Check to make sure it was wired correctly (power,ground and sender) Yes you need a tach adapter with a autogauge tach. there are two different ones. Which one you need depends on which wires you are using to trigger the box, the white or the green/violet.