[hueyhoolihan]

for diagnostic purposes, you might be able to experiment using different batteries with different voltages to power the tach independently of the car's electrical system. i've got a few sets of small and large types of jumper cables that i've used to good effect over the years to diagnose electrical problems.

i also several batteries of various voltages. 6v, 8v, 9v, 12v, etc. that have been put to good use. just the other day i was installing a new cigarette lighter in my '41 and used my spare 9v multimeter battery to test it out, as i didn't have any wire under the dash at the time to plug it into. it got hot alright!