I installed a Sun Super Tach II on my '69 396 (w/Accel dual point) Chevelle and it worked fine for 5 minutes at idle but I noticed after I pulled it into the garage the tach was showing 0 (while the engine was running). The lights still work inside and all the connections are tight, so I know it has power. What else could cause this? Is there a way to test the tach? What would fry one? Thanks.