autometer tach... body grounded ?

[Nacho-RT74]

a buddy of mine got an autometer tach 3708. He tested and worked fine. Then mounted on pillar post molding and is not working propperlly. I checked my tach 2892 what is basically the same tach, and black wire to ground goes DIRECTLY to bulb socket and not to tach chassis. I mounted below the dash frame so is attached to metal, then body grounded if needed.

maybe needs to be body grounded ? probably at his first try when got working he had the tach onto a metallic surface, making ground

he has MSD if makes some diff, what in this case I don't think so

[Just 6T9 CHGR]

All the aftermarket tach I have used had a separate black wire to connect to chassis ground

[Nacho-RT74]

not autometer!, unless I'm missing something on socket, making contact somehow with opening... I though on a socket tab linked to a black wire on socket, but trust me I didn't found anything linking socket wire with chassis.