You are wrong about the wirring, and it could blow the tach.

The two white wires go in between the coil and distributor - to replace the single low-voltage wire - this is how the tach get the count.

Tach has 5 wires:
- ground (black)
- power (also black! on true Volvo ones)
- two whites, one a bit shorter than the other
- the short goes to the coil
- the long goes to the distributor
- power to the little light-bulb (white)

I'm ALWAYS talking from an original point, and tons of SMITHS tachs are out there - some with adjustable function to accomplish both 4/6/8 cyl. etc. Genuine Volvo tachs are NOT multifunctional and can not be adjusted or calibrated to anything other than what they where ment for.