Car won't start w/ tach connected to neg on coil

[Grec]

Just finished installing the RT Engineering tach board... get everything buttoned up... and nothing. 

Starter engages but engine won't fire. Disconnect the tach lead from the coil and it fires right up...

One last bit of info... tach was working fine before I took it in to have the motor pulled and rebuilt.

Places to start looking?

[Chryco Psycho]

the Tach feed wires is obviously grounding somewhere either in the wire to the tach or inside the tach

[Grec]

Forgive my ignorance... are you taking about the 12v in or the lead that goes to the neg coil?

[Grec]

A bit of new info...

I noticed that the #8 fuse was blown. Ask the board and found out that this fuse powers, among other things, the tach. Progress! Lol.

Replaced the fuse, turned key to ACC and Zap! Fuse immediately blows again. Disconnected the 12v lead from tach and the fuse quit blowing. (This was also killing my turn signals and rev lights, which are now working again.)

So, again... the tach was working fine before I had the engine built... wondering what they might have done to kill it... Would hooking up the lead to the positive side of coil by mistake have cause the internals of the tach to fry when they went to start her up? AFIK they never touched the dash so that's the only thing I can think of...