g body tach in my 78 chev truck
 

I got this firebird tach which fits nice in my dash and wired it free hand under the hood,hot and ground wire direct to battery and to hei ign. to see if it worked and it did. Now it doesn,t want to work hooked up to a hot wire under dash, it tries but goes back to zero.

Re: g body tach in my 78 chev truck
 

Exactly which "hot wire" did you hook it into under the dash? It might be worth double checking that wire to make sure it is supplying full battery voltage.

Also double check the connections in the signal wire going out to the distributor (the tach will normally read zero if it has power & ground but no signal connected).

Re: g body tach in my 78 chev truck
 

I just hooked it up to a hot wire I found with key on.

Re: g body tach in my 78 chev truck
 

Have you tried to reconnect it to the battery like you did at first to see if it would work there.
The tach is sensitive to changes in voltage like Ray said, so you might want to find a key hot source from the fuse panel that doesn't drop the voltage.
The tach connection on the distributor might not be good, so check that as well.

There are sites on Google that describe your situation if you type in "tach works but drops to zero" you may get some hits.

Re: g body tach in my 78 chev truck
 

OK thanks, I will try these tips