I have a 2000 blazer ls with optional instrument panel with built in tac. When I first start the motor the tac needle jumps but goes back to 0 and stays there . I don't know where to even start looking for cause of problem any help would be appeciated .

 

is the check engine light on?
did you check for any codes?

 

The service engine soon light is on. I just got this blazer a couple days ago had to work this weekend. I haven't had time to get it checked yet. I'll try to get to that monday .

 

The code po340 camshaft position sensor circuit malfunction. Would this cause the tac not to work? And what book or internet site would be good source to trouble shoot this circuit? Daaaa never mind found it on this site