okay this is a weird one...

[Shirley]

The past 2 days, whenever I just start my car or when the turn signals are operating the tach needle jumps frm idle to 1500, then idle to 1500. When I am driving or not using the turn signals, everything is normal. What in the hell is the deal with this?

 

[BlownFiveLiter]

Is it just the tach needle jumping, but not the engine speed increasing? How does your voltage look at the battery and at the alternator? I see you have a relocated battery, how good is your ground, and what size cables did you use? Take a look at all your connections for corrosion, and make sure your grounds are good.

 

[Shirley]

batt voltage is steady, yes it is just the needle and no increase in engine speed. I have 4 grounding points, all are good. 1 computer ground, and 3engine and frame grounds.

 

[Black1987Stang]

My brothers 84 f250 with a 351W does that too except it goes up to like 3,000 rpms lol My guess is bad ground or a break in a wire and its using the ground from the tach to ground through instead of its own. I dont know

 

[JB66]

Try the ground wire to the instrument cluster or the tan/yellow wire that goes to the TFI module. Also, possibly corrosion on the connector.