Posessed Tach!

[reignofone]

just yesterday im cruising down the freeway, 65mph 1800rpm when out of the blue the tach jumps to 4100rpm for 4 or 5 seconds, then goes back to normal for 4 or 5 seconds, then back to 4100rpm. its constantly does this but without any problems from the engine whatsoever, it runs and drives fine! any ideas on what this might be?

 

[KENS89LX]

factory tach?

 

[kingmeirl]

loose ground to the coil, mine does the same thing

 

[reignofone]

yeah factory tach... you really think thats all it is? where do i tighten her up at? which ground wire as there are 50 under the hood

 

[1987stangman]

just yesterday im cruising down the freeway, 65mph 1800rpm when out of the blue the tach jumps to 4100rpm for 4 or 5 seconds, then goes back to normal for 4 or 5 seconds, then back to 4100rpm. its constantly does this but without any problems from the engine whatsoever, it runs and drives fine! any ideas on what this might be?

When I bought by 87' it had that same issue and ended up having to buy a whole instument cluster just to get the RPM side of it out to replace the one in the car which solved my problem.

 

[kingmeirl]

oh, im not sure about the factory tach. mine is aftermarket and it doubles the rpm reading when the ground to the coil is loose