Erratic tacho needle NA6 1990

[decitex64]

Hi
My tacho needle normally sits @ 3,250rpm when cruising @ 100kph in 5th gear.
Every now and then the needle drops to 2,750rpm for a split second and then reads the correct revs again.
The engine itself does not drop in revs, does not misfire and maintains the correct speed.
Tried cleaning some earth points in the engine bay and unplugged/plugged back various connectors last week.
It happened again this morning once on the Geelong Rd whilst driving to Apollo Bay; car ran fine otherwise all day.
Could a faulty Crank Angle Sensor be the cause?
Look forward to your comments / advice.
Cheers

[Garry]

Have you checked the earth points on the engine itself? Especially the one near the throttle body. I remember reading on Miatanet years ago about a bad earth point on the engine causing tacho problems and I think it was the one near the throttle body. Maybe a search on Miatanet will give you a bit more info.

Cheers

[93_Clubman]

might be worth checking the ECU as the NAs seem to be developing the odd electrical gremlin these days. from memory there have been a couple of NAs that developed a problem with the ECU earthing post, i.e. inside the ECU.

[Guran]

I recently noticed my tacho need jump suddenly in a similar fashion, and was wondering what it might be caused by. I've also recently replaced some old dead speakers with new ones and occasionally the righthand speaker won't work unless I up the volume (I suspect the connector is corroded). I'll be checking earth connections after reading this thread and also finding the following at miata.net.

http://forum.miata.net/vb/showthread.php?t=44128
http://forum.miata.net/vb/showthread.php?t=50425
http://forum.miata.net/vb/showthread.php?t=215350
http://forum.miata.net/vb/showthread.php?t=223193
http://www.rivercityroad.com/garage/jumpingtach.html

[NitroDann]

Haha, mines gotten very erratic lately. often oscillating up and down 200rpm. Maybe mine doesnt like spinning off the guage.

Dann