Tachometer problems, any ideas?

[bec1488]

I drive a sohc '90 240sx, and for the past few days any time I got over 3-4k rpm with any amount of throttle my tachometer goes completely nuts. It bounces around between 8k,5k, etc regardless of engine speed. This makes the car hit fuel cut at 3k rpm, and generally makes it run like crap.

So far I've replaced the tachometer unit in the gauge cluster, the ecu, and the distributor to no avail. I'm quite stumped, and I'm hoping someone has ideas on how I can locate the problem. Any useful input is appreciated, thanks.

[180fan]

sensor at the transmission

[leper421]

Disconnect the connectors for the 4th and 5th gear sensors on the transmission and see if the problem still persists.

I had a similar problem at ~5k rpm on my 91. It was a bad 5th gear sensor. The ECU thought I was in 5th no mater what gear I was in (even neutral) and was rev-limiting the engine.