I replaced the ess on my 98 12 valve tack moves now but is not accurate. Is it a bad ess or something else

 

Did you gap the ESS properly? How far off is the tach, and how do you know (scan tool or photo tach?) ? The tach is not to be considered accurate - that's why the factory service manual says not to rely on it when setting idle speed.

 

At idle 400, at 75 mph bounces from 600 to 1200. Before tack went down at 75mph tack held at 2100

 

The fluctuation at higher speeds makes me wonder if there is a wiring problem. The wiring harness turns down at the front of the engine and can rub through there and cause a short.

I'm trying to remember offhand if my tach (not tack!) is marked as low as 400 RPM. Assuming you aren't touching the throttle pedal, is the 400 reading at idle fairly constant? And you're sure that isn't a (relatively) accurate reading?

 

It fluctuates even at idle and drops completely off at times

 

I'd be looking for a wiring issue, first. Check that spot I mentioned for a rub-through. Check along the length of the harness for other possible issues. Check at the connector to be sure you aren't intermittently losing the reference or ground. Loss of either of those ESS inputs means loss of output.

Do you have a factory service manual? The FSM will have the connector pinout and expected reference voltage (I don't remember if it is 5V or 8V).