Have had a similar issue on my '96 Explorer XLT. Mine usually jumps up, not
down, with no noticeable change in actual engine RPM. Am thinking it has
something to do with the in-dash parts as a good whack on top of the dash
board (the old fix it with a hammer trick) usually settles it down for a
while. When I pull the instrument cluster to replace some burned out bulbs
I'm going to do some snooping around and see if I can spot any loose
connections or broken solder joints. Mine's been doing it for a couple of
years now, hasn't gotten any worse, nor any better either. I notice it
mostly on the highway, when I glance down and happen to see the needle jump
up or down, likely because familiarity tells me where the tach should be at
that speed. But have seen it do the same thing in park at idle.

Steve

"MACE" > wrote in message
...
>
> OE Tach on 2000 Merc MTY 5.0 acting erratic. Jumps around while driving,
> drops to 100-200 RPM at idle, but actual RPM's not noticibly lower. Kind
> of seems like its almost twitching, and fading away so to speak. Is
> there a sensor involved that I can check, or am I going to have to
> dismantle my dash? Also, is it possible that the TPS sensor or
> something like that might be starting to go? No service engine lights
> on either..78k miles.
>
> Anyone have any experience with this type of issue?
>
>
> --
> MACE