so on the way home today my tach was jumpy (engine was normal) and at one point reading 0rpm.
Its hot as hell today - had the AC full blast - engine temp was normal when this happened and didnt change.

turned off the ac and it continued breifly but seemed to clear up after.

maintenance as of late:
recent wtr pump install (the one with the brass impeller)
idler bearing already replaced
new serpentine belt

a bit of a 'whine' coming from alt or AC comp at idle
nothing visibly obvious

109k V8

uhg I hope its not the AC comp.
any ideas?

thanks

 

The tach signal comes from the alternator and when it jumps like that the alternater is going bad. But I would check the wires on the alternater first to make sure you did not knock the tach wire loose when you were working on the water pump. If they are good then replace the alternator.

Also make sure your belt is routed the correct way.

 

thanks - yeah good point Ill double check the wires tonight. I suspect its the bearings seizing as its been whining for some time.
Belts good.