Tach dropping out

[pyropyro]

2003 Outback, 75,000 miles (really was a little old ladys car) 

 

Automatic trans.

 

I've been driving it a year, typical problems, but runs great.  Yesterday, about 1 mile into a 6 mile round trip into town and back, I noticed the tach needle flitting about, dropping to peg, then working properly for a few moments.  Very eratic.  I pulled over and put trans in park, revved it to 4,000 RPM for a few seconds, and then it worked fine for the rest of the trip.

 

What might cause this?

 

I wondered about conditions.  It's been below zero at nights lately.  I've tracked snow onto the floorboards, and gotten a lot condensation on the inside of the windows which frosts over night, then when the car warms up turns to water to mopped off the inside of the widows with a towell.  Run the air condioner and heater on high with defrost. Shooting in the dark there, I doubt this really has anything to with the tach.

[TOONGA]

If condensation has gotten into the instrument cluster, then it could be causing a problem.

 

As well it could be the alternator or battery having problems.

 

TOONGA

[pyropyro]

Well, things have dried out in the cabin, humid weather is over for the time being.  The tach has behaved perfectly ever since that one incident a few days ago, so maybe that was it. 

 

Wer the alternator of battery causing the irratic tach behavior, wouldn't there be other symptoms as well?  (I'm not aware of any)

[nipper]

If it happens again (which is odd on this year, usually more a 1999 issue) you can easily remove the cluster, take it apart and clean the grounds.