The Tachometer operates off of your alternator and guages RPM via output.
Causes offluctuation could be:
1). Loose Connection. Clean and tighten on rear of alternator.
2). Loose or improperly routed Serpentine Belt. Check for tension and routing. A routing diagram is located on
www.atlanticbritish.com under Tech Tips.
3). Failing Alternator. Check output, should be 14 I believe. The Tach acts as a sort of "canary in the mine" for the alternator.
This is based on the assumption that you have a Disco.
Others may have more to add, best of luck on a fix.
