Hello.
Somethings strange happend with my friends 996 1998 when I activated the onboard computer with the PIWIS-2. Now the tachometer shows about 25/30% too much. Idle in DME shows normal but the tachometer shows 1100 rpm .
Any ideas about this issue ? It shows just the outside temp. no stalk mounted.
Have tried to turned it off again but still the same issue !

 

Try this: http://http://www.pelicanparts.com/techarticles/box_OBD_Mod/box_obd_mod.htm