My tach just started bouncing two days ago. Not by much, 100 RPM or less, happens at all engine speeds, cold or hot. Ultra Guage is fluctuating as well. Engine sounds normal, getting the same fuel economy. Driven 300 miles and several start ups, so I'm not having an alternator issue I don't believe. I'm going to pay more attention to the UG this afternoon to see by how much it is varying. No codes.

 

If the engine sounds steady, check the connection of the little white wire on the back of the alternator, clean the crud off the contacts.

 

Also check voltage, tach problems also indicate alt dying

 

ditto

 

Must have been just the lead on the back of the alt, cleaned it and all is good now.