I have a 2004 Thomas flat nose, front engine bus with a Cat 3126 engine and Allison AMD 3060 transmission. When idling and in neutral (i.e. not moving) the speedometer needle bounces between 5 MPH and about 40 MPH. Once I start moving the speedometer calms down and performs as it should.
I'm not sure if it's relevant, but the ammeter never moves (always points to 0), so I'm pretty sure it's dead. All the other gauges work fine.
Does anyone have any suggestions about what is wrong or how I can start debugging?