The same thing happened to me on an 02 Travel Supreme with the ISL 370 engine. I took it to Cummins and they found that the factory sending unit was bad. A new sending unit was installed and the ECM (electronic brain) was reprogrammed for the new sending unit. After the new sending unit was installed the dash mounted reads a constant 55 pounds of pressure, in effect turning the gauge into an "idiot light". According to Cummins and Spartan the new sending unit is incompatible with the ECM (?) and the installed gauge so I now have to install an aux gauge to replace the gauge in the dash.
According to Spartan this problem will also crop up in some Dodge Ram Pickups.
Hopes this helps
George Huggins
