Our last home, noticed that the upper air pressure gauge was at 150 psi and the rear was normal(?)
Don't remember hearing the D2 pop off. Could that be our problem. Had it replaced a couple years ago.
Echoing what I have read suggested many times here - swap the gauge inputs and see if the problem moves with the swap or not. If it doesn't, bad gauge.
Dick,
Most of our coaches have a pressure relief safety valve on the wet tank. It is usually rated at 150 psi opening pressure. If you really had 150+ psi in your air system, or if the D2 failed to cut out the air compressor, then odds are the safety valve would open. There would be NO question in your mind if this happened - it is LOUD when it opens and blows down the tank! (Don't ask how I know...)
One gauge reading high is most likely a bad pressure gauge or pressure sender, depending on what is installed in your '99 model.