June 17, 2002 Speedometer Signal and Code P0505 The speedometer signal is an extremely important input for the Delphi fuel injection system. The ECM monitors throttle position and speed signal inputs in order to properly maintain the set idle speed. If the ECM loses the speed signal for any reason while the throttle is closed, the ECM assumes the vehicle is stopped and will adjust the idle air control counts to bring engine rpm to the set idle speed. This could set a P0505 code and adversely affect engine idle.