ECU Input: Diagnostic/Test
The DIAG (diagnostic) or TEST input is used to put the ECU into a special mode that allows a service person to calibrate the throttle angle sensor. The reason that the throttle angle sensor needs to be calibrated is because a fully closed throttle is purposely designed to NOT output zero volts from the sensor. The reason for that design decision (and it is a good one) is covered in the section on the Throttle Angle Sensor.
The DIAG/TEST wire is located under the right side cover just by the seat. After removing the cover, you will find two dangling wires that look like they should be connected together. People who find these wires and connect them thinking that they must have vibrated apart will surprised to that right side dashboard display starts displaying DIAG instead of the normal information. In addition to the DIAG message, there will be a "-1", a "0", or a "+1" on the display. This information is being displayed to allow you to calibrate the throttle angle sensor.
To calibrate the sensor, loosen its mounting screws and rotate it until the display reads '0'. At this point, the voltage that is being output is the voltage that the ECU desires to correspond to a closed throttle. Tighten the screws and the calibration is complete.
The circuitry that makes up the DIAG input is shown below. R601 and C601 form a simple filter. The two diodes clamp the input voltage to safe levels in case a user plugs in incorrect wire into the input socket. R602 limits the current in the cases where the input voltage is being clamped.