C-28 FI Error Code
I am getting a C-28 error code on my 2004 DL1000. That is a Secondary Throttle Valve Actuator (STVA) circuit malfunction which can be caused by a loose connection, open circuit or short, STVA motor malfunction, or a ECM malfunction. I have pretty much ruled out a loose connection, open circuit, or short. One of the diagonistic tests for the STVA motor is to measure resistance at the STVA lead wire coupler and it is supposed to be approx. 4.8 to 7.2 ohms. Mine measures 6.2 to 6.4 but since this output appears to be used by the ECM to calculate STV opening, it seems that there should be more of a difference between the fully open and shut positions. It also appears that I only get 1.3 volts from the ECM to the STVA when the key is turned on -- which then goes to zero as soon as the ECU detects an error condition.
In contrast, the voltage sent by the ECM to the Secondary Throttle Sensory Position (STP) (and most of the other sensors) is between 4.5-5.5 volts and the resistance values from the STP that are sent back (depending on STV opening) to the ECM are supposed to be between 1.1 K and 4.3 K ohms.
Do I have a defective STVA if the resistance values are within range but barely change between the open and shut settings?