Diagnosis:

If the serial data line has been shorted to ground, the voltage at the Assembly Line Data Link (ALDL) will be 0 V. If it has been shorted to voltage, it will be over 5 V. If one of the components on the serial data line is affecting the data line voltage, the serial data voltage at the ALDL connector will be a steady value anywhere between .1 and 5 volts (e.g., 2.7V, 4.8V etc.). It has been found that a steady serial data line voltage of 4.7 to 4.9 volts usually means the BCM is the cause, and .1 to .3 volts usually means the ECM is the cause.