Code 28, Quad-Driver 2: Notes

PCM controls most components with electronic switches which complete a ground circuit when turned ON. These switches are arranged in groups of 4, called Quad-Driver Modules (QDM's) which can independently control up to 4 outputs (control module terminals). When an output is ON the terminal is grounded and its voltage normally will be low. When an output is OFF, its terminal voltage normally will be high, except for the TCC, as noted below which depends on the brake and second gear TCC switches.

QDMs are fauly protected. If a relay or solenoid coil is shorted, having very low or zero resistance, or if the control side of the circuit is shorted to voltage, it would allow too much current into the QDM. The QDM senses this and the output turns OFF or its internal resistance increases to limit current flow and protect the QDM. The result is high output terminal voltage when it sould be low. If the circuit from positive battery voltage, or the component is open, or the control side of the circuit is shorted to ground, terminal voltage will be low, even when output is commanded OFF. Either of these conditions is considered to be a QDM fault.

QDM 2 has a fault line to indicate the presence of a current fault to the PCM's central processor. A scan tool displays the status of the fault line as LOW is okay and HIGH indicates a fault.