Code 26, W Body, Quad-Driver Error
The control module controls most components with electronic switches completing a ground circuit when actuated. 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 actuated, 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, which depends on the brake and 2nd gear TCC switches.
QDM's are fault protected. If a relay or solenoid coil is shorted, having very low resistance, or if control side of circuit is shorted to voltage, it would allow too much current into QDM. The QDM senses this and turns the output off, or its internal resistance increases to limit current flow and protect the QDM. The result is high output terminal voltage when it should be low. If the circuit from battery voltage or the component is open, or the control side of circuit is shorted to ground, terminal voltage will be low, even when output is turned off. Either of these conditions is considered to be a QDM fault.
The control module ignores the fault line signal from QDM 3 to prevent false setting of Code 26 due to brake or 2nd gear TCC series switches. As a result, QDM 3 circuit problems will not cause a Code 26 although the circuits are fault protected and operate the same as QDMs 1 and 2.