Code 26 - Quad-Driver No. 1 Error: Notes

ECM controls most components with electronic switches completing a ground circuit when actuated. Switches are arranged in groups of 4, called Quad-Driver Modules (QDMs), which can independently control up to 4 outputs (control module terminals). When an output is actuated, terminal is grounded and its voltage normally will be low. When an output is off, its terminal voltage will normally be high.

QDMs 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, too much current would be allowed into QDM. QDM senses this and turns driver off or QDM's internal resistance increases to limit current flow and protect QDM. Result is high output terminal voltage when it should be low. If circuit from battery voltage or component is open or 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.

Each QDM has a separate fault line to indicate presence of a current fault to control module central processor. A scan tester displays status of each of these fault lines as "low equals okay" or "high equals fault".

NOTE: Test numbers refer to numbers on diagnostic chart.

Code 26 will set if the ECM detects the wrong voltage potential for 20 seconds. This test will begin to determine if the QDM-connected air pump relay can be controlled by the ECM. If the relay appears to operate but the air pump does not turn, diagnose mechanical/electrical portions of air pump.
This check can detect a partially shorted coil which would cause excessive current flow. Excessive current flow to a QDM will be detected as a fault and set a code.
The remaining checks will identify a circuit problem that has caused an excessive current flow or inoperative relay. If a QDM circuit check is done on a relay, it is important to identify and test the relay coil terminals of the harness connector to avoid improper diagnosis. See DRIVE CIRCUIT TEST in CODE 28, QUAD DRIVER NO. 3 ERROR.