Code 27 - Quad-Driver No. 2 Error (5.7L "F" Body): 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". A fault in QDM No. 2 will store a Code 27 in memory, but will not turn on the SERVICE ENGINE SOON light.

1. Code 27 will set if ECM detects an improper voltage potential for 26 seconds. This step begins to determine if the QDM-connected devices can be controlled by the ECM.
2. This test 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.
3. The remaining checks will identify a circuit problem that has caused an excessive current flow or inoperative solenoid. If a QDM circuit check is done on a solenoid, it is important to identify and test the solenoid coil terminals of the harness connector to avoid improper diagnosis.