Code E026, Quad-Driver (QDM) Error: Notes

The ECM is used to control several components such as those illustrated in the mini circuit schematic. ECM controls these devices through the use of a Quad-Driver module (QDM). When the ECM is commanding a component on, the voltage potential of the output circuit will be low (near zero volts). When the ECM is commanding the output circuit to a component off, the voltage potential of the circuit will be high (near battery voltage). The primary function of the QDM is to supply the ground for the component being controlled.

Each QDM has a fault line which is monitored by the ECM. The fault line signal is available on the data stream for Scan tester equipment. The ECM will compare the voltage at the QDM based on accepted values of fault line. If the QDM fault detection circuit senses a voltage other than the accepted value, the fault line will go from a low signal on the data stream, to a high signal and a Code E026 will set if applicable.

Some QDM circuits will switch from low to high normally. Examples: QDM No. 2, if A/C pressure switch closes and turns on high speed coolant fan or QDM No. 3, if brake is depressed. These conditions are normal and no Code E026 is set. These are accepted conditions. A fault on QDM No. 2 will not set a Code E026, but diagnosis may be done by ensuring A/C is turned off. Some Scan tools will cause a false Code E026 to set if engine is running and service brake is depressed for over 30 seconds.

NOTE: Test numbers refer to test numbers on diagnostic chart.

Test Number 1) The ECM does not know which controlled circuit caused the Code E026. This chart will go through each of the circuits to determine which is at fault. This test also checks SERVICE ENGINE SOON light driver and SERVICE ENGINE SOON light circuit.

Test Number 2) QDM fault symptoms:

TCC inoperative, Code E039
EGR inoperative, Codes E063, E064 and E065
Coolant fan on low speed all the time or won't come on at all
Poor driveability due to 100 percent canister purge

Test Number 3) This test will determine which circuit is out of specification. All circuits, except YC11, SERVICE ENGINE SOON light, YC9, and hot light, should have battery voltage when key is on, engine not running. Diagnostic test terminal is not grounded Terminal BB1 is ECM ground.

Test Number 4) This test will determine if problem is the circuit or the component. As factory installed ECM is protected with an internal fuse, it is highly unlikely that ECM needs replacement.