Part 1 of 3

Code 26 Chart Part 1 Of 3:

Wiring Diagram:

CIRCUIT DESCRIPTION:

The ECM is used to control several components such as those in the wiring diagram above. The 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 0 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" tool test equipment. The ECM will compare the voltage at the QDM based on accepted values of the 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 26 will set if applicable.

NOTE: Some QDM circuits will switch from "low" to "high" normally.

Examples:

QDM 2 - If A/C pressure switch closes and turns "ON" high speed coolant fan.

QDM 3 - If the brake is depressed.

These conditions are normal and no code 26 is set. These are accepted conditions. A fault on QDM #2 will not set a Code 26, diagnosis may be done by making sure the A/C is turned "OFF."

NOTE: Some Scan tools will cause a false Code 26 to be set, if the engine is running and the service brake is depressed for over 30 seconds.

Code 26 will set if:

^ System is in "CLOSED LOOP."
^ ECM detects an improper voltage level on a circuit connected to a QDM.
^ The above conditions met for 20 seconds.

NOTE: Because of all the possible color code combinations used on electrical wiring diagrams, always refer to SCHEMATIC DIAGRAMS/ELECTRICAL AND ELECTRONIC WIRING DIAGRAMS/ECM CONNECTOR IDENTIFICATION for correct color code identification of circuit.

TEST DESCRIPTION: Numbers below refer to circled numbers on the diagnostic chart.

1. The ECM does not know which controlled circuit caused the Code 26 so this chart will go through each of the circuits to determine which is at fault. This test checks the "SERVICE ENGINE SOON" light driver and the "SERVICE ENGINE SOON" light circuit.

2. QDM symptoms:

^ TCC -Inoperative - Code 39.
^ EGR - Inoperative - Codes 63, 64, 65.
^ Hot Light - "ON" all the time/"OFF" during bulb check.
^ Coolant fan on low speed all the time or won't come "ON" at all.
^ Poor driveability due to 100% canister purge.