Part 2 of 3

Code 26 Chart Part 2 Of 3:

Code 26 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 set.

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

3. This test will determine which circuit is out of specifications. All circuits EXCEPT "YC11", the SERVICE ENGINE SOON LIGHT (SES) and "YC9", the HOT LIGHT should be B+ when the key is in the "ON" position, engine not running. The diagnostic terminal is not grounded.

DIAGNOSTIC AIDS:

^ Monitor the voltage at each terminal while moving the related harness connectors, including the ECM harness. If the failure is induced, the voltage will change. This may help locate the intermittent. Check for bent pins at ECM and ECM connector terminals. If code re-occurs with no apparent connector problem, replace the ECM.