Powertrain Controls - ECM/PCM

Driver Circuits - Quad Driver "A":

CIRCUIT DESCRIPTION:
The PCM is used to control several components such as those in the wiring diagram above. The PCM controls these devices through the use of a quad-driver module (QDM). When the PCM is commanding a component "ON," the voltage potential of the output circuit will be "low" (near 0 volts). When the PCM 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 PCM. The fault line signal is available on the data stream for "Scan" tool test equipment. The PCM 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.

Code 26 will set if:
^ The system is in Closed Loop mode.
^ The PCM detects an improper voltage level on a circuit connected to a QDM.
^ The above conditions are met for 5 seconds.

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

Example:
QDM "A" - If the brake is depressed. These conditions are normal and no code 26 is set. These are accepted conditions.

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

QDM FAULT SYMPTOMS
^ 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.
^ Improper shifting.

TEST DESCRIPTION

Code 26 Diagnostic Chart (1 Of 3):

DIAGNOSTIC CHART (PART 1 OF 3)

Numbers below refer to circled numbers on the diagnostic charts.
1. The PCM 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 related codes include codes 36 and 38.

Code 26 Diagnostic Chart (2 Of 3):

DIAGNOSTIC CHART (PART 2 OF 3)

3. This test will determine which circuit is out of specification. All circuits EXCEPT 1268, 1222, and 1223 should have battery voltage with the ignition "ON" and the engine not running (diagnostic test terminal NOT grounded.)

DIAGNOSTIC AIDS
^ Monitor the voltage at each terminal while wiggling the wiring harness and connectors. If the fault is created while doing this, the voltage reading will change. This may help to locate an intermittent fault.

Code 26 Diagnostic Chart (3 Of 3):

DIAGNOSTIC CHART (PART 3 OF 3)

4. This test will determine if the problem is in the circuit or the component. The factory PCM is protected by a circuit breaker, and is highly unlikely to be the source of the problem.