Powertrain Controls - ECM/PCM

Code 26 Circuit Diagram:

CIRCUIT DESCRIPTION:
The PCM controls several components through the use of a "Quad-Driver" module integrated into the PCM. Most devices, such as solenoids or relays, receive battery power through a fused circuit to the ignition, and turn "ON", when the circuit is completed to ground. The "Quad-Driver" consists of several electronic switching circuits that provide the ground connection to complete these circuits and turn the devices "ON".

Each "Quad-Driver" has a fault line that is monitored by the PCM. When the QDM switch for a particular circuit is "OPEN", the voltage at the PCM terminal for that circuit will be high (near battery voltage), and when the device is turned "ON" (circuit completed to ground), the voltage will drop to zero or near zero volts. When a device is commanded "OFF", the PCM expects the voltage in that circuit to be high, and when a device is commanded "ON", the PCM expects the voltage in that circuit to go low. The QDM fault detection circuit monitors the voltage in the sense line for each device controlled by the QDM. If the QDM fault detection circuit "sees" a voltage other than what it expects for a device in a particular "ON" or "OFF" state, the fault line status will change, and a Code will set. The fault line signal status can be displayed on a "Scan" tool.

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.

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.

Code 26 Diagnostic Chart (2 Of 3):

DIAGNOSTIC CHART (PART 2 OF 3)

2. 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. Check for bent pins and PCM connector.

Code 26 Diagnostic Chart (3 Of 3):

DIAGNOSTIC CHART (PART 3 OF 3)

3. 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.