Powertrain Controls - ECM/PCM

Diagnostic Trouble Code 26:

Quad Driver Controlled Circuits:

CIRCUIT DESCRIPTION
The Engine Control Module (ECM) is used to control several components such as those shown in the accompanying image. The ECM controls these devices through the use of a Quad-Driver Module (QDM). When the ECM is commanding a component "ON," the voltage 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 quad-driver module is to supply the ground for the component being controlled.

Each quad-driver module has a fault line which is monitored by the ECM. The fault line signal status can be displayed on the Tech 1 scanner. The ECM will compare the voltage at the quad-driver module. If the quad-driver module fault detection circuit senses a voltage other than what is expected, the fault line status will change and a Diagnostic Trouble Code (DTC) 26 will set if applicable.

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

1. DTC 26 will set if: the ECM detects a low voltage for 5 seconds or a high voltage for 25 seconds.

This test will begin to determine if the quad-driver module connected AIR pump/bypass valve can be controlled by the ECM. If the relay/solenoid valve appears to operate but the Air Injection Reaction (AIR) system does not turn "ON," refer to:

Chart C-6A Secondary Air System Check for vehicles equipped with the VO8 option (Belt Driven Pump). Chart C-6A Secondary Air System Check - Belt Driven Pump

Or

Chart C-6B Secondary Air System Check for vehicles without the VO8 option (Electric Air Pump). Chart C-6B Secondary Air System Check - Electric Air Pump

These charts both provide further AIR diagnosis.

2. This check can detect a partially shorted coil which would cause excessive current flow. Excessive current flow to a quad-driver module will be detected as a fault and set this DTC. If excessive current flow is detected, a circuit check will be performed to isolate the device from the wiring.

3. The remaining checks will identify a circuit problem that has caused an excessive current flow or inoperative relay/solenoid valve. If a QDM circuit check is performed on a relay, it is important to identify and test the relay coil terminals of the harness connector to avoid improper diagnosis.

DIAGNOSTIC AIDS
Using a Tech 1 scanner, monitor the quad-driver module status while moving related harness connectors, including ECM harness. If the failure is induced, a fault will appear on the Tech 1 scanner. This can help locate the intermittent. Check for bent pins at ECM and ECM connector terminals. If DTC 26 reoccurs with no apparent connector problem, replace ECM.