Part 1 of 3

Code 26 (Part 1 Of 3):

Quad Driver Circuit:

CIRCUIT DESCRIPTION:

The PCM is used to control several components such as those illustrated 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 volt). 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.
The PCM detects an improper voltage level on a circuit connected to a QDM.
The above conditions met for 10.0 seconds.

NOTE: For QDM A to set a code 26, the brake switch must be closed (brake not applied). Some QDM circuits will switch from low to high normally. Example: QDM A - If the brake is depressed. This condition is normal and no code is set. Some scan tools may cause a false code 26 to be set if the engine is running and the service brake is depressed for over 30.0 seconds.

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

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.

QDM Symptoms:
- Cooling fan on low speed all the time or won't come on at all.
- Poor driveability due to 100% canister purge.