Part 1 of 3

Code 26 Part 1 Of 3:

Code 26 Wiring Diagram:

***UPDATED BY TSB # 92-U-35A DATED OCTOBER 1992



CIRCUIT DESCRIPTION:


Quad driver modules (QDM) are used to to control several components shown in the wiring diagram. When the ECM is commanding a component "ON," the QDM closes the switch completing the circuit to ground. Each QDM has a sense line and a fault line.

When a component is commanded "ON" the voltage potential on the sense line will be "low" and when the component is commanded "OFF" the voltage potential of the sense line is "HI."

Code 26 will set when the ECM is commanding a QDM 1 + 2 (A) controlled component "ON" and the voltage potential on the sense line is "HI", or if the component is commanded "OFF" and the voltage potential on the sense line is "LO." The fault line, which can be monitored with a scanner will
read "HI" under these conditions. If the ECM detects a "HI" QDM 1 + 2 (A) fault line when it expects a low one it will illuminate the "Service Engine Soon" Light and store a Code 26. QDM 3 (B) will NOT set a Code 26.


NOTICE: Code 26 will NOT set even though the fault line scans "HI" under the following conditions:

^ Ignition "ON"/engine not running.

OR

^ Vehicle not operating in second or third gear.

OR

^ Brakes applied.

Vehicles with a 3T40 transmission will read HI until the second gear switch is closed. To check, disconnect TCC connector and connect a test light between terminals "A" and "D". This should cause the scanner to display "LO" for QDM 1 + 2 (A) with the engine running.

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

1. The ECM 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 symptoms:


^ TCC Malfunction - Code 39.

^ "Check Gauges" "HOT" light - "ON" all the time/"OFF" during a bulb check.

^ Coolant fan "ON" all the time or won't come "ON" at all.

^ Poor driveability due to 100% canister purge.