Powertrain Controls - ECM/PCM

Code 26 Chart:

Code 26 Wiring Diagram:

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

CIRCUIT DESCRIPTION:


The ECM controls most components with electronic switches which complete a ground circuit when turned "ON". These switches are arranged in groups of 4, called quad-driver modules (QDM's) which can independently control up to 4 outputs (ECM terminals), although not all outputs are used. When an output is "ON", the terminal is grounded and its voltage will normally be low. When an output is "OFF", its terminal voltage will normally be high, except for the TCC, as noted below, which depends on the brake and 2nd gear TCC switches.

QDM's are fault protected. If a relay or solenoid coil is shorted, having very low or zero resistance, or if the control side of the circuit is shorted to voltage, it would allow too much current into the QDM. The QDM senses this and the output turns "OFF", or it`s internal resistance increases to limit current flow to protect the QDM. The result is high output terminal voltage when it should be low. If the circuit from B+ or the component is open or the control side of the circuit is shorted to ground, terminal voltage will be low, even when output is commanded "OFF". Either of these conditions is considered to be a QDM fault. See NOTE: below.

Each QDM has a separate fault line to indicate the presence of a current fault to the ECM's central processor. A Scan tool displays the status of each of these faults. as "LOW" = OK, "HIGH" = *Fault. Because of the brake and second gear switches in the TCC circuit, Code 26 is set under different conditions for QDM A and QDM B as follows:

^ QDM A Fault line = "HIGH" for 20 seconds or more.

^ QDM B Fault line = "HIGH" for 20 seconds or more and Brake switch signal indicates brake switch is closed and 2nd gear state switch indicates transaxle is in 2nd or 3rd gear

OR

^ TCC is commanded "ON".

*NOTE: (Auto Trans Only) QDM B will read "HIGH" until 2nd gear TCC switch closes while driving the vehicle.