hello ive been having a problem with my buick, my ses light is on at idle, shut off when i reach 20 mph and when accelerating, and comes back on while cruising at 40 and higher. the code i pulled is 26 and the car drives good shifts good too i replaced every sensor and the pcm i cant figure out the cause neither can mechanics the car idles a little rough it has new ignition module coils wires and plugs if there is anyone who recognizes these symptoms i would appreciate any advice thank you in advance

usually qdm failure is a short in the wiring to the sensors that the qdm1 power not the sensor

thank you for the reply i am hoping someone may be able to give ideas based on the symptoms

OK, found this with a search:

Part 1 of 3
Notes

DTC 26 - Quad-Driver (QDM) Circuit

Quad-Driver (QDM) Circuit


Circuit Description:

Quad-Driver Modules (QDM) are used to control the components shown in the illustration above. 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 is low and when the component is commanded 'OFF," the voltage potential on the sense line is high.

DTC 26 will set when the ECM is commanding a QDM A controlled component "ON" and the voltage potential on the sense line is high, or if the component is commanded "OFF" and the voltage potential on the sense line is low. The fault line, which can be monitored with the Tech 1, will read "HIGH" under these conditions. If the ECM detects a "HIGH" QDM A fault line when it expects a "LOW" one, it will illuminate the MIL (Service Engine Soon) and store DTC 26. QDM B will not set DTC 26.

Important

DTC 26 will not set even though the fault line scans "HIGH" under the following conditions:
Ignition "ON"/engine not running. OR
Vehicle not operating in 2nd or 3rd gear. OR
Brake applied.

Vehicles with a 3T40 transaxle will read "HIGH" until the TCC in-line switch is closed. To check, disconnect TCC connector and connect a test light between terminals "A" and "D". This should cause the Tech 1 to display "LOW" for QDM A with the engine running.

Test Description: Number(*) below refer to circled number(*) on the diagnostic chart.

The ECM does not know which controlled circuit caused DTC 26 50 this chart will go through each of the circuits to determine which is at fault. This test checks the MIL (Service Engine Soon) driver and the MIL (Service Engine Soon) circuit.
QDM symptoms:

TCC - malfunctioning - DTC 39.
"Check gauges" or hot light - "ON" all the time/"OFF" during bulb check.
Cooling fan "ON" all the time or won't come "ON" at all.
Poor driveability due to 100% canister purge.




Part 2 of 3
Notes

DTC 26 - Quad-Driver (QDM) Circuit (Part 2 Of 3)

Quad-Driver (QDM) Circuit


Circuit Description:

Quad-Driver Modules (QDM) are used to control the components shown in the illustration above. 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 is low and when the component is commanded "OFF," the voltage potential on the sense line is high.

DTC 26 will set when the ECM is commanding a QDM A controlled component 'ON" and the voltage potential on the sense line is high, or if the component is commanded "OFF" and the voltage potential on the sense line is low. QDM B will not set DTC 26.

Vehicles with a 3T40 transaxle will read "HIGH" until the TCC in-line switch is closed. To check, disconnect TCC connector and connect a test light between terminals "A" and "D". This should cause the Tech 1 to display "LOW" for QDM A with the engine running.

Test Description: Number(*) below refer to circled number(*) on the diagnostic chart.

This test will determine which circuit is out of specifications.

Diagnostic Aids:

Monitor the voltage at each terminal while moving related harness connectors, including ECM harness. If the failure is induced, the voltage will change. This may help locate the intermittent. Check for bent pins at ECM and ECM connector terminals. Also, ensure that the mid gear switch and CKT 232 is not open with the vehicle operating in park/neutral or 1st gear - This condition can cause a false DTC 26 to be set. Refer to DTC 27 chart for diagnosis. If DTC re-occurs with no apparent connector problem, replace ECM.






Part 3 of 3
Notes

DTC 26 - Quad-Driver (QDM) Circuit (Part 3 Of 3)

Quad-Driver (QDM) Circuit


Circuit Description:

Quad-Driver Modules (QDM) are used to control the components shown in the illustration above. 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 is low and when the component is commanded "OFF," the voltage potential on the sense line is high.

DTC 26 will set when the ECM is commanding a QDM A controlled component "ON" and the voltage potential on the sense line is high, or if the component is commanded "OFF" and the voltage potential on the sense line is low. The fault line, which can be monitored with the Tech 1, will read "HIGH" under these conditions. If the ECM detects a "HIGH" QDM A fault line when it expects a "LOW" one, it will illuminate the MIL (Service Engine Soon) and store DTC 26. QDM B will not set DTC 26.

IMPORTANT: DTC 26 will not set even though the fault line scans "HIGH" under the following conditions:

Ignition "ON"/engine not running. OR
Vehicle not operating in 2nd or 3rd gear. OR
Brake applied.

Vehicles with a 3T40 transaxle will read "HIGH" until the TCC in-line switch is closed. To check, disconnect TCC connector and connect a test light between terminals "A" and "D". This should cause the Tech 1 to display "LOW" for QDM A with the engine running.

Test Description: Number(*) below refer to circled number(*) on the diagnostic chart.

This test will determine if the problem is the circuit or the component. As the factory installed ECM is over current protected, it is highly unlikely that the ECM needs to be replaced.

Mistake, that was for a Skylark. admin please delete that post.

Thanks Kim, but...I think it would be best to leave it. Most vehicles share the same diagnostic/wiring/engine systems and this could be used for troubleshooting in the future.

does that mean that description has nothing to do with my car?

It'* possible what she posted will be pertinet to your application.

Let me pull out my 93 LeSabre Shop manual and see what it says. I might be able to post something from it that could help.

Thank you Kim, my 95 service manual won't cover anything you or he has.