Vehicle stalls

Tiny
LCOX4765
  • MEMBER
  • 1991 BUICK CENTURY
  • 3.3L
  • 6 CYL
  • FWD
  • AUTOMATIC
  • 176,000 MILES
When letting of the gas coming to a stop and reaching a low RPM the vehicle will stall at random. I can hear the fuel pump charging and have replaced the fuel filter and it did not stall for a week, air filter, and cleaned the carbon out of the intake. I am receiving trouble codes on a code reader, 26, 27, 28.
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Sunday, August 7th, 2016 AT 5:12 PM

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Tiny
RIVERMIKERAT
  • MEMBER
I found this. The DTC Codes 26, 27, and 28 are all related to the quad driver module circuit.

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.
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Monday, August 8th, 2016 AT 7:16 AM

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