CODE 26, QUAD DRIVER MODULE (QDM) ERROR
Note. This procedure has been revised. See appropriate TECHNICAL SERVICE BULLETIN article in the TECHNICAL SERVICE BULLETIN Section. The Technical Service Bulletin numbers are as follows: Oldsmobile 88-T-08, Pontiac 88-6-7, Buick 88-6E-2.
The ECM is used to control several components such as those illustrated in the mini circuit schematic. ECM controls these devices through the use of a QDM. When the ECM is commanding a component on, the voltage potential of the output circuit will be low (near zero volts). When the ECM 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 ECM. The fault line signal is available on the data stream for "Scan" tester equipment. The ECM will compare the voltage at the QDM based on accepted values of 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. Some QDM circuits will switch from low to high normally. Examples
QDM No. 2, if A/C pressure switch closes and turns on high speed coolant fan or QDM No. 3, if brake is depressed. These conditions are normal and no Code 26 is set. These are accepted conditions. A fault on QDM No. 2 will not set a Code 26, but diagnosis may be done by ensuring A/C is turned off.
Note. Test numbers refer to test numbers on 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 SES light driver and the SES light circuit.
- 1A) If hot light and SES light do not illuminate, check for blown 20 amp fuse. If SES light is on and hot light is off, check for burned out bulb or open in circuits No. 39 and/or 35. If circuits and ECM connections are okay, ECM is faulty and must be replaced.
- 1B) Using "Scan" tester, check coolant parameter for high temperature signal. If tester does not indicate temperature greater than 282°F (130°C), check for grounded circuit No. 35 between hot light and ECM. If circuit is okay, ECM is faulty and must replaced.
- 2) QDM fault symptoms: * TCC inoperative, Code 39. * EGR inoperative, Codes 63, 64 & 65. * Hot light, on all the time, off during bulb check. * Coolant fan on low speed all the time of won't come on at all.
- Poor driveability due to 100 percent canister purge.
- 3) This test will determine which circuit is out of specification. All circuits, except "YC11", SES light, "YC9", and hot light, should be battery voltage when key is on, engine not running. The diagnostic test terminal is not grounded.
- 4) This test will determine if problem is the circuit of the component. As factory installed ECM is protected with an internal fuse, it is highly unlikely that ECM needs replacement.
Monitor the voltage at each terminal while moving related harness connectors, including ECM harness. If failure is induced, voltage will change. This may help locate the intermittent. Check for bent pins at ECM and ECM connector terminals. If code is reset with no apparent connector problem, replace ECM.
