The ECM is used to control several components such as those illustrated above. The ECM controls these devices through the use of a Quad-Drive module (QDM). When the ECM is commanding a component "ON" the voltage potential of the output circuit will be "low"(near 0 volts). when the ECM is commanding the output circuit to a component "OFF" the voltage potential of the circuit will be "high" (near batter voltage) the primary function of the QDM is to supply the ground for the component being controlled.
Eack QDM has a faulty line which is monitored by the ECM. the faulty line signal is available on the data stream for "scan" tool test equipment. the ECM will compare the voltage at the QDM based on accepted values of the 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.
NOTICE: some QDM circuit will switch from "low" to "high" normally. examples : QDM2- if a/c pressure switch close and turn "ON" high speed coolant fan. QDM3 - if the brake is depressed. these conditions are normal and no code 26 is set. these are accepted conditions. A fault on QDM #2 will not set a code 26, diagnosis may be done by making sure the a/c is turned "off"
NOTICE: some "scan" tools will cause a false code 26 to be set if, the engine is running and the service brake is depressed for over 30 seconds.
code 26 will set if:
* system is in "closed loop"
* ECM detects an improper voltage level on a circuit connected to a QDM.
* the above conditions met for 20 seconds.
TEST DESCRIPTION: Number(s) refer to circled number(s) 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 circuit to determine which is a fault. this test checks the "service engine soon" light driver and the " service engine soon" light circuit.
2. QDM Symptoms:
* TCC - inoperative - code 39
* EGR _ Inoperative - Code 63,64,65
* HOT light - "ON" all the time/"OFF" during bulb check.
* Coolant fan an low speed all the time or won`t come "ON" at all.
* Poor driveability due to 100% canister purge.
3. This test will determine which circuit is out of specifications. ALL circuits EXCEPT "YC pin 11" (brown and white wire), the SES light and terminal "YC pin 9" (dark green wire), the "hot light" should be B+ when key is "ON", engine not running. the diagnostic test terminal is not ground.
DIAGNOSTIC AIDS:Monitor the voltage at each terminal while moving related harness connector, 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. if code reoccurs with no apparent connector problem, replace ECM.
SOURCE: GM SERVICE MANUAL.
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