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OBD I Trouble Codes

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OBD I Fault Code Detail
Code 26
Quad-Driver (QDM) Circuit
Quad driver modules (QDM) are used to control various components including, but not limited to CCP (Charcoal Canister Purge) Solenoid, Cooling Fan, TCC Solenoid, and AC Clutch.  When the ECM is commanding a component "On", the QDM closes the switch completing the circuit to the 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.  Code 26 will set if either of those conditions are reversed. 

Testing for a faulty quad driver module is a lengthy process, and should be done by a qualified techician. 

QDM failure symptoms:
-  TCC inoperative (Code 39)
-  Hot light "ON" all the time / "OFF" during bulb check
-  Coolant fan on low speed all the time, or won't come "ON" at all.
-  Poor driveability due to 100% canister purge
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