Cadillac Repair: 91 cad 5.7L, wiring harnesses, faulty computer

Question
QUESTION: ECM will not send GND.pulse signal to EGR solenoid, caused failed NOx Emission test. any ideas? Has new CTS,TPS,and MAP

         Isolate & Simplify
         byron
ANSWER: Do you know how to obtain the computer codes from this vehicle ?.

If so, Tell me what computer codes you get.

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QUESTION: no codes thru SEL or onboard diagnostic also have laptop with WINALDL software - no trouble codes
ANSWER: All you get is the flashing of the code 12 ?.

Manually ground out the egr ground circuit and see if the EGR valve operates. You should have battery voltage on one side and the ground goes back to the computer.

I am surprised that you did not get any EGR fault codes.

If you manually test the EGR valve and it works you will have to back track the ground side wire back to the computer to see if you have an open circuit , Bad connection or faulty computer.

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QUESTION: hello Tom,
  Yes-flashes code 12 only
  Grounding EGR solenoid GND ckt and EGR valve operates
  Grounding Diag. Test term. A&B gives GND to EGR solenoid with engine not running - with engine running EGR ground ckt has 200 ohms. thanks for your reply Health&happiness

Answer
Discnnect the wiring harnesses at the ECM and locate the ground wire to the EGR valve and ohm out the wire. If it tests OK, Looks like a faulty computer.

Did you manually test the egr valve ?

If you disconnect the EGR wiring harness and connect a test light between the harness connectors and with the ignition ON and engine OFF. Ground the diagnostic test terminal and the test light should flash.

If it does, replace the solenoid. If NO, Connect test light between ignition feed circuit at solenoid and a suitable engine ground and if the test light lights, You have a faulty ECM. If the test light does NOT light, you have an open ignition feed at the EGR connector.