How to test a computer transistor driver with a basic test light. This test is especially valuable when you have a faulty solenoid and cannot rely on voltage changes with the circuit activated and deactivated. Also this is the fastest way to verify wiring integrity of the control wire. A manufacturer flowchart will have you disconnect the computer and solenoid, then measure for opens and shorts with an ohmmeter. It's absolutely ridiculous that this is the standard method of testing a circuit!

Before performing this test, you must know what polarity to connect the test light to.

Watch this video to determine circuit design:

How to test a computer controlled solenoid (power/ground side identification)

Engine Performance Diagnostics chapter 3 page 12

test shown enh 50

  • how to test a computer driver
  • how to test the control wire for opens and shorts
  • how to force an output driver to turn on without a scan tool
  • how to force an EGR solenoid circuit to turn on

Tools used

  • LED test light (preferred for computer driver testing)

end faq


(Chapter 3) Transistors, Solenoids, Computer Controlled Output Testing

Related videos:

How to test a computer controlled solenoid (circuit ID with voltmeter):

How to test a computer transistor driver (using a scan tool):

 How to test a computer controlled solenoid (power/ground side identification) :

end faq

For more information on this topic, I have written a “field manual” called Engine Performance Diagnostics which is available as an eBook or paper book.

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